Kidney and urinary tract

12.1 Clinical laboratory diagnosis of kidney and the urinary tract diseases

Lothar Thomas

12.1.1 Kidney functions

• Filtration of large volumes of small solutes and water
• Clearance of potential toxins that are derived form cellular metabolism and gastrointestinal bacterial metabolism
• Maintenance of acid-base homeostasis
• Maintenance of salt and water homeostasis.

In patients with symptoms of renal disease and urinary tract disorders, serum and urine biomarkers are important investigations for diagnosis, differentiation, and therapeutic monitoring of:

• Acute kidney injury
• Chronic kidney disease (CKD)
• Congenital kidney disease
• Urinary tract infection
• Kidney stone disease.

12.1.2 Kidney disease

Kidney disease is defined as a heterogenous disorder characterized by abnormalities in the function or structure of the kidney, with implications for health. Kidney disease can be associated with /49/:

• Functional abnormalities, related to a decreased glomerular filtration rate (GFR)
• Structural abnormalities, indicated are markers for kidney damage (increased albuminuria, pathologic urine sediment, and imaging).

Kidney disease is classified according to the cause of severity (stages) of GFR and albuminuria, known as cause-GFR-albuminuria classification.

Differentiation of kidney disease /49/:

• Acute kidney injury (AKI): The disorder is defined as a subset of acute kidney disease (AKD) with an onset within 7 days before presentation
• Acute kidney disease (AKD) and chronic kidney disease (CKD) are defined by their duration: AKD less than 3 months, CKD ≥ 3 months.

Clinical evaluation of AKD and CKD /49/:

• Testing of patients at increased risk, even in the absence of clinical symptoms
• Detection of AKD and CKD with the use of guideline-recommended criteria
• Evaluation of cause and stages (cause-GFR-albuminuria classification)
• Assessment of prognosis (risk) for kidney disease progression and complications
• Implementation of risk-based therapy.

Refer to Tab. 12.1-1 – Tests for renal disease.

An isolated elevated serum creatinine or a positive test strip result in urine should, initially, be seen as a screening result and be confirmed with repeated testing and more specific tests. In patients with suspected renal disease such results should be considered to be relevant, and they should be verified with further examinations and, particularly, by clinical findings.

For renal disease screening, reference intervals of biomarkers are definitive. More sensitive tests (renal ultrasound, albuminuria testing) and clinical investigations are needed for symptomatic patients with underlying illness that also effects renal function (diabetes, hypertension), and individuals with a relevant family history.

12.1.3 Acute kidney injury (AKI)

AKI is a complication of hospitalized patients with acute illness. AKI is defined by an acute decrease in kidney function and is assessed on the basis of the GFR, but is not limited, to ARF. AKI may be functional, mediated by hemodynamic factors (prerenal AKI) or from obstructed urine excretion (postrenal AKI) or may result from intrinsic processes involving one or more renal structures, but is not limited to ARF. AKI may be the result of sepsis, shock or the accumulation of nephrotoxins.

In this contribution, AKI and ARF are treated separately.

AKI is related to a decreased GFR (e.g., retention of creatinine) or a reduction of urine output and development of metabolic disturbances such as metabolic acidosis and hyperkalemia /2/.

ARF encompasses various specific and non-specific etiologies /1/:

• specific kidney diseases e.g., acute interstitial nephritis, acute glomerular and vasculitic renal disease
• non-specific conditions e.g., ischemia toxic injury
• extrarenal pathology e.g., prerenal azotemia and acute postrenal obstructive nephropathy.

No specific pharmacologic therapy is effective in patients with established AKI and the care of such patients is limited to supportive treatment, including renal replacement therapy /3/.

12.1.3.1 Definition of AKI

Recommendations for the definition and classification of AKI were developed by nephrologists and intensive care specialists. AKI is redefined as a subset of ARF, with an onset within 7 days before presentation. The AKI is based upon serum creatinine determination, the urine output, and the GFR. Refer to Tab. 12.1-2 – Definition of acute kidney failure according to the AKI classification.

For the diagnosis and classification of AKI a pre morbid serum creatinine baseline value must be known for application as a component of the AKI criteria /4/. Since a pre morbid creatinine value is often not available, the estimation of a baseline serum creatinine value by use of the Modification of Diet in Renal Disease (MDRD) equation is recommended assuming a lower limit of normal glomerular filtration rate of 75 [mL × min^–1 × (1.73 m²)^–1].

Acute kidney injury is a common condition in patients with liver cirrhosis. It occurs in up to 50% of hospitalized patients with cirrhosis and in 58% of such patients in the intensive care unit. The initial mechanism in the pathogenesis is portal hypertension and increased intrahepatic resistance due to distortion of the liver architecture. Vasodilatation results in a reduction of the effective arterial blood volume that will activate the neurohumeral system (renin-angiotensin-aldosterone), the sympathetic, and the arginine-vasopressin system leading to Na⁺ und water retention and ascites formation. Fractional excretion of Na⁺ below 0.1%, for urea below 21% and excretion of less than 440 mg/L albumin differentiates cirrhosis caused AKI from acute tubular necrosis /50/.

12.1.4 Acute renal failure (ARF)

Acute renal failure is differentiated in acute prerenal failure, acute renal failure, and acute postrenal failure.

12.1.4.1 Acute prerenal failure

Prerenal ARF is by far the most common cause of ARF /5/. The most systemic factor is a response to renal hypoperfusion caused by sepsis and septic shock. The integrity of the kidney tissue is maintained. Prerenal ARF can complicate any other underlying disease that causes a reduction in the effective blood volume. Vasoconstrictor mechanisms are activated by the hypovolemia, in order that the blood pressure be maintained. The kidneys normally respond to changes in renal perfusion pressure through an autoregulatory process which keeps the renal blood flow (RBF) and the glomerular filtration rate (GFR) constant. This occurs with a gradual expansion of the preglomerular capillaries, mediated by angiotensin, prostaglandins and nitric oxide (NO). A simultaneous vasoconstriction of the post glomerular capillaries, mediated by angiotensin II, preserves the glomerular capillary hydrostatic pressure. A tubulo-glomerular feedback mechanism stabilizes the GFR and the delivery of fluid to the distal tubule.

Prerenal ARF is triggered by drugs that cause a disturbance in the autoregulation of renal blood flow (RBF) and the GFR, or by renal hypoperfusion following vomiting, diarrhea, bleeding, diuretic therapy or massive burns. Treatment with ACE-inhibitors triggers ARF in 6–23% and in up to 38% of patients with bilateral and unilateral renal artery stenosis, respectively. Angiotensin receptor blockers can also trigger ARF /6/.

12.1.4.2 Acute renal failure (ARF)

ARF and chronic kidney disease (CKD) are differentiated by their duration /7/. ARF with or without AKI can be superimposed on preexisting CKD. Kidney disease is classified according to the cause and severity (stages) of GFR and albuminuria. ARF is a syndrome in which the principle within the kidney and typical structural changes can be seen on microscopy /8/.

The following disturbances due to tubular necrosis occur /49/:

• Decrease in the GFR
• Reduction in the concentrating capability of the kidneys
• Proteinuria and interstitial fibrosis
• Reduced tubular Na⁺ reabsorption.

Two components are responsible for an acute decrease in the GFR:

• A vascular component, which comprises pre-glomerular vasoconstriction with a decrease in the GFR, decreased blood flow in the external medulla and an activation of the tubuloglomerular feedback mechanism
• A tubular component; it involves tubular obstruction, the trans tubular back flow of ultra filtrate, and interstitial inflammation.

Histologically, the tubular cells manifest vacuolization, the loss of brush border membrane and desquamation of the cells into the lumen. Interstitially, edema with mild leukocyte infiltration occurs. As the damage increases, the tubular cells desquamate from the basement membrane and aggregate in the tubular lumen. In the kidney cortex many tubular cells are injured only to a sub-lethal extent, and may regenerate after restored re perfusion. Following the desquamation of necrotic and apoptotic cells and the accumulation of monocytes, quiescent kidney cells are activated and generate tubular cells. In the next phase poorly differentiated epithelial cells are activated, and they pre-differentiate to tubular cells. In the last phase, survived tubular cells proliferate and the regeneration of the tubule takes place along with the renewal of its regular functions /9/.

For further information refer to:

• Tab. 12.1-3 – Etiology of acute kidney failure
• Tab. 12.1-4 – Acute renal failure: clinical and laboratory findings
• Tab. 12.1-5 – Diagnostically significant laboratory tests in acute renal failure.

12.1.4.3 Acute postrenal ARF

Obstruction to urine output causes the so-called post renal renal failure, which is the most common cause of acute functional impairment in ambulatory patients /9/. Post renal acute renal failure is often secondary to prostatic hypertrophy. Frequent cases are bladder neck obstruction from an enlarged prostate, ureteric obstruction from pelvic tumors or retroperitoneal fibrosis, papillary necrosis or large calculi. In addition, in their medical history these patients report alternating anuria, oliguria and polyuria as well as, occasionally, hematuria.

12.1.5 Chronic kidney disease

Chronic kidney disease (CKD) affects 10–16% of the World’s population. CKD is defined as a sustained estimated glomerular filtration rate (GFR) of less than 60 [mL × min^–1 × (1.73 m²)^–1] or a urinary albumin excretion of 30 mg or more per day, or both, for more than 3 months. Most cases result from disorders of the filtration barrier of the kidney. CKD is defined as /10/:

• Reduction of the glomerular capillary surface area (number of glomerula decreased) available for filtration. Reduction of the glomerular filtration rate (GFR) is an indicator of the decrease in the glomerular filtrate
• Reduction of the intrinsic permeability of the capillary wall by altering its selective permeability. Albuminuria is a manifestation of reduction of the intrinsic permeability of the capillary wall.

CKD comprises /10/:

• Decreased GFR below 60 [mL × min^–1 × (1.73 m²)^–1]. The GFR is considered to be the best marker of renal function and is decreased in a broad spectrum of kidney diseases. A GFR decrease is associated with a higher risk of CKD complications in comparison to individuals with a normal GFR. Patients with end stage renal disease have a GFR < 15 [mL × min^–1 × (1.73 m²)^–1]. The GFR < 60 [mL × min^–1 × (1.73 m²)^–1] for definition of CKD was chosen because it lies just below one half of the GFR value in young adults, namely 125 [mL × min^–1 × (1.73 m²)^–1].
• Markers of kidney damage (one or more): albuminuria, urine sediment abnormalities, electrolyte and other abnormalities due to tubular disorders, abnormalities detected by histology, structural abnormalities detected by imaging, history of kidney transplantation.
• The time frame of 3 months is a measure of the chronic disease. If the time frame cannot be documented, CKD is not confirmed.

The definition of CKD is also valid for newborns and for children up to 18 years of age. However, the following exceptions apply to children below 2 years:

• The time frame of 3 months does not hold for newborns
• For the criterion of GFR < 60 [mL × min^–1 × (1.73 m²)^–1] an age-appropriate value must be utilized
• For the criterion of albuminuria ≥ 30 mg/24 h, an age-appropriate value must be utilized.

12.1.5.1 Staging of chronic kidney disease

CKD is classified according to the Kidney Disease Global Outcomes (KDIGO) based on cause ( C ), GFR category (G) and albuminuria category (A). The classification is designated as CGA /10/.

Cause

The cause of CKD is based on presence or absence of systemic disease and the location within the kidney of observed or presumed pathologic anatomic findings (Tab. 12.1-7 – Classification of CKD based on primary renal disease or systemic disease). The cause of disease is included in the classification of CKD because of its fundamental importance in predicting the outcome of CKD and choice of cause-specific treatment.

GFR category

The GFR category is assigned as shown in Tab. 12.1-8 – GFR categories in CKD.

Albumin category

Albuminuria is included as an additional expression of severity of disease not only because it is a marker of the severity of injury but also because albuminuria itself strongly associates with progression of kidney disease (Tab. 12.1-9 – Albuminuria categories in CKD).

12.1.5.2 Prognosis of CKD

In predicting the risk for outcome of CKD the cause of CKD, the GFR category, the albuminuria category, other risk factors and co morbid conditions have to be evaluated /10/. The relative strength of each of these factors for kidney disease endpoints will vary for each complication or outcome of interest. Risk for kidney disease and endpoints is individually driven by an individual patient’s diagnosis.

Independent of the cause of CKD, the GFR and albuminuria are risk multipliers of further complications of CKD, such as overall mortality and mortality due to cardiovascular disease (CVD). On their own, neither the GFR categories nor the albuminuria categories enable prognostic statements. Risk associations for the progression of CKD, CVD-associated mortality and kidney failure are, however, possible with the combination of GFR and albuminuria categories (see Fig. 12.9-1 – Assessment of the course of the glomerular filtration rate (GFR) and of albumin excretion in type 1 diabetes mellitus). The lower the GFR and the higher the degree of albuminuria at the time of the CKD diagnosis, the more marked is the progression of CKD.

12.1.5.3 Clinical symptomatology in CKD

Most renal diseases only manifest clinical symptoms or significant laboratory findings in the CKD stage. Irreversibility is usually already present and the therapy consists of retarding progression to kidney failure. In some diseases, however, the CKD is reversible, or a therapy (immunosuppressive therapy of glomerulonephritis) can achieve a partial improvement in the kidney injury and function /11/.

The decrease in GFR leads to metabolic and endocrine complications such as metabolic acidosis, anemia, bone mineralization disturbances, increased cardiovascular and overall mortality. The final stage of CKD (end stage renal disease, ESRD) is the state of kidney failure that can only be treated with kidney replacement therapy (dialysis or kidney transplant). Proteinuria, hypertension, diabetes mellitus, race, and ethnicity are strong risk factors for progression of CKD to ESRD and the higher ESRD incidence among men than among women is most pronounced in elderly patients. Declines in renal function, measured by creatinine clearance, occur in two thirds of healthy older adults over time but progress to ESRD in only 1–2% of these individuals /14/.

The 5-year survival rates among dialysis patients are low (Europe 40.5%, USA 55.2%) and has remained constant during the past decade /13/. CKD is a powerful risk factor for poor survival. The mortality rates are considerably higher among CKD patients than among the general population (17.7 versus 5.5/100 patient years) /12/.

Cardiovascular disease (CVD) is the single largest cause of mortality, responsible for 40–50% of deaths among CKD patients. CKD patients are more likely to die of CVD than to develop ESRD, with mortality rates estimated to be 10 to 30-fold higher in dialysis patients as compared to the healthy population /12/.

CKD patients have an elevated prevalence of traditional risk factors for cardiovascular mortality such as atrial fibrillation, hypertension, diabetes mellitus, obesity, hypercholesterolemia as well as of newer risk factors like low grade inflammation (CRP ≥ 3 mg/L), abnormal lipoproteins and markers of oxidative stress. However, some CKD patients do not have hypertension or dyslipidemia and nonetheless manifest elevated cardiovascular risk /12/.

Finally, mineral and bone disorders develop early in CKD patients and biochemical abnormalities are common (increased phosphate and parathyroid hormone, decline of vitamin D). Diseases such as insulin resistance and amenorrhea are also diagnosed in CKD. Also meaningful are neurological diseases that cause muscle weakness and sensory disturbances in the extremities and, in acute renal failure, are associated with altered mental status and encephalopathy.

12.1.5.4 Diabetes mellitus

In Europe and in the US Diabetes mellitus is the most frequent cause of chronic kidney disease (CKD), while in developing countries inflammatory renal disease such as glomerulonephritis and interstitial nephritis are common. About one-third of type 1 diabetes patients and nearly half of the patients with diabetes type 2 develop endstage kidney disease. Diabetic disease frequently results in a progressive decline in glomerular filtration rate (GFR) and loss of plasma proteins which produce proteinuria and albuminuria. Estimated GFR and albuminuria are the key biochemical parameters for monitoring patients with CKD. In a study /55/ circulating proteins that modulate TGF-β signaling were measured. The results revealed associations between progression to CKD and increased concentrations of neuroblastoma suppressor of tumorgenicity 1 protein (NBL1).

The association of CKD with diseases is described in Tab. 12.1-10 – Chronic kidney disease in association with systemic disease.

12.1.5.5 Progression of CKD and acute kidney injury

Patients with CKD, particularly elderly individuals, are more likely to experience acute renal failure (ARF) and ARF is a risk factor for progression to ESRD /11/. In ischemia associated hypoxia developing ARF is characterized by a temporary decrease in GFR, reduced urine-concentrating ability, proteinuria, impaired Na⁺ processing and interstitial fibrosis. In elderly individuals, in the absence of other kidney insults glomerular capillary density and interstitial peri tubular vascular density decrease and are exacerbated by acute injury. The progression of CKD is not, apparently, a smoothly proceeding continuous event but rather one that occurs in phases.

In CKD a continuous decline in the number of nephrones and a compensatory hypertrophy of the remaining nephrones develops. The result is, initially, compensatory glomerular hyper filtration, which is contingent upon a rise in the capillary pressure and an increase in capillary flow. With time, however, and as a consequence of elevated capillary pressure, sclerosis of the nephrones along with a decrease in performance occurs.

12.1.5.6 Laboratory findings in CKD, and prediction of CKD progression

In CKD patients GFR and albuminuria should be assessed at least annually. Small fluctuations in GFR are common and are not necessarily indicative of progression. CKD progression is defined based on one of more of the following /15/:

• Decline in GFR category (G1 ≥ 90; G2 60–89; G3a 45–59; G3b 30–44; G4 15–29; G5 < 15 [mL × min^–1 × (1.73 m²)^–1]. A certain drop in eGFR is defined as a drop in GFR category accompanied by a 25% or greater drop in eGFR from baseline.
• Rapid progression is defined as a sustained decline in eGFR of more than 5 [mL × min^–1 × (1.73 m²)^–1] per year.

See Tab. 12.1-8 – GFR categories in CKD and Tab. 12.1-9 – Albuminuria categories in CKD.

12.1.5.7 Hemodialysis

The incidence of CKD is > 300 per 1 million individuals per year, and the prevalence of individuals with renal replacement therapy is 804 per million. The optimum timing of dialysis treatment is still a matter of debate, and mainly based on clinical judgement and clinical experience. K/DOQI guidelines suggest that the estimation of estimated GFR in conjunction with the evaluation of the nutritional and clinical status of the patient should guide decision making /16/. A problem is, however, that the initiation of hemodialysis at higher, as compared with lower, eGFR values is associated with increased mortality. In the literature widely differing eGFR values were published for the initiation of dialysis, e.g., ≤ 6–15 [mL × min^–1 × (1.73 m²)^–1]. Since, however, the eGFR is dependent not only on renal function, but also on nutritional status, muscular mass and fluid overload, the European Best Practice Guidelines do not recommend the eGFR as an exclusive marker for the optimal timing of dialysis but rather, the evaluation of the patient’s co morbidities. In addition, the ratio of the urea clearance to the creatinine clearance is recommended (see Section 12.6.5 – Clinical significance).

Inflammation, hyper hydration and CVD are determining mortality factors in dialysis patients. Therefore, the following are determined as risk markers:

• For the monitoring of hyper hydration, surrogate markers such as inter dialytic weight gain, ultrafiltration rate and blood pressure
• CRP as inflammation marker
• NT-proBNP as a stress marker of the cardiac wall
• Cardiac troponin for the detection of ischemic heart muscle injury.

12.1.5.8 Medications and monitoring after kidney transplantation

Patients who underwent kidney transplantation, have better survival, improved cognition, and less economic burden than those who continue with dialysis. The mortality rates within months after transplantation are elevated in re-transplantation, ABO incompatible transplantation, age of the patient (older than 60 years), elevated levels of glucose, increased AST, and hypoalbuminemia /11/. Acute rejection within the first year after transplantation is primarily T-cell rejection. Episodes of acute rejection are common after kidney transplantation and are confirmed by histologic diagnosis after kidney biopsy. Donor specific antibodies appearing after transplantation represent a preformed or memory response causing antibody-mediated rejection. The incidence of clinical acute rejection is 10 to 15% and of subclinical rejection 5 to 10%, respectively during the first year /12/. Post-transplantation viral infection e.g., Cytomegalo virus, BK virus (human Polyoma virus) can be prevented by vaccinations admistered before transplantation.

Laboratory tests for monitoring patients after kidney transplantation are:

• Chemistry: urea, creatinine, glucose, albumin, total protein. AST, ALT, total bilirubin, Na⁺, K⁺ and chloride
• Hematology: complete blood count, thrombocytes
• Medications: Calcineurin inhibitors (cyclosporine or tacrolimus)
• Antimetabolites, e.g., mycophenolate mofetil, mycophenolic acid, azathioprine (during pregnancy)
• Glucocorticoid

12.1.5.9 Fibroblast growth factor 23

Fibroblast growth factor 23 (FGF23) plays an important role in the pathophysiology of chronic kidney disease. FGF23 lowers 1,25(OH)₂D₃ and has a central role in vitamin D metabolism. CKD elevated levels of FGF23 appear to be associated with poor cardiovascular outcomes. Acceleration of ageing has been associated with the increase in phosphate and alteration of the FGF23. Elevation in phosphate levels, hyperparathyroidism and elevated concentrations are common in CKD. Long-term elevated concentrations of FGF23 and intact parathyroid hormone (PTH) during CKD can result in adverse effects on the cardiovascular system /51/.

12.1.6 Glomerulonephritis (GN)

The kidneys are perfused by some 25% of the cardiac output. Daily, 180 liters of primary urine are formed, and this is accomplished by filtration at the glomerular filtration barrier. Damage to the glomerular capillary wall by altering its selective permeability is diagnosed by measuring albuminuria. Albuminuria, like reduction in the GFR, is associated with increased cardiovascular morbidity and mortality and even in the absence of diabetes and hypertension /16/.

The filtration barrier in the glomerulus (Fig. 12.1-1 – Structure of the renal filtration mechanism) which separates the blood and urine compartments, enables only the passage of a limited amount of plasma proteins. The filtration barrier is composed of the glomerular endothelium, the glomerular basement membrane (GBM) and the podocyte (glomerular-visceral epithelial cell). Podocytes extend long processes toward the GBM to which they affix by cell surface adhesion proteins (α₃β₁ integrin and dystroglycan). The foot processes of adjacent podocytes interdigitate and are separated by narrow spaces (30–40 nm) that are bridged by a porous membrane called the slit diaphragm. The diaphragm contains pores freely permeable to plasma proteins. The integrity of the slit diaphragm is one of the principal determinants of the perm-selective properties of the glomerular filtration barrier.

The NPHS1 gene encodes the protein immunoglobulin superfamily protein nephrin, a major constituent of the slit diaphragm, and the NPHS2 gene encodes podocin. Mutations of the NPHS1 gene are diagnosed in severe forms of the nephrotic syndrome and mutations of the NPHS2 gene in familial segmental glomerulosclerosis and the sporadic steroid refractory nephrotic syndrome in children /16/.

Glomerulonephritis (GN) is a disease with intraglomerular inflammation and cellular proliferation. It is associated with hematuria. Numerous disorders which, strictly speaking, are believed to be of inflammatory genesis, are concealed under the term GN. Because, however, the transition to non-inflammatory glomerulopathy is smooth, the terms GN and glomerulopathy are often used as synonyms. The distinction between primary and secondary GN is clinically more relevant, however. This is often possible only through the exclusion of a systemic disease or the investigation of a kidney biopsy. Primary and secondary GN with and without proliferative changes are distinguished /17/ (Tab. 12.1-11 – Types of glomerular diseases).

Anti-glomerular basement membrane (anti-GBM) disease can also cause pulmonary and renal disease. Circulating anti-GBM antibodies produce pulmonary symptoms only after disruption of the alveolar endothelium occurs and the antibodies are permitted to access the alveolar basement membrane; thereafter the inflammatory response mediates damage and disease.

12.1.6.1 Primary glomerulonephritis

In primary GN, also often called idiopathic GN, the origin of the pathogenesis is in the kidneys and the systemic manifestations are the result of the renal dysfunction. The most common form of primary GN is IgA nephropathy /18/. Post-streptococcal GN is also regarded as primary GN, in spite of the fact that, in the strict sense of the term, it cannot be classified as idiopathic.

IgA nephropathy

IgA nephropathy or Schönlein-Henoch Purpura Nephritis is the most common primary glomerulonephritis The annual incidence is 2.5 per 100,000 individuals. IgA nephropathy usually present with hematuria and evidence of an antecedent or concurrent upper respiratory infection.

Several factors are involved in the pathogenesis of IgA nephropathy:

• elevated concentration of mucosal circulating galactose-deficient IgA1
• IgG- or IgA-autoantibodies are produced
• pathogenic immune complexes (galacose deficient IgA1–autoantibodie) are formed
• immuncomplexes are deposited in the glomerular mesangium, causing glomerular injury
• During formation of immuncomplexes cytokines of T-cells promote B cell class switching to IgA1-producing plasma cells, while lectins and alternative complement system cause tissue injury.
• Albuminuria and hematuria are the laboratory diagnostic and clinical signs of IgA nephropathy.

Patients treated with immunosuppressive therapy (including glucocorticoids) had a greater reduction of proteinuria compared with those who received placebo /47/.

 

12.1.6.2 Secondary glomerulonephritis

Secondary GN results from renal involvement associated with systemic disease /18/. Diabetic nephropathy and lupus nephritis are examples of multisystem disease-dependent GN. Renal lesions are considered diffuse if they affect more than 50% of the glomeruli, and as focal if the number is lower. Global lesions affect the entire nephron, while with segmental lesions only a part of the nephron is involved. Due to leukocyte infiltration, proliferative glomerulopathies have an increased number of glomerular cells. Crescent-shaped glomerular lesions are those in which the Bowman’s capsule is filled with proliferating epithelial cells and infiltrating monocytes.

The clinical course of GN is as follows:

• Acute nephritic syndrome. This manifests clinically as edema and hypertension within a period of days to a few weeks and can develop into acute kidney failure. These classical clinical symptoms are evident in the proliferative forms of GN.
• Rapid progressive GN (RPGN) associated with a rapid decline in renal function. RPGN can result in end-stage renal failure within a period of days to a few weeks /17/. The pathological hallmark of this disease is the presence of cellular crescents surrounding most glomeruli. RPGN is, like the acute nephritic syndrome, the consequence of immune-mediated proliferative GN, which can be either of primary or secondary nature occurring as a complication of systemic disease. From a laboratory diagnostic standpoint, the urinalysis is nephritic.
• Primary chronic GN, a latent form of the disease that progresses slowly over a period of years. The indicators for this form are as follows: elevated creatinine, proteinuria or hematuria, and anemia.

12.1.6.3 Laboratory findings in glomerulonephritis

Increase in creatinine and nephritic urinalysis with macroscopic hematuria (frequent), dysmorphic erythrocytes, erythrocyte casts, sub nephrotic proteinuria (< 3 g/24 h). In non-proliferative forms of GN, such as focal segmental glomerulosclerosis, the clinical and laboratory findings are less pronounced.

In IgA nephropathy, the laboratory findings include mild hematuria, proteinuria of > 0.5 g/24 h and a GFR of > 30 [mL × min^–1 × (1.73 m²)^–1].

12.1.6.4 Nephrotic syndrome (NS)

The NPHS1 genes code for the protein nephrin, the glomerular slit diaphragm which, along with podocin, regulates signal transmission (Fig. 12.1-1 – Structure of the renal filtration mechanism). Mutations in NPHS1 are responsible for the development of a severe congenital nephrotic syndrome. The nephrotic syndrome is defined as protein excretion of > 3.5 [g × 24 h^–1 × (1.73 m²)^–1]. Severe proteinuria is present and the consequences are, independent of the underlying disease, uniform: Na⁺ retention with edema, hyperlipidemia, elevated risk of thrombosis and susceptibility to infections. Diabetic nephropathy is the most frequent cause of the nephrotic syndrome /19/.

The most important non-diabetic kidney diseases associated with a nephrotic syndrome are the primary forms of GN which include minimal change GN, focal segmental glomerulosclerosis, membranous GN, and vasculitis/necrotizing GN.

12.1.6.4.1 Laboratory findings in nephrotic syndrome

Proteinuria, serum albumin < 25 g/L, often only with protein excretion > 10 g/L, also a decrease in antithrombin < 75%, hyperlipidemia with either raised cholesterol only or combined elevated cholesterol and triglycerides. The VLDL, IDL und LDL fraction and Lp(a) are elevated, the HDL fraction is usually normal /20/.

12.1.6.5 Minimal change disease (MCD)

MCD is a podocytopathy and is the cause of the nephrotic syndrome in 76% of affected children and 20% of affected adults. The steroid-resistant form is caused by a mutation in the NPHS2 gene. Under light and fluorescence microscopy, the glomeruli appear to be normal. The clinical presentation is sudden with the nephrotic syndrome and, occasionally, with hematuria and hypertension. Secondary MCD occurs in non-Hodgkin lymphoma and is drug-induced. Medications that can cause MCD are listed in Tab. 12.1-12 – Drugs with tubulo-interstitial, glomerular and vascular toxic effects /21/. Some MCD cases convert into focal segmental glomerulosclerosis.

12.1.6.6 Focal segmental glomerulosclerosis (FSGS)

FSGS is, likewise, a podocytopathy. A few or many glomeruli manifest segmental sclerosis. With immunofluorescence, some of the glomeruli show segmental glomerular IgM and C3. FSGS can be primary in nature and is then based upon a mutation in the NPHS2 gene, or it may be secondary (e.g., drug-induced). FSGS is similar to MCD clinically, although its onset is less sudden, it is associated with nephrotic proteinuria, mild hematuria and, occasionally, hypertension. The frequency of FSGS of the nephrotic syndrome is 8% in children and 15% in adults. Medications can lead to immunological and non-immunological injury. Segmental capillary obliteration occurs due to the accumulation of foam cells, matrix accumulation and adhesion to the Bowman’s capsule.

12.1.6.7 Membranoproliferative glomerulonephritis (MPGN)

MPGN is a pathological entity, characterized by subepithelial immune complex depositions and corresponding changes in the glomerular basement membrane. Histologically, characteristic thickening of the capillary wall is seen. Patients are often in their 4th to 5th decade of life, but one third is below the age of 16 or older than 60. Clinically, the patients present with a nephrotic syndrome, micro hematuria, temporary hypertension and mild renal insufficiency. Membranous GN can also be linked to neoplasms. In elderly patients with neoplasms or a nephrotic syndrome this link should, therefore, be taken into consideration. In addition, in order to exclude multiple myeloma, the urine should be tested for monoclonal immune protein excretion /22/.

12.1.6.8 Vasculitis/necrotizing glomerulonephritis

An inflammation of the vessel wall, the capillaries or the glomeruli is present. Vasculitis of the small kidney vessels leads to necrotizing or crescentic glomerulonephritis (the histological criteria are capillary fibrinoid necrosis and crescents). Based upon immunohistology, this form is differentiated into the categories with antibodies to the glomerular basement membrane, the immune complex form, and pauci-immune vasculitis. Most cases of pauci-immune vasculitis are associated with the presence of anti neutrophil cytoplasmic antibodies (ANCA). ANCA vasculitis can occur in a renally isolated manner or within the context of Wegener’s granulomatosis or Churg-Strauss syndrome. The patients present with rapidly progressing GN with hematuria, erythrocyte casts, proteinuria and acute renal failure. Fever, joint pain and myalgia are often present. Most cases are idiopathic in nature, while some are drug-induced (Tab. 12.1-12 – Drugs with tubulo-interstitial, glomerular and vascular toxic effects).

12.1.7 Tubulo-interstitial disease

The renal tubulo-interstitial compartment is the space between the basement membrane of the glomeruli, the tubuli and the peri tubular capillaries. The compartment comprises 4–7% of the healthy renal cortex and consists of fibroblasts and dendritic cells. The exchange of substances between the blood and the primary urine in the tubular lumen includes the passage through the tubulointerstitium. Tubulointerstitial diseases are differentiated into acute tubular necrosis and tubulointerstitial nephritis. In the latter, acute, chronic and crystal-induced forms and primary and secondary forms are distinguished (Tab. 12.1-13 – Types of tubulointerstitial nephritis).

12.1.7.1 Acute tubular necrosis

Acute tubular necrosis is defined as acute kidney failure with tubular injury in the absence of significant or vascular renal pathology. A sudden rise in creatinine occurs, sometimes in combination with microscopic hematuria and mild proteinuria. The etiology is that of acute kidney failure. Medications that are utilized in cancer therapy play an important role as a cause of acute tubular necrosis.

12.1.7.2 Tubulointerstitial nephritis (TIN)

TIN is characterized by inflammation of the renal interstitium and the tubuli, resulting in interstitial edema, acute tubular injury or interstitial fibrosis with tubular atrophy /22/. Mechanisms that cause TIN can be attributed to direct cytotoxic reactions as well as systemic inflammatory or immunological reactions. Direct cytotoxic mechanisms, as in analgesic nephropathy and lead poisoning-related nephropathy, are dose and time-dependent. Of the functional renal disorders, 5–10% are based upon primary TIN. The largest proportion (85%) is drug-induced, while 10% are due to an infectious process (e.g., Epstein-Barr virus infection) and 5% are idiopathic /23/.

Acute TIN

Clinically, acute TIN is similar to acute tubular necrosis. Histologically, interstitial edema and tubular injury are present, along with a mixed infiltrate composed of mononuclear cells and also, in some cases, of eosinophilic granulocytes (if drug-induced). Clinically, symptoms of hypersensitivity as well as exanthema and eosinophilia are also often diagnosed; these are relatively specific attributes. The causes of acute TIN are medications (Tab. 12.1-12 – Drugs with tubulo-interstitial, glomerular and vascular toxic effects), infections and autoimmune diseases (M. Sjögren). In contrast to acute tubular necrosis, leukocyturia is present and not uncommonly in drug-induced TIN, eosinophiluria of > 5%. The incidence of TIN in patients with ESRD is 26–42% /22/.

Chronic TIN

The chronic TIN involves tubulointerstitial fibrosis, which is an integral feature of the structural changes in chronic progressive renal insufficiency /23/. The accumulation of extracellular matrix in the tubulointerstitial space is mediated by myofibroblasts. These are formed from localized fibroblasts, tubular epithelial cells, periadventitial cells and, probably, from mesenchymal stem cells and endothelial cells as well. The fibrosis usually precedes the tubulointerstitial infiltration with mononuclear inflammatory cells. Proteinuria is one of many triggering events of glomerular or vascular disease, which transfer the disease process to the interstitium. The elevated protein filtration is likely toxic at the level of the tubular cells and induces the activation of chemokines and cytokines and increased expression of adhesion molecules. All of this leads to an augmented influx of inflammatory cells into the interstitium. The result is tubular atrophy with a gradual decline in kidney function.

Crystal-induced TIN

It is not uncommon, since many drugs tend to precipitate as crystals within the tubules (Tab. 12.1-12 – Drugs with tubulointerstitial, glomerular and vascular toxic effects).

12.1.7.3 Laboratory findings in tubulo-interstitial disease

In acute TIN, early symptoms include decreased urine concentration, sterile leukocyturia, leukocyte casts, erythrocyturia, erythrocyte casts, and occasionally macro hematuria /22/. Neither serum creatinine nor the eGFR provide a hint with regard to etiogenesis, but they do allow an important prognostic statement to be made.

In chronic TIN, in addition to a gradual decrease in the GFR, glucosuria, phosphaturia and acidification disorders of urine due to disturbed NH₄⁺ formation may be detectable (see Section 8.8.4 – Disturbances of chloride excretion).

A typical finding, regardless of the etiology of the TIN, is tubular proteinuria, caused by inadequate tubular reabsorption of low molecular weight proteins such as α₁-microglobulin. Total protein excretion is < 2.5 g/24 h and, in the chronic form, usually < 1 g/24 h, and in random specimen < 1 g/g creatinine.

The diagnosis of tubular proteinuria can be made semi quantitatively using SDS polyacrylamide gel electrophoresis, and quantitatively with the fractional clearance of β₂-microglobulin or α₁-microglobulin, relative to the creatinine excretion (see also Section 12.7 – Cystatin C).

12.1.8 Hepatorenal syndrome (HRS)

HRS is a fully reversible impairment of renal function in patients with severe hepatic failure. The prevalence in patients with liver cirrhosis and ascites is 18% after 1 year and increases to 39% at 5 years. Renal failure is a serious complication in patients with advanced liver cirrhosis and occurs in about 20% of patients hospitalized with decompensated liver cirrhosis /24/. In about 70% impaired renal function is caused by pre renal failure due to gastrointestinal hemorrhage, bacterial infection and hypovolemia. In about 30% of cases renal failure is caused by intrarenal etiology. HRS is due to renal hypo perfusion causing reduced filtration fraction. Vasodilation in the splanchnic region is the main trigger of hypo perfusion /24/. The vasodilation is associated with increased formation of vasodilators and vascular hypo responsiveness to vasoconstrictors.

12.1.8.1 Laboratory findings in hepatorenal syndrome

Creatinine based formulae overestimate the true glomerular filtration rate, especially in patients under 50 years and in those with ascites /25/. Refer to Tab. 12.7-2 – Diagnostic significance of cystatin C in impaired renal function.

12.1.9 Acute pyelonephritis

Acute pyelonephritis denotes inflammation of the renal pelvis and kidney /26/. Pyelonephritis usually occurs when enteric bacteria enter the bladder and ascend to the kidneys. Clinical presentation manifests suddenly with signs and symptoms of systemic inflammation (fever, chills, and malaise) and bladder inflammation (urinary frequency, urgency, and dysuria). Approximately 10% of septicemia cases originate from pyelonephritis.

12.1.9.1 Laboratory findings in acute pyelonephritis

The confirmatory test is the urine culture, which yields 10,000/mL or more colony-forming units of a uropathogen, usually E. coli. The white blood cell count shows granulocytosis, urinalysis is positive for leukocyte esterase and nitrites. The estimated GFR is decreased.

12.1.9.2 Glomerular hyperfiltration and hypertrophy

Obesity is a risk factor for chronic kidney disease (CKD) as obesity aggravates the prognosis of all kidney diseases, regardless of etiology /52/. Sustained glomerular hyperfiltration and hypertrophy can lead to glomerular injury and progressive kidney damage. A range of pathological mechanisms underlying this association include adipose tissue, chronic inflammation, dyslipidemia, insulin resistance, oxidative stress and the renin-angiotensin aldosterone system. Glomerular hyperfiltration results from increased renal blood flow and intra glomerular hypertension, inducing mechanical stress on the filtration barrier and postfiltration structures. Injured glomeruli increase in size before sclerosing and collapsing. Kidney biopsy can help identifying the underlying cause of kidney disease and assess disease progression and treatment effectiveness. Glomerular hypertrophy, as indicated by a maximal glomerular diameter of ≥ 242.3 μm has been shown to be a significant predictor of poor renal prognosis with IgA nephropathy /53/. The maximal glomerular diameter is not an indicator of compensatory glomerular hypertrophy but of renal damage itself /54/.

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12.2 Glomerular filtration rate (GFR)

Lothar Thomas

Functional abnormalities of the kidney are related to a decreased GFR. Kidney diseases are classified according to the cause and severity (stages) of GFR and albuminuria (known as cause-GFR-albuminuria classification, or CGA staging) /18/. Most kidney diseases do not have symptoms or findings until later in their course and are detected only when they are chronic.

Diseases that reduce glomerular capillary surface are or that alter the intrinsic permeability of the capillary wall reduce the GFR. Although downstream compensatory mechanisms maintain glomerular-tubular balance and regulate fluids, electrolytes, and the acid-base balance at physiologic levels, even small reductions in the GFR are associated with increased cardiovascular morbidity and mortality /15/. The same applies to albuminuria.

The kidney has excretory, endocrine and metabolic functions. The GFR is one component of the excretory function, but is widely accepted as the best overall index of kidney function because it is generally reduced after widespread structural damage and most other kidney functions decline in parallel with GFR in chronic kidney disease (CKD) /1/. Excretory, endocrine and metabolic functions decline together in most CKDs. This applies to:

• Water and electrolyte metabolism
• Acid-base homeostasis
• The synthesis of 1,25 (OH)₂D and erythropoietin
• The metabolism of low molecular weight proteins and insulin
• The excretion of metabolites like creatinine, urea, uric acid, and phosphate.

With reduction of the GFR due to CKD, all these functions are affected; a process that proceeds in parallel and, likely, in intervals rather than in a continuous manner /1/. It therefore makes sense to measure the GFR as the equivalent of renal function /1/. The KDIGO refers to a GFR < 60 [mL × min^–1 × (1.73 m²)^–1] as decreased GFR and a GFR < 15 [mL × min^–1 × (1.73 m²)^–1] as kidney failure. Physiologically, the GFR decreases with age. In individuals over 50 years, this occurs at the rate of 13 [mL × min^–1 × (1.73 m²)^–1] per decade of life. Thus, some elderly people have a GFR < 60 [mL × min^–1 × (1.73 m²)^–1] and a prognostic dilemma emerges in the range of 60–45 [mL × min^–1 × (1.73 m²)^–1].

12.2.1 Determination of GFR using exogenous filtration markers (measured GFR; mGFR)

GFR is measured indirectly through the concept of clearance, which is defined as the equivalent of plasma volume from which a filtration marker would have to be totally removed to account for its rate of excretion in urine per unit of time. Clearance is calculated by dividing the excretion rate of a filtration marker by its plasma concentration (C_x = U_xV/P_x) where U_x and P_x are urine and plasma concentrations respectively, of substance X, and V is urine flow rate (Tab. 12.2-1 – Clearance formula). When the filtration marker is freely filtered by the kidney, then C_x = GFR. Exogenous filtration markers are inulin, iothalamate, iohexol, EDTA and DTPA (Tab. 12.2-2 – Determination of measured GFR using exogenous filtration markers (mGFR)). The gold standard for determining the mGFR is inulin clearance, but this is the most complex method. Due to their complexity, methods that use exogenous filtration markers are usually only employed as confirmatory tests.

12.2.1.1 Reference interval

Refer to Ref. /6, 8 / and Tab. 12.2-3 – GFR reference intervals based on inulin clearance. The mGFR is associated with considerable inter individual and intraindividual variability. The day-to-day variability depends on the diurnal variation, protein intake and physical activity. Healthy young white males have an average mGFR of 130, while that of females is 120 [mL × min^–1 × (1.73 m²)^–1]. The lower limit for men is 90 and for women is 85 [mL × min^–1 × (1.73 m²)^–1].

With increasing age, the mGFR decreases, but there is no age group-specific reference interval for adults, particularly not for elderly and old people. The US National Kidney Foundation has, therefore, set the threshold for chronic kidney disease, independent of age, at < 60 [mL × min^–1 × (1.73 m²)^–1] /7/.

The GFR is based on the body surface area of 1.73 m² of a person weighing 75 kg and differentiated into six categories according to the KDIGO (Tab. 12.2-4 – GFR categories according to KDIGO).

12.2.2 Evaluation of GFR using endogenous filtration markers

GFR estimating equations are recommended for the evaluation of kidney function for routine clinical care.

12.2.2.1 Filtration marker serum creatinine

Current clinical guidelines /1/ recommend:

• Estimated GFR (eGFR) based on serum creatinine (eGFR_Cr) as the initial diagnostic test (Fig. 12.2-1 – Relationship between the glomerular filtration rate (GFR) and the serum creatinine concentration)
• A measured clearance (mGFR) or estimated GFR based on serum cystatin C (eGFR_Cys) as a confirmatory test or the combination of serum cystatin C and creatinine (eGFR_Cr-Cys) as confirmatory test. GFR estimates based on both filtration markers are likely to be more precise than estimates based on either marker alone, by minimizing errors due to non GFR determinants /2/.

12.2.2.2 Determination of eGFR using creatinine (eGFR_cr)

KIDGO recommendations for clinical laboratories /1/:

• Measure serum creatinine using a specific assay with calibration traceable to the international standard reference materials and minimal bias compared to isotope-dilution mass spectrometry reference methodology
• Report eGFR_cr in addition to the serum creatinine concentration in adults and specify the equation used whenever reporting eGFR_cr
• Report eGFR_cr in adults using the CKD-EPI creatinine equation. An alternative creatinine-based GFR estimating equation is acceptable if it has been shown to improve accuracy of GFR estimates compared to the CKD-EPI creatinine equation.
• Report serum creatinine concentration be rounded to the nearest whole number when expressed as standard international units (μmol/L) and rounded to the nearest 100th of a whole number when expressed as conventional units (mg/dL)
• When reporting eGFR_cr the result be rounded to the nearest whole number and relative to a body surface area of 1.73 m² in adults using the units mL/min/1.73 m²
• Understand clinical settings in which eGFR_cr is less accurate
• Values of eGFR_cr < 60 [mL × min^–1 × (1.73 m²)^–1] should be reported as decreased.

12.2.2.2.1 GFR estimating equations /21/

• The 2012 CKD-EPI creatinine equation is employed for adults and applies the variables of age and gender /17/. eGFR [mL × min^–1 × (1.73 m²)^–1] = 142 × min (S_Cr/k, 1)^α × max (S_Cr/k, 1)^–1,200 × 0.9938^age × 1.012 [female].
• The 2012 CKD-EPI Cystatin equation is employed for adults and applies the variables of age and gender (GFR_crea-cys and GFR_cys)
• Race-free 2021 CKD-EPI equations (eGFR_cr and eGFR_cr-cys).
• Cystatin C-based equation (eGFR_cys) of the European Kidney Function Consortium (EKFC) /19/. The authors demonstrate that the EKFC equations improve the accuracy of GFR estimation in select cohorts from Europe.

Based on the 2012 equations, the GFR and the progression of renal impairment is differentiated into categories specified in Tab. 12.2-4 – GFR categories according to KDIGO.

The European Federation of Laboratory Medicine (EFLM) /20/ recommends not to implement the race-free Chronic Kidney Disease Epidemiology Consortium (CKD-EPI) equation in European laboratories and to keep the 2012 version of CKD-EPI equation, without applying a race correction factor.

12.2.2.2.2 Determination of eGFR_cr in children

The criteria for CKD of GFR < 60 [mL × min^–1 × (1.73 m²)^–1] does not apply to children below 2 years of age in whom an age appropriate value should be applied. Since it is not until 2 years of age that one expects to see body surface area adjusted to GFR values comparable to seen in the adult the CKD-EPI equation can not be applied in children of lower age. The most robust pediatric eGFR equations, which have been derived from iohexol clearance, emerge from the CKDs study (Tab. 12.2-6 – Equations for estimation of GFR in children using serum creatinine) /1/. Values of eGFR_cr < 60 [mL × min^–1 × (1.73 m²)^–1] should be reported as decreased if evaluated with pediatric equations in children below the age of 2 years.

The Kidney Disease Improving Global Outcomes (KDIGO) guidelines recommend the use the Creatinine-based GFR in Children (CKID) equation to estimate GFR. However, this switch from the CKID_serum to CKD-EPI equation in young adults lead to sudden eGFR increases. Implausible changes in eGFR can result in large overestimation (bias +21 [mL × min^–1 × (1.73 m²)^–1] in the 18–20 age group and poor precision. The Lund-Malmö-Revised equation, and an age-dependent weighted average of CKID and CKD-EPI resulted in much smaller changes in eGFR at the transition (change 0.6, –2.1, –0.9 and –1.8 [mL × min^–1 × (1.73 m²)^–1] /19/. In the Lund-Malmö-Revised equation the eGFR is plotted against the age for children, adolescents, and young adults /19/.

12.2.2.2.3 Determination of eGFR_cys in adults

KDIGO suggests measuring cystatin C in adults with eGFR_cr 45–59 [mL × min^–1 × (1.73 m²)^–1] who do not have markers of kidney damage in cases where confirmation is required. Use a GFR estimating equation to derive GFR from serum cystatin C (eGFR_cys) rather than relying on the serum cystatin C concentration alone.

KDIGO recommendations for clinical laboratories /1/

• Measure serum cystatin C using an assay with calibration traceable to the international standard reference material
• Report eGFR from cystatin C in addition to the serum cystatin C level in adults and specify the equation used whenever reporting eGFR_cys and eGFR_cr-cys
• Report eGFR_cys and eGFR_cr-cys in adults using the CKD-EPI cystatin C equations, respectively, or alternative cystatin C-based estimating equations if they have been shown to improve accuracy of GFR estimates compared to the CKD-EPI eGFR_cys and CKD-EPI eGFR_cr-cys
• Report serum cystatin C concentration rounded to the nearest 100th whole number when expressed as conventional units (mg/dL)
• When reporting eGFR_cys and eGFR_cr-cys the result be rounded to the nearest whole number and relative to a body surface area of 1.73 m² in adults using the units mL/min./1.73 m²
• Levels of eGFR_cys and eGFR_cr-cys < 60 [mL × min^–1 × (1.73 m²)^–1] should be reported as decreased.

12.2.2.1.4 Determination of the eGFR_cys in children

The advantage of the cystatin C-based equation over the creatinine-based equation is that it is less subject to the effects of age, sex, and race. Cystatin C should be determined immunonephelometrically. The serum cystatin C value should be reported together with the eGFR_cys. The eGFR_cys is calculated according to the equation /9/:

eGFR_cys [mL × min^–1 × (1.73 m²)^–1] = 70.69 × (S_cys)^–0.931

The equation has good accuracy; 82.6% and 37.6% of the children fall within the 30% range and 10% range of the iohexol clearance. The majority of children with a GFR < 60 [mL × min^–1 × (1.73 m²)^–1] had either structural abnormalities of the kidneys or kidney damage, which become apparent with urine or serum analyses. In contrast to adults, an isolated GFR reduction occurs rarely.

Equations, specified for gender, creatinine and cystatin C, are shown in Tab. 12.2-8 – CKD-EPI eGFR_Cr-Cys equations.

Data from the National Health and Nutrition surveys suggest that 3.6% of U.S. adults would be classified as having chronic kidney disease solely on the basis of a creatinine based GFR estimate of 45–59 [mL × min^–1 × (1.73 m²)^–1]. In a study /10/ participants whose eGFR_cr was 45–74 [mL × min^–1 × (1.73 m²)^–1] , the eGFR_cr-cys equation improved the classification of measured GFR as either 60 [mL × min^–1 × (1.73 m²)^–1] or greater than or equal to 60 [mL × min^–1 × (1.73 m²)^–1] and correctly classified 16.9% of those with an eGFR_cr of 45 to 60 [mL × min^–1 × (1.73 m²)^–1] as having a GFR of ≥ 60 [mL × min^–1 × (1.73 m²)^–1].

Individuals with eGFR below 60 [mL × min^–1 × (1.73 m²)^–1] have markedly elevated risks for death, cardiovascular disease and end stage renal disease compared with individuals with eGFR > 60 [mL × min^–1 × (1.73 m²)^–1] /1/. The consensus of the KDIGO Work Group was therefore that the large group of individuals with eGFR_cr 45–59 [mL × min^–1 × (1.73 m²)^–1] without markers of kidney damage, but with eGFR_cr-cys ≥ 60 [mL × min^–1 × (1.73 m²)^–1] could be considered not to have CKD.

12.2.3 Progression of CKD

In patients with CKD there is variability in the presence or rate of decline of kidney function /1/. The rate at which the decline occurs also varies based on the underlying population, cause of CKD, presence of albuminuria, co morbidities, age and kidney damage markers (Tab. 12.2-9 – Kidney damage markers).

12.2.3.1 Definition, identification, and prediction of CKD progression

The annual decline in GFR in various populations is shown in Tab. 12.2-10 – Annual decline of GFR in various populations.

Progression of CKD is based on one or more of the following /1/:

• Decline in GFR category (Tab. 12.2-4 – GFR categories according to KDIGO). A certain decrease in eGFR is defined as a drop in GFR category accompanied by a 25% or greater decline in eGFR from baseline
• Rapid progression is defined as a sustained decline in eGFR of more than 5 [mL × min^–1 × (1.73 m²)^–1] and year
• The confidence in assessing progression is increased with increasing number of serum creatinine measurements and duration of the follow-up.

Factors associated with CKD progression:

• Factors associated with CKD progression are: cause of CKD, level of GFR, level of albuminuria, age, sex, race/ethnicity, elevated blood pressure, hyperglycemia, dyslipidemia, smoking, obesity, history of cardiovascular disease, and ongoing exposure to nephrotic agents.

Evaluation of CKD progression:

• Assess GFR and albuminuria at least annually in patients with CKD. Assess GFR and albuminuria more often for individuals at higher risk progression, and/or where measurement will impact therapeutic decisions.
• Recognize that small fluctuations in GFR are common and are not necessarily indicative of progression.

12.2.4 Influence factor GFR decrease

With a decrease in GFR, reduced excretion and metabolism of endogenous and exogenous substances, as well as potentially toxic reactions, may occur. Refer to:

• Tab. 12.2-11 – Influencing variables in chronic kidney disease
• Table 28.2-3 – Oncogenes. oncogene products and tumor suppressor genes in cancer patients.

12.2.5 Referral to specialists

KDIGO recommends referral to specialist kidney services for people with CKD in the following circumstances /1/:

• Acute renal failure or abrupt sustained fall in GFR
• GFR below 30 [mL × min^–1 × (1.73 m²)^–1]
• A consistent finding of significant albuminuria (albumin/creatinine ratio ≥ 300 mg/g (≥ 30 mg/mmol) or albumin excretion ≥ 300 mg/24 h
• Progression of CKD
• Urinary red cell casts, erythrocytes > 20 per high power field sustained and not readily explained
• CKD and hypertension refractory to treatment with 4 or more antihypertensive agents
• Persistent abnormalities of serum potassium
• Recurrent extensive nephrolithiasis
• Hereditary kidney disease.

12.2.6 Indexing GFR for body surface area

The GFR is expressed in [mL × min^–1 × (1.73 m²)^–1]. Indexing GFR for body surface area (BSA) is meaningful in individuals with a normal constitution, but not in obesity or cachexia /12/. Examples:

• In obesity, a GFR indexed on BSA describes erroneously low clearance values. Thus, in a cohort of obese individuals, the measured GFR was 101 ± 24 mL/min., but only 76 ± 16 [mL × min^–1 × (1.73 m²)^–1], indexed on the BSA. The estimated GFR according to the MDRD equation was 90 ± 22 [mL × min^–1 × (1.73 m²)^–1].
• In individuals with cachexia, a GFR indexed on BSA describes erroneously high clearance values. Thus, the measured GFR was 68 mL/min., but indexed on the BSA it was 80 [mL × min^–1 × (1.73 m²)^–1].

12.2.7 Pathophysiology

The mean number of nephrones is 860,000 ± 370,000 per kidney. The mean single-nephron GFR is 80 ± 40 nL per minute /13/. A higher single-nephron GFR is independently associated with larger nephrones and more glomerulosclerosis and arteriosclerosis than would be expected for age. The single-nephron GFR varies little according to age, sex and height (if ≤ 190 cm), despite substantial variation in the number of nephrones. The number of nephrons declines with age, owing to nephrosclerosis; but the single-nephron GFR in the remaining non sclerotic glomeruli does not increase with age.

Certain acquired risk factors for CKD (e.g., obesity) or inherent risk factors for CKD (e.g., family history for end-stage renal disease) are associated with a higher single-nephron GFR. Obesity is a risk factor for CKD and increases total GFR /13/. Nephrosclerosis exceeding that expected for age decreases the number of nephrons but is generally accompanied by a compensatory higher single-nephron GFR, which maintains the total GFR /13/.

CKD, which is defined as an eGFR below 60 [mL × min^–1 × (1.73 m²)^–1] and/or an albuminuria ≥ 30 mg/24 h, is associated with an increased cardiovascular disease mortality risk. Findings support the concept that eGFR and albuminuria are, over the entire range, associated with cardiac injury /14/.

References

1. Kidney Disease: Improving Global Outcome (KDIGO) CKD Work Group. KDIGO clinical practice guideline for the evaluation and management of chronic kidney disease. Kidney Int Suppl 2013: 3: 1–150.

2. Levey AS, Eckfeldt JH. Using glomerular filtration rate estimating equations: clinical and laboratory considerations. Clin Chem 2015; 61: 1226–9.

3. Davies DF, Shock NW. The variability of measurement of inulin and diodrast tests of kidney function. J Clin Invest 1950; 249: 491–5.

4. Stevens LA, Levey AS. Measured GFR as a confirmatory test for estimated GFR. J Am Soc Nephrol 2009; 20: 2305–13.

5. James T, Lewis AV, Tan GD, Altmann P, Taylor RP, Levy JC. Validity of simplified protocols to estimate glomerular filtration rate using iohexol clearance. Ann Clin Biochem 2007; 44: 369–76.

6. Piepsz A, Tondeur M, Ham H. Revisiting normal ⁵¹Crethylenediaminetetraacetic acid clearance values in children. Eur J Nucl Med Mol Imaging 2006; 33: 1477–82.

7. National Kidney Foundation-K/DOQ1. Clinical practice and guidelines for chronic kidney disease: evaluation, classification, and stratification. Am J Kidney Dis 2002; 39 (Suppl 1): pg. 1–266.

8. Schwartz GJ, Munoz A, Schneider MF, Mak RH, Kaskel F, Warady BA, et al. New equations to estimate GFR in children with CKD. J Am Soc Nephrol 2009; 20: 629–37.

9. Schwartz GJ, Schneider MF, Maier PS, Moxey-Mims M, Dharnidharka VM, Warady BA, et al. Improves equations estimating GFR in children with chronic kidney disease using an immunonephelometric determination of cystatin C. Kidney Int 2012; 82: 445–53.

10. Inker LA Schmidt CH, Tighiouart H, Eckfeld JH, Feldman HI, Kusek JW, et al. Estimating glomerular filtration rate from serum creatinine and cystatin C. N Engl J Med 2012; 367: 20–9.

11. Furth SL, Abraham AG, Jerry-Flunker J, Kaskel F, Make R, Schwartz G, et al. Metabolic abnormalities, cardiovascular disease risk factors, and GFR decline in children with chronic kidney disease. Clin J Am Soc Nephrol 2011; 6: 2132–6.

12. Delanaye P, Mariat C, Cavalier E, Krzesinski JM. Errors induced by indexing glomerular filtration rate for body surface area: reductio ad absurdum. Nephrol Dial Transplant 2009; 24: 3593–6.

13. Denic A, Mathew J, Lerman LO, Lieske JC, Larson JJ, Alexander MP, et al. Single-nepron glomerular filtration rate in healthy adults. N Engl J Med 2017; 376: 2349–57.

14. Martens RJH, Kimenai DM, Kooman JP, Stehouwer CDA, Tan FES, Bekers O, et al. Estimated glomerular filtration rate and albuminuria are associated with biomarkers of cardiac injury in a population-based cohort study: The Maastricht study. Clin Chem 2017; 63: 887–97.

15. Miller CG, Kaufman HW, Levey AS, Straseski JA; Wilhelms KW, H-Y Yu, et al. National kidney Foundation Laboratory Engagement Working Group recommendations for implementing the CKD-EPI 2021 race-free equations for estimated glomerular filtration rate: practical guidance for clinical laboratories. Clin Chem 2022; 68 (4): 511–20.

16. Killeen AA, Horowitz GL. New equations for estimating glomerular filtration rate. Clin Chem 2022; 68 (4): 491–3.

17. Inker LA, Eneanya ND, Coresh J, Tighiouart H, Wang D, Sang Y, et al. New creatinine- and cystatin C-based equations to estimate GFR without race. N Engl J Med 2021; 385: 1437–49.

18. Levey AS, Grams ME, Inker LA. Uses of GFR and albuminuria level in acute and chronic kidney disease. N Engl J Med 2022; 386 (22): 2120–8.

19. Pottel H, Björk J, Bökenkamp A, Berg U, Äsling-Monemi K, et al. Estimating glomerular filtration rate at the transition from pediatric to adult care. Kidney Int 2019; 95: 1234–43.

20. Delanaye P, Schaeffner E, Cozzolino M, Langlois M, Plebani M, Ozben T, et al. The new race-free chronic kidney disease Consortium (CKD-EPI) equation to estimate glomerular filtration rate: is it applicable in Europe? A position statement by the European Federation of Clinical Chemistry and Laboratory Medicine (EFLM). Clin Chem Lab Med 2023; (60): 44–7.

21. Karger AB. GFR estimating Equations-a work in progress. Clin Chem 2023; 69 (9): 951–3.

12.3 Urinalysis

Christian Thomas, Lothar Thomas

In kidney and urinary tract patients, an effective diagnostic strategy in urine examinations should start from medical need followed by standard procedures for collection, transport, and examinations. Standardized procedures are needed for consistent reference intervals and interpretation of results /1/. Procedures for urine collection, transport and analysis are published in the European Analysis Guidelines /2/.

Urinanalysis plays an important role in diagnosis and monitoring of urinary tract and kidney pathology and can be divided into chemical and urine sediment analysis. The guidelines recommend a diagnostic algorithm for urinalysis (Fig. 12.3-1 – Algorithm for urinalysis in individuals with suspected renal and urinary tract disorders). Applications of artificial intelligence and mechanized urinanalysis may alter the algorithms in future /21/.

Low-value urinanalysis is often obtained before procedures despite guidelines from the infectious disease societies of the countries. In a study /18/ among more than 13 million procedures, 25% were preceded by urinalysis within 30 days Among procedures for which urinalysis were obtained, 89% were not indicated. Across procedure categories, between 5.8% and 28% of these urinalysis were followed by an antibiotic prescription. The study found that preprocedural urinalysis were common, but the vast majority were not plausibly indicated. Especially the risk of inappropriate antibiotic use, which can result in adverse drug reactions (antibiotic resistance) and increased risk of infections (Clostridioides difficile) bear the risk of complications.

12.3.1 Indication

Medical indications for urinalysis according to the European Urinalysis Guidelines /2/:

• Suspicion or clinical evidence of acute or chronic kidney disease, either primary or secondary to a systemic disease such as diabetes mellitus, metabolic syndrome, hypertension, gestosis of pregnancy, drug abuse, autoimmune disease, as well as the assessment of their course
• Decrease in the glomerular filtration rate < 60 [mL × min^–1 × (1.73 m²)^–1]
• Suspicion, clinical symptoms or situations suggestive of urinary tract infection as well as the assessment of its course
• Suspicion of post renal disease and the assessment of its course (e.g., kidney stones, urinary tract obstruction)
• Glucosuria in patient groups (e.g., pregnant women, children, emergency patients)
• Assessment of the course in diabetics (e.g., children, in addition to blood glucose monitoring, for the detection of glucosuria and ketonuria)
• Detection and assessment of the course of a metabolic disorder in patients with vomiting, diarrhea, acidosis/alkalosis, ketosis, or recurrent stone formers.

12.3.2 Specimen

Preparing the patient prior to specimen collection, selecting the appropriate type of urine specimen, using the best possible means to transport the sample, and starting the urinalysis within a reasonable amount of time are all important pre analytical requirements for obtaining accurate urinalysis results.

12.3.2.1 Patient preparation

Preparation must begin with the patient being instructed on the evening before the specimen is to be collected. The patient should be given the following information:

• The reasons why the urine specimen needs to be collected
• What type of urine specimen is to be collected
• The procedures to be followed when collecting the urine specimen. The information should ideally be given to the patient both verbally and in writing. Detailed information on urine collection procedures can be found in reference /2/.

The information provided to the patient depends on the tests that are indicated.

• For example /2/, if a sensitive urine screening is required, the urine must be highly concentrated with a urinary flow rate of no more than 20–50 mL/h. In this case, it is important that the patient fast for 8–12 hours before the specimen is collected in order to reduce diuresis.

12.3.2.2 Urine specimens

A first morning urine specimen is collected.

• If the morphology of the particulate components (erythrocytes, leukocytes, or casts) is important, then urine that has incubated only for 1–2 hours in the bladder is preferred.

A second morning urine specimen suffices for the following purpose:

• If bacterial infection is supposed, the bacteria should be given 4–8 hours of growth time in the bladder in order to ensure a high analytic sensitivity. First morning urine is therefore the preferred type of specimen. Antibiotics should be discontinued beforehand.

A 24 hour urine specimen is preferred if calcium excretion is to be measured, since calcium excretion while the patient is in bed is over twice as high as in the middle of the day.

In order to keep the urine sample as free as possible of contaminants, patients should refrain from sexual intercourse for 1 day before the urine specimen is collected. This is because contamination of the urine with vaginal or prostate secretions and semen cause increased cell counts and protein levels.

The following time-related urine specimens are used for urinalysis /2/:

• First-void urine: this is the first urine of the morning after an overnight bed rest. The patient should not take fluids during the night. The urine collected must have remained in the bladder for at least 4 hours, even if the bladder was emptied during the night. The first morning void urine is the standard urine specimen used for urinalysis since it is concentrated and acidic. Compared with daytime urine samples, these specimens contain better preserved particulate components. Bacterial growth is also more significant due to the long time the urine remains in the bladder. First morning urine specimens are most often collected in clinics, but outpatient collection is also possible if the urine is brought in the laboratory quickly for testing.
• Second-void urine: this is a single urine specimen collected 2–4 hours after voiding the first morning urine. The quality of second-void urine is affected by movement before the specimen is collected, as well as by eating and drinking. To improve the quality of the urine, it is recommended that a glass of about 200 mL of water be taken after 10:00 p.m., with the patient subsequently fasting until the specimen is collected. The second morning urine is used for testing on outpatients.
• Timed urine collection: 24-hour urine collection is frequently used. Collection can begin at any time. The bladder must be emptied before the collection starts; all urine for the next 24 hours is then voided into the collection container.
• Overnight urine collection: before the patient goes to bed, the bladder is emptied and the time recorded. All urine passed during the night is collected in the urine container and the time and total volume are recorded in the morning.
• Random urine specimens: these spot urine specimens can be obtained at any time of the day, and no reference can therefore be made to the time of day, volume or patient preparation. This type of sample should only be used in acute situations, and caution should be exercised when evaluating the results since they are associated with an elevated incidence of false-positive or false-negative results.

12.3.2.3 Procedures for urine collection

When performing microbiology testing on urine, the procedures used to collect the urine specimens are important because the specimen must not be contaminated with commensal bacteria. A distinction is made between the following types of urine /2/:

• Mid-stream urine: the first portion of the urine is discarded due to contamination from commensal bacteria in the urethra; then, 50–100 mL of urine is passed into the specimen cup. The remainder of the urine is also discarded. The area around the urethra on the glans penis or vaginal introitus is washed with water prior to urination. In this way, the number of false-positive bacterial cultures can be reduced by 20% /2/.
• First-void urine: this type of urine is suitable for molecular biology assays for(e.g., Chlamydia trachomatis) but not for determining bacterial counts
• Single catheter urine: is collected after the insertion of a bladder catheter. This method of collection is particularly suited for children with no bladder control.
• Indwelling catheter urine: indwelling catheter urine is collected when a catheter is replaced or by puncturing an existing indwelling catheter. The collection bag must not be punctured.
• Suprapubic aspiration urine: this type of urine can be used to confirm the presence of a urinary tract infection.
• Bag urine: this method is used with children. The urine specimen should be collected within one hour of the bag being attached onto the patient.
• Urostomy specimens (urine specimens from an ileal conduit): since chronic infection and bleeding are common in urostomies, it is recommended that the urine be collected after a sterile catheter has been used to clean the stoma.

12.3.2.4 Collection container, preservation of sample

Collection container

The collection container must have a volume of at least 50–100 mL, as well as an opening of at least 5 cm to allow both males and females to urinate directly into the container. For 24-hour total urine samples, the volume of the container must be 2–3 liters, and the contents must be protected from light.

Sample preservation

No stabilization is required for urine chemistry testing with test strips, as long as the test is performed within 24 hours and the urine is stored at 4–8 °C. Failure to refrigerate the urine frequently results in a false-positive nitrite reaction. For urine particle analysis, the urine must be refrigerated immediately if it cannot be tested within one hour of collection. However, the leukocyte count is questionable after 2–4 hours, even if the urine has been refrigerated /3/. For bacterial tests, the urine must not be more than 2 hours old. If testing cannot be performed within this time, the sample can be stored for up to 24 hours at 4–8 °C. Adding boric acid (1 g per liter of urine) makes it possible to stabilize the leukocyte and bacteria counts for 24 hours at + 20 °C. However, borate will inhibit the growth of Pseudomonas spp.

12.3.2.5 Visual inspection and odor of urine

Inspecting the color and turbidity of the urine can provide certain types of information about existing diseases and should not be neglected. Normal urine is yellow and has an aromatic odor. Tab. 12.3-1 – Characteristic appearances of urine shows the various colors of urine and their causes, while the characteristic odors associated with metabolic disorders are listed in Tab. 12.3-2 – Characteristic odors of urine in metabolic diseases.

12.3.3 Method of determination

The urine assays used to diagnose diseases of the kidney and the urinary tract include (Tab. 12.3-3 – Urine tests for evaluation of disorders of the kidney and the urinary tract):

• Multiple test strips
• Total protein excretion
• Plasma protein excretion (e.g., albumin, IgG and α₁-microglobulin)
• Osmolality or relative density (specific gravity)
• Microbiology urine examination
• Creatinine determination to evaluate the excretion rates for proteins, hormones, or other analytes independently of the elimination of water.

12.3.3.1 Multiple test strips (dipsticks)

Rapid methods provide the user with a fast, reliable response to an individual patient. Because of easy handling of equipment rapid methods, like multiple test strips, are used to screen for renal and urinary tract disorders. The test strips can be read with the naked eye or automated spectrophotometric reading can be used for larger series.

When testing for hematuria and/or leukocyturia, the sample tested must contain urine that has not been centrifuged but is well mixed. Testing should be performed within two hours after collection of the sample.

It must be noted that the precision of the test strip assay is only moderate to good, and is poorer than is generally assumed /4/. Spectrophotometric reading is also no more precise than the eye of a good investigator /5/.

The test strips provide qualitative or semi-quantitative results. The detection limits are set with screening requirements in mind, and therefore provide a high degree of diagnostic specificity in order to exclude as many healthy individuals as possible.

For urology and nephrology purposes, a urine test strip should contain the following screening assays: protein, erythrocytes, leukocytes, pH, specific gravity, and nitrite. Other test fields, such as urobilinogen, bilirubin, glucose, and ketones are less suitable for screening as they lack diagnostic specificity.

When screening for renal and urinary tract disorders, the test strip fulfills the requirements for the preselection of individuals who are free of disease based on pH, leukocytes, and erythrocytes. The test strip is not sufficiently sensitive to detect mild proteinuria, and is totally unsuitable for detecting the excretion of free light chains. Overall, studies have found that test strip results are accurate in 71–92% of tests for those renal and urinary tract disorders that manifest themselves in the form of changes in the urine detectable by diagnostic laboratory testing /6/.

Reliability

Requirements for test strip assays need to be evaluated at the following two points of their measuring range /2/:

• At the limit of detection (L_D), which represents the analyte concentration for which the test field begins to show positive results
• At the confirmation limit (L_C), which is the analyte concentration for which almost all measurements are positive
• There is a grey zone between L_C and L_D and the concentration ratio L_C/L_D should be 5
• The following level of reliability can be expected: a false-positive rate of < 10% at the L_D analyte concentration, and a false-negative rate of < 5% at the L_C concentration.

12.3.3.2 Microscopy of urine

The microscopy of urine for clinically significant particles in urine sediment, or the standardized particle count in centrifuged or uncentrifuged urine, is diagnostically meaningful /7/:

• In symptomatic patients
• In patients with chronic diseases of the kidneys and the urinary tract
• As a follow-up evaluation of a positive test strip finding in proteinuria and micro hematuria.

The morphological assessment of erythrocytes, casts and epithelia can provide clues as to whether, for example, the genesis of a disease is renal or post renal. For microscopic urine testing and standardized particle counting with urine chemistry analyzers, refer to Section 12.8 – Erythrocytes, leukocytes, casts in urine.

12.3.3.3 Biochemical urine examinations

The measurement of total protein in the urine is meaningful for the quantitative determination of proteinuria, since test strips are only semi quantitative and insensitive. By means of the follow-up determination of marker proteins, the etiogenesis of proteinuria can be differentiated into pre renal, renal and postrenal /8/. Renal proteinuria is differentiated into glomerular, tubular, and miscellaneous forms with the use of marker proteins. Marker proteins represent a characteristic size category for each form of proteinuria, and are present in the urine under normal conditions in concentrations that are easy to measure using immunonephelometric or immunoturbidimetric methods. Identifying marker protein levels makes it possible to classify proteinuria according to the various types. This testing also allows conclusions to be drawn regarding the location of any renal damage, but not regarding etiology. Refer also Section 12.9 – Urinary proteins.

12.3.3.4 Microbiological urine examinations

Urine from symptomatic patients with microscopic evidence of bacteriuria in uncentrifuged, native urine will usually have a microbe count of > 10⁵/mL and should be cultured. Positive nitrite and leukocyte esterase test are the two standard dipstick analyses used for microbial urine analysis and can indicate urinary tract infections. The nitrite test is positive for those bacteria that can reduce nitrate to nitrite and detects bacteria at levels of 10⁵ colony forming units per mL (cfu/mL). This also applies to all urine samples with a leukocyte count of ≥ 10/μL in uncentrifuged urine /9/. Gram-positive pathogens such as Enterococcus or Staphylococcus do not produce nitrites and leukocyte esterase level may not be high enough to be detected early in an infection /19/. For these reasons, it is not recommended to treat a positive nitrite or a positive esterase test alone as an indication of urinary tract infection.

The urinary tract infection prevalence in children 2 to 24 months of age with a fever without obvious source is 5%. The diagnostic sensitivity of leukocyturia (≥ 10 leukocytes/μL) for a urinary tract infection is about 95% and a specificity of 88%. In the absence of leukocyturia, the likelihood of a urinary tract infection decreases from 5% to 0.2–0.4% /10/.

Medical indications for microbiology investigation of urine include /2/:

• Identifying the relevant pathogen
• Locating the focus of infection
• Optimizing the chemotherapeutic elimination of the pathogen.

Urine bacterial culture with identification of species and susceptibility testing

The culture methodology on plates is common in the clinical bacteriological laboratory /2/. A 1-μL disposable loop inoculum is used for routine practice. Quantitative culture is performed on a relatively nonselective agar plate, such as Cystine-Lactose Electrolyte Deficient (CLED) medium. In addition, culture on blood agar is performed as an optimum approach.

The following should be evaluated (Tab. 12.3-4 – Interpretation of microbiological urine findings):

• Significant concentration of colony forming units
• Whether the culture is a mono culture or a mixed culture
• Species and type of the bacteria
• Susceptibility testing of the bacteria.

Asymptomatic bacteriuria

The term describes a condition with pathogenic or facultative pathogenic bacteria in two consecutive urine samples and a bacterial count of ≥ 10⁵/mL. At the same time, leukocyturia of ≥ 10 cells per view field in the urine sediment or ≥ 10/μL in the uncentrifuged urine are indicative of a local defense reaction. If the defense responses are disrupted, a urinary tract infection can develop /11/.

Asymptomatic bacteriuria occurs particularly frequently in diabetic patients, pregnant women, patients with organ transplants and elderly individuals. It should be taken into account that beyond the 85th year of life, asymptomatic bacteriuria is present in 10% of women and 5% of men; in individuals with an indwelling catheter this occurs quite regularly.

Mixed cultures

Most uncomplicated urinary tract infections result from one bacterial species /12/. The isolation of more than one organism from a single specimen of urine must always be interpreted in the light of features suggesting either true infection (presence of leukocytes) or contamination (presence of squamous epithelial cells). The demonstration of a maximum of two types of uropathogenic bacteria with ≥ 10⁵ CFU/mL speaks in favor of an infection. The likelihood is augmented if, at the same time, leukocyturia is present or if the pathogen was already repeatedly isolated. In this case lower bacterial counts are also indicative of an infection.

According to the European Association of Urology /20/ urinary tract infections are classified in:

• Asymptomatic bacteriuria (ABU) in adults: a common disorder that corresponds to a commensal colonisation. ABU is defined by a midstream sample of urine showing bacterial growth ≥ 10⁵ colony forming units (cfu)/mL in two consecutive samples. Treatment of ABU should be performed only in cases of proven benefit.
• Uncomplicated cystitis: The disease is defined as acute, sporadic or recurrent cystitis limited to non-pregnant women with no known relevant anatomical or functional abnormalities within the urinary tract or comorbidities.In patients presenting with typical symptoms of an uncomplicated cystitis urine analysis (i.e. urine culture, dip stick testing, etc.) leads to only minimal increase in diagnostic accuracy. However, if the diagnosis is unclear dipstick analysis can increase the likelihood of an uncomplicated cystitis diagnosis. Taking a urine culture is recommended in patients with atypical symptoms, as well as those who fail to appropriate antimicrobial therapy.
• Recurrent urinary tract infections (rUTIs). Recurrent UTIs are recurrences of uncomplicated or complicated UTIs, with a frequency of at least three UTIs/year or two UTIs in the last six months. Although rUTIs include both lower tract infection (cystitis) and upper tract infection (pyelonephritis), repeated pyelonephritis should prompt consideration of a complicated aetiology. Diagnosis of rUTI should be confirmed by urine culture.
• Uncomplicated pyelonephritis. The disease is defined as pyelonephritis limited to non-pregnant, pre-menopausal women with no known relevant urological abnormalities or comorbidities. Urinanalysis including the assessment of white and red blood cells and nitrite is recommended for routine diagnosis. In addition, urine culture and antimicrobial susceptibility testing should be performed.
• Complicated urinary tract infections (cUTIs). A cUTI occurs in an individual in whom factors related to the host (e.g., underlying diabetes or immunosuppression) or specific anatomical functional abnormalities related to the urinary tract (e.g., obstruction) are believed to result in an infection. A cUTI is associated with clinical symptoms (e.g., dysuria, urgency, frequency, flank pain, costovertebral angle tenderness, suprapubic pain and fever). Urine culture is the recommended method to determine the presence or absence of clinically significant bacteriuria in patients suspected having a cUTI.
• Catheder-associated urinary tract infection (cUTI). These UTIs occur in an individual whose urinary tract is currently cathederised or has been cathederised within the past 48 hours. Microbiologically, catheder associated urinary tract infection is defined by microbial growth of ≥ 10³ cfu/mL of one or more bacterial species in a single catheder urine specimen or in a midstream voided urine specimen from a patient whose urethral, subrapubic, or codom catheder has been removed.
• Urosepsis. The disease is diagnosed when clinical evidence of infection is accompanied by signs of systemic inflammation, presence of symptoms of organ dysfunction and persistent hypotension associated with tissue anoxia. Most commonly the condition develops in compromised patients.
• Urethritis. The disorder can be of either infectious or non-infectious origin. Criteria are mucoid, mucopurulent or purulent urethral discharge. Gram or methylene-blue stain of urethral secretions demontrate inflammation. Five or more polymorphonuclear leukocytes per high power field is the cut-off for the diagnosis of urethritis. Gonococcal urethritis is differentiated from non-gonococcal urethritis. All sexual partners at risk should be assessed and treated whilst maintaining patient confidentiality.

12.3.4 Comments and problems

Proteinuria

The average coefficient of variation (CV) of one individual (CV within subject) is 40–50% at the upper reference interval value in health. In discrimination between health and disease (diagnostic testing) the average CV between-subject is about 25% for total protein and 75% for albumin in 24-hour output /2/.

Creatinine excretion

The average CV between-subject is about 20% giving an average CV for albumin/creatinine ratio of about 40%, and maximally 80%. This within-subject variation results in estimate of maximally allowable imprecision ≤ 20%. It would allow discrimination between twofold and threefold changes in excretion rates (analyte/creatinine ratios) in multiple monitoring of the same patient, by using the equation for critical difference DC = α × CV, where α = 2.77 at a statistical probability p < 0.05 /2/

In discrimination between health and disease (diagnostic testing) the average CV between-subject for creatinine in 24-hour excretion is about 25–30%.

Automated urinalysis systems

Some automated urinalysis systems have borderline trueness. In comparison to microscopy true classifications of leukocytes and erythrocytes were only above 85% and 72%, respectively. False negative casts were above 70% for all tested systems /16/. Automated urinanalysis uses flow cytometers, in which urinary particles are counted and classified on their impedance, forward scatter, side fluorescence, and depolarized side scattered light characteristics. Non-ionized radiological contrast media e.g., iomepol (Iomeron), iohexol (Omnipaque), iodixanol (Visipaque) contain a large portion of iodine (3–6 atoms). The agents show a high refractive index. An analytical interference on white blood cell count, red blood cells, cellular casts, epithelial cells, and urinary conductivity was observed using an urinary fluorescence flow cytometer. The urinary sediment parameters were falsely lowered with higher iomepol concentrations /17/.

References

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2. European Urinalysis Guidelines: Summary. Scand J Clin Lab Invest 2000; 60: 1–96.

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4. Kierkegaard H, Feldt-Rasmussen U, Hoerder M, Andersen HJ, et al. Falsely negative urinary leukocyte counts due to delayed examination. Scand J Clin Lab Invest 1980; 40: 249–61.

5. Winkens RAG, Leffers P, Degenaar CP, Heuben AW. The reproducibility of urinalysis using multiple reagent test strips. Eur J Clin Chem Clin Biochem 1991; 29: 813–8.

6. Guder WG, Heidland A. Urinalysis. Report on a workshop conference. J Clin Chem Clin Biochem 1986; 24: 611–20.

7. Brody LH, Sallady JR. Urinalysis and urinary sediment. Med Clin North Am 1971; 55: 243–66.

8. Hofmann W, Regenbogen C, Edel H, Guder WG. Diagnostic strategies in urinalysis. Kidney Int 1994; 46 (Suppl 47): pg. 111– 4.

9. Graham JC, Galloway A. The laboratory diagnosis of urinary tract infection. J Clin Pathol 2001; 54: 911–9.

10. Finnell SME, Carroll AE, Downs SM. Technical report– Diagnosis and management of an initial UTI in febrile infants and young children. Pediatrics 2011; 128: e749–e770.

11. Fünfstück R, Stein G. Asymptomatische Bakterurie. Nieren- und Hochdruckkrankheiten 2007; 36: 269–76.

12. Pfister W. Klinische Relevanz mikrobiologischer Befunde. Nieren- und Hochdruckkrankheiten 2007; 36: 258–61.

13. The National Collaborating Center for Chronic Conditions (UK). Chronic Kidney Disease 2009. www.nice.org.uk/Guidance/CG73.

14. Collier G, Greenan MC, Brady JJ, Murray B, Cunningham SK. A study of the relationship between albuminuria, proteinuria and urinary reagent strips. Ann Clin Biochem 2009; 46: 247–9.

15. Hooton TM, Roberts PL, Cox ME, Stapleton AE. Voided midstream urine culture and acute cystitis in premonopausal women. N Engl J Med 2013; 369: 1883–91.

16. Bartosova K, Kubicek Z, Franekova J, Louzensky G, Larikova P, Jabor A. Analysis of four automated urinalysis systems compared to reference methods. Cin Lab 2016; 62: 2115–23.

17. Oyaert MN, DeBuyzere ML, Verstraete KL, Speeckaert MM, Delanghe JR. Iodine containing contrast media and urinary flow cytometry: an unknown interference in automated urine sediment analysis. Clin Chem Lab Med 2021; 59 (8): e335–e337.

18. Shenoy ES, Giuriato MA, Song Z. Prevalence, costs, and consequences of low-value preprocedural urinanalyse in the US. JAMA Internal Medicine 2021; 181: 1533–4.

19. Harris M, Fasolino T. New and emerging technologies for the diagnosis of urinary tract infections. Lab Med 2021. doi: 10.1515/labmed-2021-0085

20. Bonkat G, Bartoletti R, Bruyere f, Cai T, Geerlings SE, Köves B, et al. EAU Guidelines onurological infections. https://uro­web.org/guideline/urological-infections

21. De Bruyne S, De Kesel P, Oyaert M. Applications of artificial intelligence in urinalysis: Is the future already here? Clin Chem 2023; 69 (12): 1348–60.

12.4 Creatinine

Lothar Thomas

12.4.1 Indication

Assessment of renal function:

• Symptomatic patients (proteinuria, leukocyturia, erythrocyturia, hypertension, diabetes mellitus, metabolic syndrome, hyperuricemia)
• Clinical suspicion of acute renal failure or chronic kidney disease and its course
• Extrarenal diseases with diarrhea, vomiting, profuse sweating
• Acute disease conditions (e.g., preoperative and post-operative, in intensive care)
• Sepsis, shock, poly trauma
• Hemodialysis treatment
• Pregnancy
• Diseases with increased protein metabolism (e.g., chronic inflammation, multiple myeloma, acromegaly)
• Therapy with renally excreted drugs that are potentially nephrotoxic if administered in too high a dosage.

12.4.2 Method of determination

12.4.2.1 Higher order reference materials and reference measurement procedure

Isotope dilution gas chromatography-mass spectrometry (IDGC-MS) is designated by the NCCLS and the Joint Committee on Traceability in Laboratory Medicine as a premium reference method for the determination of creatinine /1/. LC-MS is of equal value. The Laboratory Working Group of the National Kidney Disease Education Program (NKDEP) for improving serum creatinine measurements of NIST has developed the standard reference material SRM 967 for the calibration of the creatinine methods /2/. The certified creatinine concentrations of the two pools are 1 mg/dL (88.4 μmol/L) and 4 mg/dL (354 μmol/L).

12.4.2.2 Jaffé method

Principle: in alkaline medium creatinine reacts with alkaline picrate to form a red-orange solution which is measured spectrophotometrically. The absorption of the solution is, within a certain range, proportional to the creatinine concentration. The Jaffé method is implemented with different modifications and the kinetics of the reaction product that emerges is determined /3/. Undiluted serum and plasma is used in the assays. The reason for the many modifications is the low analytical specificity of the Jaffé method (i.e., about 50 pseudo creatinines, also referred as non-creatinine chromogens form a reaction product with alkaline picrate similar to that produced by creatinine). In comparison to true creatinine, some pseudo creatinines react faster while some others react more slowly with alkaline picrate. Following the addition of the sample, the reaction proceeds in three phases. In the first phase mainly the fast pseudo creatinines react; in the second phase, true creatinine; and subsequently, the slow pseudo creatinines. The change in absorption in the second phase is measured at 509 nm. It is proportional to the true creatinine concentration in the sample. Dependent upon the type of pseudo creatinines and the assay these lead to an intercept. To account for the sensitivity of alkaline picrate-based methods to non-creatinine chromogens, some manufacturers have adjusted the calibration to minimize the pseudo creatinine contribution (compensated methods), others did not (non-compensated methods) /4/.

• Compensated methods: the pseudo creatinine contribution of plasma proteins is minimized by introducing a negative offset to compensate the positive intercept found in the correlation. Thus, for example, one manufacturer compensated Jaffé assay, 0.208 mg/dL (18 μmol/L) is automatically subtracted from each result /4/.
• Non-compensated methods: the manufacturers do not compensate for pseudo creatinine but rather they attempt, with the reaction conditions, to capture a minimum of pseudo creatinines.

12.4.2.3 Enzymatic method

The enzymatic creatinine methods are based on similar reactions with two incubation periods /5/

Principle: During the first incubation period serum or plasma is added to reagent one, which contains creatinase, sarcosine oxidase and catalase to remove endogenous creatine The second incubation step starts when reagent two, which contains creatininase and peroxidase, is added to the mixture. The formed peroxide then reacts with a manufacterer-dependent chromogen to initiate a colour change, the intesity of which is proportional to the concentration of creatinine in the sample. Refer to Fig. 12.4-1 – Principle of the creatinine PAP color test.

Creatinine determination in whole blood /6/

The determination is performed with blood gas analyzers or point of care instruments. The blood gas analyzer uses an amperometric biosensor (electrode) based on the enzymatic conversion of creatinine to creatine, then sarcosine, followed by a reaction that generates H₂O₂ from sarcosine.

Creatinine UV test /7/

Principle: creatinine imino hydrolase catalyzes creatinine degradation to N-methyl hydantoin and ammonia. The ammonia thus formed is determined in a subsequent enzymatically catalyzed reaction by measuring the absorption increase of NADH at 340 or 366 nm.

12.4.3 Specimen

Serum, plasma (heparin, EDTA, citrate): 1 ml

Whole blood (point of care)

12.4.4 Reference interval

The reference intervals for serum creatinine are dependent on the method of determination mentioned in Tab. 12.4-1 – Serum creatinine reference intervals.

12.4.5 Clinical significance

For the assessment of renal function, the KDIGO recommendations state that the serum creatinine value, along with the estimated GFR (eGFR), should be reported. The CKD-EPI equation is recommended for the calculation of the eGFR (see also Section 12.2 – Glomerular filtration rate (GFR)) /11/. The creatinine value is of importance in the progression of chronic kidney disease due to the acceptable intraindividual variation. In addition the creatinine level contributes to the establishment of whether or not the renal function is in a steady state.

Intraindividual and inter individual variation

In healthy individuals the intraindividual variation in serum creatinine is minimal, while the inter individual variation is high. Therefore the reference interval is large. As a result, in an individual with low basal creatinine value and an impaired GFR, the creatinine level has to be increased by 13 times the intraindividual variation before the upper reference interval value is exceeded /12/.

The reasons for the high inter individual variation and the insensitivity to early changes in renal function are (Tab. 12.4-2 – Limitations of serum creatinine):

• Differences in muscle mass and thus in creatinine formation. Given the same GFR, men have a higher creatinine level than women, muscular individuals have higher levels than less muscular individuals, and younger individuals higher levels than elderly /13/.
• Varying intake of meat. One kilogram of meat contains 2–5 g of creatine, which is converted to creatinine during cooking. Food is the source of 15–30% of the daily renal creatinine excretion. Thus, for example, the creatinine value increases from 1.0 mg/dL (88 μmol/L) to 2.0 mg/dL (177 μmol/L) 3 hours following the ingestion of a large portion of goulash /14/.

The intraindividual variation of serum creatinine is lower than the MDRD value and the creatinine clearance (Tab. 12.4-3 – Variation in creatinine, cystatin C and MDRD values). Diseases and conditions with changes in serum creatinine levels are listed in Tab. 12.4-4 – Diseases and conditions that can cause a change in serum creatinine. The relationship between GFR and serum creatinine is shown in Fig. 12.4-2 – Relationship between glomerular filtration rate and serum creatinine.

12.4.6 Comments and problems

Reference interval

The upper and lower reference interval values depend on age, gender, body weight, and the method of determination. Over a long period of time, creatinine concentrations in an individual vary only to a small extent. Creatinine does not manifest a circadian rhythm.

Method of determination

Within the range of 0.2–2.0 mg/dL (18–176 μmol/L), the non-compensated procedure provides falsely high creatinine values. With the Jaffé method, the compensation of the non-creatinine chromogens is critical, particularly in children. Overcompensation occurs occasionally and incipient renal insufficiency is not identified. Therefore, it is generally recommended that exclusively enzymatic methods be used for the determination of creatinine in children.

Jaffé method: the Jaffé modifications are to a varying extent prone to interference and unspecificity. Kinetic methods require a high degree of stability with regard to temperature and pH. Generally, they measure pseudo creatinines more than the endpoint methods, because in these latter tests the serum proteins are removed beforehand by precipitation or dialysis. Falsely elevated values that are clinically relevant are determined in the range of below 2 mg/dL (177 μmol/L) are caused by ketone bodies, glucose, fructose, ascorbic acid, HbA (1.88 mmol/L) and HbF (2.03 mmol/L) /20/. Especially in samples from children with creatinine values of around 0.5 mg/dL (44 μmol/L) the values are raised by 8–27% /21/.

The following cephalosporins may interfere with the Jaffé method and thus cause elevated creatinine concentrations: cefoxitin, cephalothin, cefatril, cefazolin. Those that are believed not to interfere are moxalactam, cefoperazone, cefotaxime, ceftazidime, cephalexin, cefradine /22/. Flucytosine also causes increased creatinine levels.

Substances like acetoacetic acid react to some extent faster with alkaline picrate than creatinine while others, such as glucose, react slower. Depending on the modification employed, they are measured as creatinine and falsify the measured value. In the early reaction phase, fast-reacting substances contribute to the color reaction; during the middle phase it is almost exclusively creatinine that does so; in the late phase the reaction is influenced by slowly reacting non-creatinine chromogens.

The interference by bilirubin in the Jaffé method, varies according to the different analysis systems and can be mathematically corrected only to a limited degree. Up to a bilirubin concentration of 35 mg/dL (600 μmol/L), this interference can be eliminated by treatment of the icteric samples with peroxidase, bilirubin oxidase or ferrocyanide.

Enzymatic method: the PAP method shows excessively low creatinine levels in the presence of bilirubin concentrations > 7 mg/dL (120 μmol) /7/. The UV test is believed to measure falsely low creatinine in samples of bilirubin concentrations ≥ 12.1 mg/dL (207 μmol/L) /23/. Falsely low values are obtained with calcium dobesilate; the same is true, but only with high doses, of metamizole, ascorbic acid and α-methyldopa /7/.

With the UV creatinine test, the addition of a constant quantity to the creatinine concentration occurs due to high glucose values, amino acids and ammonia in the sample /16/. Dopamine and dobutamine lead to falsely low creatinine values by up to 67% if the creatinine to dopamine ratio in the sample is 4 : 1, expressed on a molar basis. Even a ratio of 50 :1 can lead to reductions of > 25%. The interference occurs only if the blood is collected via an indwelling intravenous tube /24/.

IgM paraproteinemia causes on the enzymatic creatinine method an undetectably low creatinine result. The IgM precipitation which occurs during incubation with reagent one results in increased sample turbidity causing an absorbance several orders of magnitude greater than the absorbance peak of the post-PEG precipitation sample /5/.

Separation of the cellular constituents

The erythrocytes should be separated from the serum within 16 hours from the time of the blood collection, otherwise an increase of 29% (21–63%) is measured after 48 hours with the Jaffé reaction /25/.

Biological variation

In a study /29/ the within-subject biological variation (CV) of creatinine and different eGFR equations in people with chronic kidney disease (CKD) and people without CKD was assessed. The CV creatinine was higher in people without CKD than in those with CKD (6.4% vs 2.5%) owing primarily to the more profound effect of meat consumption on creatinine variability in individuals with lower baseline creatinine concentrations.

Interferences

Enzymatic creatinine is not free of interferences. Low values should be considered in patients with unexplained low enzymatic creatinine result. The reason is IgM paraprotein negative interference. Investigations to identify an interference include dilution studies, polyethylene precipitation, mass spectrometry, or an alternative method such as Jaffe, or an enzymatic method from a different platform /30/.

Stability

In serum and plasma, no changes for up to 7 days at room temperature in closed containers /26/.

12.4.7 Biochemistry and physiology

Creatine, the precursor of creatinine, is produced by the liver and following its release is taken up by the muscles and other tissues. A man weighing 70 kg has a creatine pool of some 120 g, 98% of which is stored in the muscles. Creatine accounts for 20–30% while the remainder is phosphocreatine. The metabolism of creatine and creatinine is shown in Fig. 12.4-3 – Creatine and creatinine metabolism.

Phosphocreatine serves as an energy store for the muscles; during muscle contraction, chemical energy is transformed into mechanical energy by the cleavage of phosphocreatine /27/.

Creatinine is created by the non enzymatic dehydration of muscular creatine (Fig. 12.4-4 – Transformation of creatine into creatinine). In men, some 1.5% of the creatine pool is transformed daily into creatinine, representing a quantity of 1.8 g. The creatine and creatinine pools are increased by creatine that is ingested with food.

The size of the creatinine pool is determined by the muscular mass. As determining factors with regard to muscle mass, age and gender therefore have a marked influence on the creatinine pool. Further biological factors are dietary creatine intake and protein ingestion.

The intake of creatine and creatinine through meat consumption increases creatinine in the body. 1 g of beef contains 3.5 mg of creatine, of which 18–65% is transformed into creatinine during the process of cooking.

Limited protein intake diminishes the creatinine pool since arginine and glycine, as precursors of creatine synthesis, are not available (Fig. 12.4-3 – Creatine and creatinine metabolism) /27/.

Because the muscle mass is diminished in myopathy, in cases of leg or arm amputation, and in intensive care patients, serum creatinine is low to normal and urinary creatinine excretion is reduced. Serum creatinine exceeds the upper reference value only when the GFR is markedly reduced.

Serum creatinine can only be used for the estimation of GFR if a steady state is present (i.e., the synthesis rate of creatinine equals its elimination rate). In order to check whether or not this situation is present, two determinations should be performed 24 hours apart. Differences of more than 10% indicate that a steady state is not present /28/.

In impaired renal function, GFR of 80–40 [mL × min^–1 × (1.73 m²)^–1], the GFR is overestimated with rising serum creatinine concentrations if creatinine clearance is measured. This is due to the fact that the elimination of creatinine is not only due to glomerular filtration is also excreted via the intestinal mucosa. In the intestines, creatinine is apparently metabolized as a result of the action of bacterial creatininase.

An approximately linear correlation between serum creatinine concentrations and the GFR exists only with an impairment of the GFR in the range of 40–20 [mL × min^–1 × (1.73 m²)^–1], which represents a serum creatinine range of some 2–4 mg/dL (177–354 μmol/L) in a person weighing 75 kg /15/.

References

1. Joint Committee on Traceability in Laboratory Medicine. Database of higher order reference materials and reference measurement procedure for laboratory medicine and in vitro diagnostics. JCTLM Reference measurement, 2006.

2. NIST. Certificate of analysis, standard reference material 967, creatinine in human serum. JCTLM Reference material, 2007.

3. Moss GA, Bondar RJ, Buzzelli DM. Kinetic enzymatic method for determining serum creatinine. Clin Chem 1975; 21: 1422–6.

4. Panteghini M on behalf of the IFCC Scientific Division. Enzymatic assays for creatinine: time for action. Clin Chem Lab Med 2008; 46: 567–72.

5. Flowers KC, Tuddenham E, Leiva A, Garrison L, Morris JE, Cromwell T, Boa FG. Negative interference from immunoglobulin M paraproteinaemia on the Roche enzymatic creatinine method. Ann Clin Biochem 2022; 59 (3): 205–10.

6. Korpi-Steiner N, Williamson EE, Karon BS. Comparison of three whole blood creatinine methods for estimation of glomerular filtration rate before radiographic contrast administration. Am J Clin Pathol 2009; 132: 920–6.

7. Toffaletti J, Blosser N, Hall T, Smith S, Tompkins D. An automated dry-slide enzymatic method evaluated for automated enzymatic determination of plasma creatinine. Clin Chem 1983; 29: 684–7.

8. Ceriotti F, Boyd JC, Klein G, Henny J, Queralto J, Kairisto V, Parthegine M. Reference intervals for serum creatinine concentrations: assessment of available data for global application. Clin Chem 2008; 54: 559–65.

9. Pottel H, Vrydags N, Mahieu B, Vandewynckele E, Croes K, Martens F. Establishing age/sex related serum creatinine reference intervals from hospital laboratory data based on different statitical methods. Clin Chim Acta 2008; 396: 49–55.

10. Arzideh F, Wosniok W, Haeckel R. Reference limits of plasma and serum creatinine concentrations from intra-laboratory data bases of several German and Italian medical centres. Comparison between direct and indirect procedures. Clin Chim Acta 2010; 411: 215–21.

11. Kidney Disease: Improving Global Outcome (KDIGO) CKD Work Group. KDIGO clinical practice guideline for the evaluation and management of chronic kidney disease. Kidney Int Suppl 2013: 3: 1–150.

12. Perrone RD, Madias NE, Levey AS. Serum creatinine as an index of renal function: new insights into old concepts. Clin Chem 1992; 38: 1933–53.

13. Salive ME, Jones CA, Guralnik JM, et al. Serum creatinine levels in older adults: relationship with health status and medications. Age and Ageing 1995; 24: 142–50.

14. Jacobson FK, Christensen CK, Mogensen CE, Heilskov NSC. Evaluation of kidney function after meals. Lancet 1980; i: 319.

15. Filser D, Ritz E. Serum cystatin C concentration as a marker of renal dysfunction in the elderly. Am J Kidney Dis 2001; 37: 79–83.

16. Ferry N, Caillette A, Goudable J, Denicola C, Pozet N. Creatinine determination in peritoneal dialysis: what method should be used. Nephrol Dial Transplant 1996; 11: 2282–7.

17. Jungers P, Chauveau D. Pregnancy in renal disease. Kidney Int 1997; 52: 871–85.

18. Kierdorf HP, Seeliger S. Acute renal failure in multiple organ dysfunction syndrome. Kidney Blood Press Res 1997; 20: 164–6.

19. Andreev E, Koopman M, Arisz L. A rise in plasma creatinine that is not a sign of renal failure: which drugs can be responsible? J Internal Med 1999; 246: 247–52.

20. Soldin SJ, Henderson L, Hill JG. The effect of bilirubin and ketones on reaction rate methods for measurement of creatinine. Clin Biochem 1978; 11: 82–6.

21. Cobbaaert CM, Baadenhuisjen H, Weykamp CW. Prime time for enzymatic creatinine methods in pediatrics. Clin Chem 2009; 55: 549–58.

22. Kroll MH, Koch TR, Drusano GL, Warren JW. Paint of interference with creatinine assays by four cephalosporin-like antibiotics. Am J Clin Pathol 1984; 82: 214–6.

23. Spaett R, Gässeler N. Enzymatische Creatininbestimmung mit der Creatininiminohydrolase. Eine kritische Evaluation. Ärztl Lab 1990; 36: 136–44.

24. Saenger AK, Lockwood C, Snozek CL, Milz TC, Karon BS, Scott MC, et al. Catecholamine interference in enzymatic creatinine assays. Clin Chem 2009; 55: 1732–6.

25. Ford L, Berg J. Delay in separating blood samples affects creatinine measurement using the Roche kinetic Jaffe method. Ann Clin Biochem 2008; 45: 83–7.

26. Heins M, Heil W, Withold W. Storage of serum or whole blood samples? Effects of time and temperature on 22 serum analytes. Eur J Clin Chem Clin Biochem 1995; 33: 231–8.

27. Wyss M, Kaddurah-Daouk R. Creatine and creatinine metabolism. Physiological Reviews 2000; 80: 1107–1213.

28. Rutherford WE, Blondin J, Miller JP, et al. Chronic progressive renal disease: rate of change of serum creatinine concentration. Kidney Int 1977; 11: 62–70.

29. Hilderink JM, van der Linden N, Kimenai DM, Litjens EJR, Klinkenberg LJJ, Aref BM, et al. Biological variation of creatinine, cystatin C, and eGFR over 24 hours. Clin Chem 2018; 64: 851–60.

30. Panteghini M. Enzymatic assays for creatinine: time of action. Clin Chem Lab Med 2008; 46: 567–72.

12.5 Creatinine clearance (Cl_Cr)

Lothar Thomas

The Cl_Cr is not a valid method for the assessment of the glomerular filtration rate (GFR). Only relative changes – and not absolute – in the GFR can be well assessed. It is, therefore, recommended that in symptomatic patients and in at risk patients with suspicion of impaired renal function, clinicians should order the measured GFR (mGFR). This method correlates closely with the real GFR (see Section 12.2 – Glomerular filtration rate (GFR)). If the GFR and the Cl_Cr are known, the Cl_Cr alone can, subsequently, be employed as a relative criterion for the assessment of the GFR course /1/.

In the following the Cl_Cr will, in spite of the above-mentioned recommendation, be discussed in detail, because it is recommended in numerous drug studies in spite of a certain lack of reliability /2/.

12.5.1 Indication

• Determination whether the GFR is almost normal, slightly or severely reduced
• Assessment of a change in GFR under medication with potentially nephrotoxic substances
• For the assessment of the necessity of dialysis in combination with urea clearance in patients with end stage renal failure (see Section 12.6 – Urea nitrogen.

12.5.2 Test protocol

1. Collection of 2 mL of blood for the determination of creatinine.

2. Collection of 24-hour urine (possibly 4-hour or 2-hour collections). The collection period for the 24-hour urine sample includes one daytime and one nighttime period. It starts at 7:00 or 8:00 a.m. At first the bladder is emptied. This urine is not included in the collection. As of this point in time until the very same time the following morning (any urine voided then is included as well), all urine is collected. A urinary flow rate of > 1 mL/min. should be maintained by an adequate fluid intake.

3. Determination of creatinine in the collected urine.

4. Calculation of the clearance value. Besides the volume of urine collection, the height and body weight of the patient need to be known as well.

Clearance results are related to 1.73 m² body surface area of a person weighing 75 kg. The body surface area of the patient is derived from a nomogram using height and body weight. (Fig. 12.5-1 – Nomogram for determining the body surface area). The clearance is calculated according to the formula shown in Tab. 12.5-1 – Creatinine clearance formula.

Some laboratories employ the 4-hour urine collection for the calculation of the Cl_Cr, especially under therapy with nephrotoxic medication. A clearance value is calculated on each of 3 successive days.

12.5.3 Specimen

• Serum, obtained at the beginning and at the end of the collection period: 1 mL
• Urine without additives (volume to be measured): 5 mL

12.5.4 Reference interval

Refer to Ref. /3, 4, 5, 6/ and Tab. 12.5-2 – Creatinine clearance reference intervals.

12.5.5 Clinical significance

The Cl_Cr does not permit the measurement of the GFR; rather, it provides an approximation of it’s magnitude. For practical clinical concerns this is sufficient in order to clarify the following questions /7/:

• Detection of a reduced Cl_Cr and including an approximate but clearly reduced GFR, in particular for the recognition of the creatinine-blind range of the GFR of 80–40 [mL × min^–1 × (1.73 m²)^–1], that is to say when the serum creatinine value is likely still within the reference range
• Assessment of the course of the GFR by serial measurements of the Cl_Cr (e.g., under therapy with potentially nephrotoxic substances)
• Establishment of when a patient with end stage renal failure will become dialysis-dependent, which can be the case as of a Cl_Cr of less than 10–20 [mL × min^–1 × (1.73 m²)^–1].

In contrast to inulin clearance, which is the gold standard for the determination of GFR, Cl_Cr provides values that are too high. The cause of this is that in contrast to inulin, in healthy individuals 10–14% of the creatinine that appears in the urine is actively secreted by the tubules. Therefore, even a normal GFR already produces falsely elevated values if Cl_Cr is used. The overestimation of the GFR with Cl_Cr increases with the impairment of the renal function. The absolute difference between Cr_Cl and GFR (Cr_Cl-GFR) is, however, greatest in the GFR range of 80–40 [mL × min^–1 × (1.73 m²)^–1] (Tab. 12.5-3 – Relationship between the clearances of creatinine and inulin depending on the GFR) /8/.

According to a study /9/, 42% of patients with an impaired GFR of 70–61 mL had normal Cl_Cr, while in those with impairment down to 60–51 mL this was still the case in 23% of the patients. Only with a Cl_Cr above 115 [mL × min^–1 × (1.73 m²)^–1], a reduction in the GFR was no longer found /3/.

With serum creatinine values ≥ 3 mg/dL (265 μmol/L), Cl_Cr is superfluous. In such cases the GFR is below 20 [mL × min^–1 × (1.7 m²)^–1], and the relative deviation of the Cl_Cr to the GFR (Cl_Cr/GFR) is double (Tab. 12.5-3). This is due to the increasing tubular secretion and the intestinal elimination of creatinine.

In the assessment of the Cl_Cr, the age dependence of the GFR must be taken into consideration. In adults the following relationship exists between the GFR and age /10/:

GFR = 157 – (1.16 × age in years)

12.5.5.1 Urinary creatinine excretion

The completeness of a 24-hour urine collection can be estimated roughly by means of the quantity of excreted creatinine /11/. The excretion values for adults are listed in Tab. 12.5-4 – Creatinine excretion in adults; in children they are calculated according to the following formula /10/:

mg creatinine/kg body weight/24 h = 15.4 + (0.46 × age in years)

12.5.6 Comments and problems

Method of determination

The Cl_Cr correlates better with the GFR in 24-hour urine collections than in shorter collection periods. In comparison with 24-hour samples, urine samples collected at daytime periods result in an increase in the Cl_Cr/GFR ratio (Tab. 12.5-3 – Relationship between the clearances of creatinine and inulin depending on the GFR) /10/.

For the determination of urinary creatinine concentrations, the specimen has to be diluted 1: 20 to 1 : 50 with water. In order to determine serum creatinine for Cl_Cr, blood should be obtained before and after the 24-hour urine collection, and the serum should be analyzed in the same test run. The results are only used for averaging if the difference is less than 10% of the highest value. If the difference is greater, no clearance calculation should be performed.

The method for the determination of creatinine has a considerable influence on the Cl_Cr. If the Jaffé method for serum and urine creatinine determination is employed, it must be noted that within the reference interval, with the use of a non-compensated method 20% of the measured value is attributed to non-creatinine chromogens. These are, however, so diluted in the urine that they do not interfere.

If serum and urinary creatinine is determined with the enzymatic method, only true creatinine is measured and the serum creatinine level is lower than that determined with the non-compensated Jaffé method. Therefore, a higher Cl_Cr is determined with the insertion of the creatinine value into the clearance formula. Since the Cl_Cr reference intervals were usually established with the Jaffé method, it is recommended that the enzymatically established creatinine value in the concentration range of up to 1.5 mg/dL be raised by 20% to achieve comparability with the reference ranges described in literature /12/.

Urine collection

The most critical errors made in determining the Cl_Cr are insufficient bladder emptying and incomplete collection of the urine. Often, the first morning urine is already added to the collection container (instead of being discarded). Loss of urine also occurs if urine is spontaneously voided during defecation. Occasionally, the patient forgets that at the end of the collection period the bladder contents needs to be completely emptied into collection container. Precise instruction about the urine collection procedure is an indispensable prerequisite to the implementation of a Cl_Cr.

Reference interval

The Cl_Cr has high interindividual variability (Tab. 12.4-3 – Variation in creatinine, cystatin C and MDRD values).

References

1. Payne RB. Creatinine clearance and glomerular filtration rate. Ann Clin Biochem 2000; 37: 98–9.

2. Hasslacher C. Praxis-Leitlinien der Deutschen Diabetes-Gesellschaft. Diabetische Nephropathie. Diabetes und Stoffwechsel 2002; 11, Suppl 2: 17–9.

3. Schirmeister J, Willmann H, Kiefer H, Hallauer W. Für und wider die Brauchbarkeit der endogenen Creatininclearance in der funktionellen Nierendiagnostik. Dtsch Med Wschr 1964; 89: 1640–7.

4. Apple F. Creatinine clearance: enzymatic vs. Jaffé determinations of creatinine in plasma and urine. Clin Chem 1986; 32: 388–90.

5. Kampmann J, Siersbaek-Nielsen K, Kristensen M, Hansen JM. Rapid evaluation of creatinine clearance. Acta Med Scand 1974; 196: 517–20.

6. Schwartz GJ, Feld LG, Langford DJ. A simple estimate of glomerular filtration rate in full-term infants during the first year of life. J Pediatr 1984; 104: 849–54.

7. Schirmeister J. Standpunkte: Creatininclearance. Diagnostik 1984; 17: 9.

8. Shemesh O, Golbetz H, Kriss JP, Myers BD. Limitations of creatinine as a filtration marker in glomerulopathic patients. Kidney Int 1985; 28: 830–8.

9. Kim KE, Questi G, Ramirez O, Brest AN, Swartz C. Creatinine clearance in renal disease: a reappraisal. Brit Med J 1969; 4: 11–14.

10. Watkins DM, Shock NW. Agewise standard value for Cin, CPAH and TMPAH in adult males. J Clin Invest 1965; 34: 969–73.

11. Filler G, Browne R, Seikaly MG. Glomerular filtration rate as a putative »surrogate end-point« for renal transplant clinical trials in children. Pediatr Transplant 2003; 7: 18–24.

12. Kertai MD, Boersma E, Bax JJ, et al. Comparison between serum creatinine and creatinine clearance for the prediction of postoperative mortality in patients undergoing major vascular surgery. Clin Nephrol 2003; 59: 17–23.

12.6 Urea nitrogen (BUN)

Lothar Thomas

The terms urea and blood urea nitrogen (BUN) are used in medical diagnostics as synonyms. Urea is calculated from the BUN value by multiplication with the factor 2.14 or BUN is calculated from the urea value by multiplication with the factor 0.46.

Conditions with an elevated serum urea value are referred to as azotemia. Pre renal, renal and post renal azotemia are distinguished.

12.6.1 Indication

• Differential diagnosis of acute renal failure (ARF) using the urea/creatinine ratio
• Differential diagnosis of ARF using fractional urea clearance
• Assessment of end stage renal failure, because the signs of uremic intoxication, particularly the gastrointestinal signs, correlate better with serum urea than with creatinine concentrations
• Assessment of metabolic status in intensive care and dialysis patients, since urea concentrations are representative of protein metabolism
• Assessment of the urea distribution volume in dialysis patients.

12.6.2 Method of determination

Urease-Berthelot reaction

Principle: urea is hydrolyzed to ammonium ions and CO₂ with urease (urea amidohydrolase, EC 3.5.1.5). The ammonium ions are quantitated in a reaction with phenol and sodium hypochlorite, thus forming a blue dye which is measured spectrophotometrically between 530 and 570 nm /1/.

Diacetyl monoxime method

Principle: the condensation of diacetyl with urea forms the chromogen diazine. The specimen added to diacetyl monoxim forms a pink dye which is stabilized by the addition of thiosemicarbazide and iron(III) chloride. The pink dye is quantified spectrophotometrically /2/.

Urease UV method

Principle: urea is hydrolyzed to ammonium ions and CO₂ with urease (urea amidohydrolase, EC 3.5.1.5). In the presence of large excess of 2-oxoglutarate, the ammonium ions arising from the hydrolysis of urea are stoichiometrically consumed by the GLD reaction. Thus, the decrease in NADH concentration, measured by the change in absorbance at 340 nm is proportional to the amount of urea in the specimen (Tab. 12.6-1 – Urease UV method) /3/.

Additional methods

They include the hydrolysis of urea by urease. Either the change in pH or in conductivity, or the formation of ammonia, is measured with an NH₄⁺ electrode.

12.6.3 Specimen

Serum, plasma (no ammonium heparin), urine: 1 mL

12.6.4 Reference interval

Refer to Ref. /4, 5/ and Tab. 12.6-2 – Reference intervals for urea nitrogen (BUN).

12.6.5 Clinical significance

Essentially, three factors determine the serum urea concentration:

• Renal perfusion and thus the quantity of excreted water. In diuresis, re diffusion of urea in the distal tubule is minimal, a lot of urea are excreted in the urine, serum urea levels are low. In anti diuresis (e.g., in the case of thirst, exsiccosis or oliguric heart failure) urea re diffuses in the tubules at increased rate and hence serum urea increases.
• Urea synthesis rate. It depends on the daily protein intake and the amount of endogenously metabolized protein. Urea concentrations of up to 100 mg/dL (16.7 mmol/L) can occur, particularly in combination with thirst or febrile illnesses.
• Value of the glomerular filtration rate (GFR). A persistently elevated serum urea concentration indicates a significantly impaired GFR. With normal protein intake of approximately 75 g/day and normal renal perfusion, concentrations higher than the reference interval are not found until the GFR declines to below 30 [mL × min^–1 × (1.73 m²)^–1].

Because of the dependence on renal perfusion and protein intake, serum urea is not suitable for the first-time diagnosis of incipient renal insufficiency but is useful for monitoring the course in cases associated with a more clearly reduced GFR.

12.6.5.1 Urea/creatinine ratio

In acute renal failure pre renal and post renal disorders can be distinguished from the renal cause by determining the urea/creatinine ratio /6, 7/. With normal daily protein intake of some 1 g/kg body weight, the urea/creatinine ratio in healthy individuals is approximately:

• 35, if both are expressed in mmol/L
• 25, if both are expressed in mg/dL
• 12, if urea is determined as BUN and if both are expressed in mg/dL.

The clinical interpretation of the urea/creatinine ratio is shown in Tab. 12.6-3 – Serum urea/creatinine ratios in various diseases and conditions.

12.6.5.2 Fractional excretion of urea nitrogen (FE_UN)

In acute renal failure (ARF) fractional Na⁺ excretion (FE_Na) is used in the diagnosis of ARF to distinguish between the two main causes of ARF, pre renal state and acute tubular necrosis (ATN) (see Section 8.8.3 – Fractional sodium excretion (FE_Na)). However, many patients with pre renal disorders receive diuretics, which decrease Na⁺ reabsorption. In contrast, the FE_UN is primarily dependent on passive forces and is therefore less influenced by diuretic therapy.

A spectrum of investigations such as the urea/creatinine ratio, urine Na⁺concentration, urine osmolality, the serum/urine creatinine ratio, and the FE_Na are frequently used markers.

The FE_UN is a marker for the determination of the excreted fraction of glomerularly filtered urea-N. The FE_UN is calculated according to the following formula:

U: urine; S: serum; UN, urea nitrogen in mg/dL; creatinine in mg/dL

A low FE_Urea (≤ 35%) was found to be a more sensitive and specific index than FE_Na in differentiating between ARF due to pre renal azotemia and that due to ATN, especially if antibiotics have been administered /8/.

12.6.5.3 Urea in advanced chronic kidney disease

In advanced chronic kidney disease, up to 60% of urine creatinine can be the result of tubular secretion, and the relationship between the factional excretions of creatinine and inulin may fluctuate between 1.2 and 2.1 in cases with GFR of 20–25 [mL × min ^–1 × (1.73 m²)^–1] /9/. For the calculation of the GFR in these patients, the average value of the urea and creatinine clearance is recommended. It is considered by some nephrologists to be a precise indicator of a GFR of < 15 [mL × min^–1 × (1.73 m²)^–1] and of the likelihood of a necessity of dialysis /10/.

GFR [mL × min^–1 × (1.73 m²)^–1] = (C_Cr + C_Urea)/2

The urea clearance is performed like the creatinine clearance and, in each case, inserted into the above formula in [mL × min^–1 × (1.73 m²)^–1].

The behavior of urea in certain diseases and conditions is shown in Tab. 12.6-4 – Diseases and conditions that cause elevated serum urea.

12.6.5.4 Subnormal urea values

Subnormal urea concentrations are less meaningful diagnostically than elevated values, and can appear in cases of severe liver disease, in low-protein nutrition and in long-term treatment with hypotonic solutions. Physiologically, low urea values occur in children and in pregnant women.

12.6.6 Comments and problems

Reference interval

Within the reference interval, men have higher values than women. Low values are found in pregnant women, with low-protein nutrition, and following the intraoperative transfusion of plasma expanders. The circadian rhythm of the serum urea level is not higher than the methodological imprecision.

Possible methodological errors

The urease GLD method is specific; interferences occur due to contamination with ammonia. The diacetyl monoxime method is relatively nonspecific. Creatinine, allantoin, arginine, and some proteins are also determined.

Using the urease GLD method with serum start, urea values increase if the serum is stored (e.g., at 37 °C for 3 days). Some commercially produced control sera contain noteworthy quantities of ammonia (ammonium sulphate precipitation). Like the urease/Berthelot method, the serum start procedure provides higher urea concentrations than the method with a urease start.

Drugs

Ascorbic acid, sulfonylurea, guanethidine, thiazides, sulfonamides, chloramphenicol and dextran containing plasma expanders can produce artificially high values, mainly with the diacetyl monoxime method.

Stability

At room temperature for 2 days, at 4 °C for 1 week /14/.

12.6.7 Biochemistry and physiology

Urea is the final product of protein and amino acid metabolism; it is synthesized in the liver. During protein catabolism, the proteins are split into amino acids and de aminated. The ammonia thus produced is converted to urea inside the mitochondria via a series of reactions that are jointly referred to as urea cycle (see Fig. 5.1-1 – Organization and regulation of the hepatic and renal ammonia metabolism). On the average, dietary protein contains 16% nitrogen. Of this nitrogen, over 90% is not used for metabolic processes but is converted to urea. In adults, some 16 g of urea are formed daily /15/.

Most of urea is eliminated renally via glomerular filtration. Approximately 40–60% of the filtered urea is reabsorbed, independently of the tubular flow rate, in the proximal tubule. The re diffusion in the distal tubule is dependent upon the urine flow and is controlled by the anti-diuretic hormone. Thus, during diuresis approximately 40% of the urea that arrives at the distal tubule is reabsorbed and in anti diuresis this value increases to 70% /16/.

In pre renal and post renal ARF, in contrast to renal ARF the tubular urine flow is decreased. Due to the increased re diffusion of urea in the distal tubule and the increased creatinine secretion, a disproportionate rise in plasma urea relative to creatinine, as well as an increase in the urea/creatinine ratio, occurs.

In childhood protein catabolism is diminished, because of higher growth-related protein requirements. Average urea values are, therefore, generally lower than in adults. In pregnant women, the subnormal urea values are due to the increased protein requirement of the fetus and to renal hyper perfusion.

Because of the dependence on protein metabolism and the complex process of renal excretion the reference interval for serum urea is broad. Urea determination is, therefore, a less specific and less sensitive parameter for the assessment of incipient GFR impairment.

The upper reference interval value for serum is not exceeded until the GFR declines by 75%. With more marked impairment of the renal function, serum urea correlates better with the GFR. With a decrease of the GFR to 10% of normal, the serum urea concentrations increases by approximately 10 fold.

A correlation of serum urea with the GFR is, however, reliable only if the urea level is not influenced by extrarenal factors. For instance in chronic renal failure with polyuria or in the case of diarrhea, vomiting and hepatic insufficiency, the urea value may be less elevated than anticipated. In contrast, it is higher than expected if in addition to chronic renal failure, oliguria, excessive protein intake, heart failure or gastrointestinal hemorrhage are also present.

High protein intake leads to urea induced diuresis. This is associated with high urine osmolality of 700–900 mmol/kg. The urea induced diuresis can lead to a decrease in extracellular fluid volume combined with hypernatremia. For the clinician, this results in an apparently contradictory situation including hypernatremia together with a maximally concentrated urine at the same time /7/.

References

1. Fawcett JK, Scott JE. A rapid and precise method for the determination of urea. J Clin Pathol 1960; 13: 156–9.

2. Marsh WH, Fingerhut B, Miller H. Automated and manual direct methods for the determination of blood urea. Clin Chem 1965; 11: 624–7.

3. Kerscher L, Ziegenhorn J. Urea. In: Bergmeyer HU. Methods of enzymatic analysis. Weinheim: VCH, 1985: III, 444.

4. Rodger R, Laker M, Fletcher K, White TF, Heaton A, Ward MK, et al. Factors influencing normal reference intervals for creatinine, urea, and electrolytes in plasma as measured with a Beckman Astra 8 analyzer. Clin Chem 1985; 31: 292–5.

5. Soldin SJ, Hicks JM. Pediatric reference ranges. Washington: AACC-Press, 1995: 135.

6. Schuster VL, Seldin DW. Renal clearance. In: Seldin DW, Giebisch G (eds). The kidney. New York: Raven, 1992: 951–78.

7. Star RA. Pathogenesis of diabetes insipidus and other polyuric states. In: Seldin DW, Giebisch G (eds). Clinical disturbances of water metabolism. New York: Raven, 1995: 211–24.

8. Carvounis CP, Nisar S, Guro-Razuman S. Significance of fractional excretion of urea in the differential diagnosis of acute renal failure. Kidney International 2002; 62: 2223–9.

9. Bauer JH, Brooks CS, Burch RN. Renal function studies in man with advanced renal insufficiency. Am J Kidney Dis 1982; 3: 30–5.

10. Lubowitz H, Slatopolsky E, Shankel S, et al. Glomerular filtration rate: Determination in patients with chronic renal disease. JAMA 1969; 199: 100–4.

11. Harten J, Hay A, McMillan DC, McArdle CS, O’Reilly DStJ, Kinsella J. Postoperative serum urea is associated with 30-day mortality in patients undergoing emergency abdominal surgery. Ann Clin Biochem 2006; 43: 295–9.

12. Morgan DB, Carver ME, Payne RB. Plasma creatinine and urea creatinine ratio in patients with raised plasma urea. Brit Med J 1977; 2: 929–32.

13. Buchborn E. Erhöhte Harnstoffwerte im Blut. Monatskurse Ärztl Fortbild 1970: 20: 328–31.

14. Heins M, Heil W, Withold W. Storage of serum or whole blood samples? Effects of time and temperature on 22 serum analytes. Eur J Clin Chem Clin Biochem 1995; 33: 231–8.

15. Atkinson DE. Functional roles of urea synthesis in vertebrates. Physiological Zoology 1992; 65: 243–67.

16. Gillin AG, Sands JM. Urea transport in kidney. Semin Nephrol 1993; 13: 146–54.

12.7 Cystatin C

Lothar Thomas

Cystatin C is a non glycosylated basic protein (MW 13.36 kDa) and is found in a variety of biologic fluids. Cystatin C is endogenously produced at a constant rate, freely filtered in the glomerulus, neither reabsorbed nor secreted in the renal tubule, and not extra renally eliminated. All endogenous filtration markers are affected by factors other than GFR (non-GFR determinants), including generation, renal tubular reabsorption and secretion, and extrarenal elimination. In comparison with creatinine, cystatin C is less influenced by muscle mass and has often been proposed to be more accurate than creatinine for estimation of GFR in subgroups of the population /1/.

In addition, cystatin C is a novel risk factor for cardiovascular events and increasing concentrations are associated with higher mortality in patients with acute coronary syndromes /2/.

12.7.1 Indication

The 2012 Kidney Disease Improving Global Outcomes (KDIGO) guideline suggest measuring cystatin C in adults with eGFR_Cr 45–59 [mL × min^–1 × (1.73 m²)^–1] who do not have markers of kidney damage if confirmation of chronic kidney disease (CKD) is required /3/.

Further indications are:

• Patients with assumed moderate impairment of the GFR, such as in hypertension, diabetes mellitus, metabolic syndrome, hyperuricemia, cardiovascular disease, liver disease, obstructive uropathy
• Children and elderly individuals (≥ 70 years)
• Suspicion of acute renal failure
• Monitoring of renal function in transplant patients; using the eGFR_Cr–Cys
• Monitoring of the renal function under cytostatic therapy, e.g., cisplatin, carboplatin.

12.7.2 Method of determination

Particle-enhanced homogeneous immunoassays using latex or polystyrene particles which are coated with specific antibodies to cystatin C. Two different versions of particle-enhanced assays are offered /4/:

• Particle-Enhanced Immuno Turbidimetric Assay (PETIA)
• Particle-Enhanced Nephelometric Assay (PENIA).

PETIA

Principle: polyclonal rabbit antibodies to cystatin C are covalently attached to carboxylate-modified uniform latex particles /4/. The increase in absorbance at 340 nm produced by the agglutination reaction is then measured after 240 seconds to give an endpoint value. Lyophilized recombinant human cystatin C is used as calibrator.

PENIA

Principle: serum cystatin C is measured by a nephelometric immunoassay based on rabbit mono specific anti-human cystatin C antiserum covalently coated with 80-nm diameter chloromethyl styrene particles (latex reagent) /5/. This refers to a fixed time method, in which the formation of agglutinates is measured following incubation of diluted samples with the latex reagent. These scatter the infrared light (840 nm) of a diode. Changes in the scattered light signal are transformed into mg/L using a calibration curve.

A certified reference material, ERM-DA471/IFCC, is available for cytatin C.

12.7.2.1 KDIGO recommendations for laboratories that measure cystatin C

Recommendations:

• Measure cystatin C using an assay with calibration traceable to the international standard reference material
• Report eGFR_cys in addition to the serum cystatin C concentration in adults and specify the equation used whenever reporting eGFR_cys and eGFR_creat-cys
• Report eGFR_cys and eGFR_creat-cys in adults using the 2012 CKD-EPI cystatin C and 2012 CKD-EPI creatinine-cystatin C equations respectively
• eGFR_cys and eGFR_creat-cys levels less than 60 [mL × min^–1 × (1.73 m²)^–1] should be reported as decreased.

For calculation and clinical assessment of eGFR_cys and eGFR_creat-cys refer to Chapter 12.2 – Glomerular filtration rate.

12.7.3 Specimen

Serum, plasma (heparin, EDTA): 1 mL

12.7.4 Reference interval

Refer to Ref. /6, 7, 8, 9, 10, 11/ and Tab. 12.7-1 – Reference intervals for cystatin C.

12.7.5 Clinical significance

Cystatin C is an additional marker for the estimation of renal function and also for prediction of cardiovascular risk.

12.7.5.1 Cystatin C and renal function

Serum cystatin C has been proposed as a better marker of chronic kidney disease (CKD) than serum creatinine. The most important reasons are /12/:

• Cystatic C- based GFR-estimating equations do not require the use of vague terms like race and sex
• Cystatic C- based GFR-estimating equations are useful for both children and adults, including the elderly
• Cystatic C- based GFR-estimating equations are superior to creatinine-based equations in predicting end-stage renal disease, cardiovascular manifestations, hospitalization and death
• Cystatin C is required to diagnose the new syndrome shrunken pore syndrome with its high mortality and morbidity, even in the absence of reduced GFR.

Different studies in patients with reduced measured GFR (mGFR) < 80 [mL × min^–1 × (1.73 m²)^–1] have found a better correlation of mGFR to cystatin C than to serum creatinine. In some cases it was possible to diagnose CKD as early as stage 2 /13, 14/. If the diagnostic sensitivity of cystatin C and creatinine was set, in each case, at 100% for the recognition of a GFR of < 80 [mL × min^–1 × (1.73 m²)^–1], the diagnostic specificity of cystatin C was 75%, while that of creatinine was 0% /4/. In a study, for the detection of mild impairment of GFR < 72 [mL × min^–1 × (1.73 m²)^–1], the diagnostic sensitivity of cystatin C was 71.4% and of creatinine it was only 52.4% /14/.

Cystatin C is more accurate than creatinine in subgroups of the population, including vegetarians, those with limb amputation, muscle wasting or chronic disease /1/.

The diagnostic sensitivity of cystatin C for the diagnosis of CKD is higher than for creatinine because of its lower inter individual variation. Thus, a creatinine value at the lower reference interval value must increase by 13 times the standard deviation of the intra individual variation to exceed the upper reference interval value; with cystatin C, only a 4-fold rise is necessary /15/.

The cystatin C-based eGFR_cys is not more accurate for routine GFR estimation than eGFR_Cr. The eGFR based on the combination of creatinine and cystatin C (eGFR_Cr-Cys) is more accurate than either alone, reflecting the lesser influence of non-GFR determinants of either marker when both are used /16/.

The KIDGO recommends the determination of the eGFR_cys or eGFR_cr-cys in patients with eGFR_cr in the range of 45–59 [mL × min^–1 × (1.73 m²)^–1] /3/. Data from the CKD-EPI showed improved accuracy in GFR estimation using eGFR_Cr-Cys compared to either marker alone. In patients with eGFR_Cr 45–59 [mL × min^–1 × (1.73 m²)^–1], the combined equation correctly reclassified 16.8% of those with eGFR_Cr 45–59 [mL × min^–1 × (1.73 m²)^–1] to mGFR ≥ 60 [mL × min^–1 × (1.73 m²)^–1] /17/. The consensus was therefore that individuals with eGFR_Cr 45–59 [mL × min^–1 × (1.73 m²)^–1] without markers of kidney damage, but with eGFR_cys or eGFR_cr-cys ≥ 60 [mL × min^–1 × (1.73 m²)^–1] could be considered not to have CKD.

Co morbidities such as solid-organ transplant strongly influence the relationship between mGFR, eGFR_Cr, eGFR_cys or eGFR_cr-cys. Among the 3 CKD-EPI equations, eGFR_Cr-Cys performed most consistently across potential kidney donors and transplant recipients /3, 18/. The three CKD-EPI equations are depicted in Section 12.2 – Glomerular filtration rate (GFR).

Recently a simple cystatin C-based equation for estimation of GFR comprising only two variables, cystatin C concentration and age was introduced /19/:

eGFR = 130 × cystatin C^–1.069 × age^–0.117 –7

The new equation includes age, a single coefficient (exponent) for serum cystatin C (–1.069), and a value for extrarenal elimination of cystatin C [7 mL × min^–1 × (1.73 m²)^–1] across the range of GFR. By contrast to the CKD-EPI eGFR_cys the new equation does not require specification of sex and takes into account extrarenal cystatin C elimination /19/.

The diagnostic value of cystatin C for the detection of impaired kidney function and for the assessment of their course is shown in Tab. 12.7-2 – Diagnostic significance of cystatin C in impaired renal function.

12.7.5.2 Cystatin C in children

Cystatin C concentrations in serum during the first weeks of life are approximately twice as high as in adults; they then fall continuously and reach adult values by the end of the first year of life /10/. At the age of < 4 years, serum creatinine is in the range of 0.2–0.4 mg/dL (17.7–35.4 μmol/L) and, because of insensitivity of the Jaffé method, slight changes are difficult to detect. These children benefit from the determination of cystatin C.

12.7.5.3 Cystatin C, age and metabolic disturbances

Because there is a constant relationship between the mGFR and the cystatin C serum level, the age-dependent decrease in the GFR leads to a rise in concentrations. However, metabolic abnormalities, which occur frequently in old age, also contribute to the rise in cystatin C. Thus, in the Third National Health and Nutrition Examination Survey /20/, individuals ≥ 60 years with a cystatin C level of > 1.0 mg/L (PENIA), but without stage 3 or 4 CKD, had a higher prevalence of elevated uric acid, phosphate, homocysteine, CRP and fibrinogen and more often a reduced hemoglobin level than those with a lower cystatin C value. Hence, elevated cystatin C may be an indicator of preclinical metabolic disease.

12.7.5.4 Discrepant results of serum creatinine and cystatin C

Factors that are not associated with the GFR may influence serum cystatin C concentrations.

Corticosteroids

High-dose corticoidsteroids stimulate cystatin C production. The eGFR_cys indicates a false low clearance. Glucocorticoids affect the extrarenal metabolism of cystatin C. This was demonstrated in asthmatics and in organ transplant patients under corticosteroid therapy /21/.

Thyroid gland hormones

Hyperthyroidism is associated with a reversible increase in cystatin C production. The eGFR_cys indicates a false low clearance. Untreated hypothyroidism leads to decreased cystatin C concentration. The eGFR_cys indicates a false increased clearance. Treatment causes a 14% increase in cystatin C in hypothyroid patients and a 21% decrease in the hyperthyroid group, producing similar final concentrations /22/.

Other factors

In a study involving 3418 patients in whom the measured GFR was determined with ¹²⁵I-iothalamate or ⁵¹Cr EDTA in comparison to cystatin C and creatinine the results were in summary /21, 23/:

• After adjustment for GFR, cystatin C was 4,3% lower for every 20 years of age, 9,2% lower for female gender but only 1.9% lower in blacks
• Diabetes was associated with 8.5% higher levels of cystatin C and 3.9% lower levels of creatinine
• Higher C-reactive protein and white blood cell count and lower serum albumin were associated with higher levels of cystatin C and lower levels of creatinine. Adjustment for age, gender and race had a greater effect on the association of factors with creatinine than cystatin C.
• Low creatinine levels in the presence of cystatin C concentrations at or above the reference interval are suggestive of muscle wasting, while the opposite may indicate very high muscle mass or excessive intake of cooked meat., fish or creatinine supplements (e.g., body builders) /23/.
• Although cystatin C is predominantly excreted via the kidney, there is some hepatic elimination accounting for about 5% at normal. The relative contribution of extrarenal elimination increases with declining GFR and underlies the observation that cystatin C levels do not exceed 10 mg/L even in anuric patients. Thus, compared to creatinine, the rise in cystatin C is stronger with mildly to moderately impaired renal function (CKD stage 2-3), whereas the opposite applies to severe and end-stage renal failure (CKD stage 4-5) /23/.

12.7.6 Comments and problems

Specimen

The differences in cystatin C values in serum, heparin and EDTA plasma are slight and, therefore, all three specimens can be used. Estimated GFR based on capillary cystatin C concentrations is higher than the venous cystatin C based GFR. The limits of agreement between estimated GFR based on capillary and venous measurements exceed ± 20% at the upper reference concentration, which hampers the usefulness of capillary sampling /24/.

Method of determination

For PENIA one manufacturer provides the laboratory with instruments and reagents. For PETIA there are several manufacturer of reagents, but these are adapted to various analytical platforms. The calibrators are adjusted to the reference preparation ERM-DA471/IFCC with a concentration of 5.48 mg/L in the dissolved state /25/. The analytical imprecision of PENIA and PETIA and the upper reference interval values for individuals aged 70 and older were comparable /24/.

Although some manufacturers have clearly improved their calibration protocols relative to ERM-DA471, most of them failed to meet the criteria for acceptable cystatin C measurement /28/.

Reference interval

It is reported predominantly that in the age group of 1–49 years, the reference interval is stable. Newborns have significantly higher values which decrease during the first year in life. From the age of 50 onwards, a continuous rise in cystatin C occurs, due to reduction in the GFR /24/. A few publications hint at gender-specific differences, but the majority denies their existence /14/.

Biological variation

In a study 24-h profiles of cystatin C and its derived estimates of GFR were determined in people with and without CKD. The CV of cystatin C was in the same range for both people. Despite differences in the biological variation the reference change values of all eGFR equations were within 13 to 20% in both study groups /29/.

Interference factors

PENIA: hemolysis > 6–12 g Hb/L, bilirubin > 24.5 mg/dL (418 μmol/L), triglycerides > 800 mg/dL (10 mmol/L), rheumatoid factors > 2000 kIU/L /27/.

PETIA: hemolysis > 1 g Hb/L, bilirubin > 9–40 mg/dL (120–700 μmol/L), triglycerides > 700 mg/dL (8 mmol/L), rheumatoid factor > 300 kIU/L /26/.

Stability

7 days at room temperature, 1–2 months at –20 °C, at least 6 months at –80 °C /26/.

12.7.7 Biochemistry and physiology

Cystatin C, also termed γ-trace and post-γ-globulin, is a protease inhibitor belonging to the cystatin super family. Eleven proteins from this protease inhibitor family are known; clinically cystatin C is the most important of these. It is believed that cystatin C neutralizes proteases that are released from lysozymes or that originate from dying cells.

Cystatin C has a MW of 13,359 Da, is composed of 120 amino acids, is non-glycosylated, and contains two disulfide bonds. It is a basic protein with a pI of 9.3. Because it is synthesized as a pre protein, this is an indication of its extracellular function /27/. In bodily fluids, the highest concentrations are found in seminal plasma (41–62 mg/L) and in the cerebrospinal fluid (3.2–12.5 mg/L), the lowest concentrations are found in the urine (0.03–0.29 mg/L).

Cystatin C has been demonstrated in all organs and in all nucleated cells; it is synthesized by the cells at a relatively constant rate. There are, as yet, no concrete indications that infections, cancer, or exogenous substances may have a significant effect on cystatin C levels in serum. Only high concentrations of dexamethasone stimulate the formation of cystatin C in cell culture. In addition, the extreme doses that are used for immunosuppression lead to elevated values in transplant patients.

The higher reliability of cystatin C, in comparison with serum creatinine, for the assessment of the GFR is based on the following facts /27/:

• As demonstrated in rat studies, 94% of cystatin C undergoes glomerular filtration as compared to ⁵¹Cr-EDTA clearance. There is only minimal and serum concentration-independent extrarenal clearance.
• Following glomerular filtration, cystatin C is metabolized by the proximal tubular cells and is not reabsorbed in the intact form
• In the presence of an intact tubular epithelium, there is no secretion of cystatin C from the renal vessels into the tubuli.

References

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2. Taglieri N, Koenig W, Kaski JC. Cystatin C and kardiovascular risk. Clin Chem 2009; 55: 1932–43.

3. Kidney Disease: Improving Global Outcome (KDIGO) CKD Work Group. KDIGO clinical practice guideline for the evaluation and management of chronic kidney disease. Kidney Int Suppl 2013: 3: 1–150.

4. Kyhse Andersen J, Schmidt C, Nordin G,Andersson B, Nilsson-Ehle P, Lindström V, Grubb A. Serum cystatin C, determined by a rapid, particle-enhanced turbidimetric method, is a better marker than serum creatinine for glomerular filtration rate. Clin Chem 1994; 40: 1921–6.

5. Mussap M, Ruzzante M, Varagnolo M, Plebani M. Quantitative automated particle-enhanced immunonephelometric assay for the routinary measurement of human cystatin C. Clin Chem Lab Med 1998; 36: 859–65.

6. Montini G, Cosmo L, Amici G, Mussap M, Zacchello G. Plasma cystatin C values and inulin clearances in premature neonates. Pediatr Nephrol 2001; 16: 463–4.

7. Bökenkamp A, Domanetzki M, Zinck R, Schumann G, Brodehl J. Reference values for cystatin C serum concentrations in children. Pediatr Nephrol 1998; 12: 125–9.

8. Norlund L, Fex G, Lanke J, von Schenck H, Nilsson JE, Leksell H, et al. Reference intervals for the glomerular filtration rate and cell proliferation markers: serum cystatin C and serum β₂-microglobulin/cystatin C ratio. Scand J Clin Lab Invest 1997; 57: 463–70.

9. Bahar A, Yilmaz Y, Unver S, Gocmen I, Karademir F. Reference values of umbilical cord and third-day cystatin C levels for determining glomerular filtration rates in newborns. J Int Med Res 2003; 31: 231–5.

10. Fischbach M, Graff V, Terzic J, Bergere V, Oudet M, Hamel G. Impact of age on reference values for serum concentration of cystatin C in children. Pediatr Nephrol 2002; 17: 104–6.

11. Finney H, Newman DJ, Price CP. Adult reference ranges for serum cystatin C, creatinine and predicted creatinine clearance. Ann Clin Biochem 2000; 37: 49–59.

12. Grubb A. Cystatin C is indispensable for evaluation of kidney disease. eJIFCC 2017; 28: 268–76.

13. Newman DJ, Thakkar H, Edwards RG, et al. Serum cystatin C, measured by automated immunoassay: a more sensitive marker of changes in GFR than serum creatinine. Kidney Int 1995; 47: 312–8.

14. Keevil BG, Kilpatrick ES, Nichols SP, Maylor PW. Biological variation of cystatin C: implications for the assessment of glomerular filtration rate. Clin Chem 1988; 44: 1535–9.

15. Inker LA, Schmid CH, Tighioart H, Eckfeldt JH, Feldman HI, Greene T, et al. Estimating glomerular filtration rate from serum creatinine and cystatin C. N Engl J Med 2012; 367: 20–9.

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12.8 Erythrocytes, leukocytes, casts in urine

Christian Thomas, Lothar Thomas

The detection of hematuria, leukocyturia and proteinuria are early indicators of kidney and urinary tract disorders. Evidence of casts provides an indication of a pathological process in the kidney.

Screening for hematuria and leukocyturia includes the use of:

• Test strips with test fields for erythrocytes/hemoglobin and leukocytes
• Particle examination by means of microscopy or flow cytometry.

In asymptomatic patients the test strip screening precedes particle examination and this is only performed if the test yields a positive result either for erythrocytes, leukocytes, protein, and glucose. The diagnostic sensitivity of the test strips is adequate for the detection of clinically significant erythrocyturia and leukocyturia /1/.

Depending on the clinical concerns in the presence of proteinuria, hematuria or leukocyturia, the following further examinations may be useful for the location of the underlying injury:

• Macroscopic inspection of the urine before and after centrifugation
• Particle analysis
• Investigation for the presence of dysmorphic erythrocytes
• Urine cytology if hematuria is present, for the diagnosis of tumor cells.

12.8.1 Indication

Test strips

Screening during the first examination for the exploratory exclusion of a kidney and urinary tract disease.

Particle examination

• Patients with symptoms of a kidney and urinary tract disease
• If the test strip indicates a positive finding (e.g., proteinuria, erythrocyturia, leukocyturia, and glucosuria).

Dysmorphic erythrocytes

Differentiation between renal and extrarenal hematuria.

Urine cytology

Diagnosis and monitoring of bladder tumors.

12.8.2 Method of determination

For the detection of hematuria and/or leukocyturia, a sample of uncentrifuged, well-mixed urine is examined within 2 hours following collection.

Urinary test strip and sediment analysis integrated with intelligent verification criteria can help to select samples that need manual review. In a study /37/ application of intelligent verification criteria resulted in an increase in specificity from 85.5% to 96.8% and from 88.2 to 94.9% for red blood cells and white cells, respectively.

12.8.2.1 Erythocyte examination

Principle of reagent strip test

The test detects pseudo-esterase activity of erythrocytes. The reagent pad contains an organic hydro peroxide, which oxidizes a highly sensitive chromogen to a blue dye through catalytic pseudo-esterase activity. The presence of erythrocytes is recorded as spots on the test field, while hemoglobin or myoglobin are recorded as a homogeneous appearance of color on the reagent pad. For erythrocyte count of 10 × 10⁶/L, the diagnostic sensitivity of the test strip is 70–80% /2/. The specificity of the red blood cell detection is, in comparison with chamber counting, only seemingly diminished, since some of the cells in the urine lyse and the test strips provide a positive result, while chamber counting shows a negative outcome.

12.8.2.2 Leukocyte examination

Principle of reagent strip test

Neutrophil granulocytes and macrophages contain the enzyme indoxyl esterase. The reagent pad on the test strip contains an indoxyl ester. The ester is cleaved in the presence of leukocytes. The indoxyl produced is oxidized to indigo blue under the influence of atmospheric oxygen which results in a color change on the test pad from beige to blue /3/. The detection limit of the test strip is 20 × 10⁶ leukocytes/L and the accuracy 80–90% when compared with microscopic leukocyte count. At a cell count of 100 × 10⁶ leukocytes/L, the diagnostic sensitivity is 95%. At the limit of detection, the diagnostic specificity is 80–90% /4/.

12.8.2.3 Particle examination

Clinically significant particles in the urine include /4/:

• Erythrocytes as indicators of hematuria. These can result from a general bleeding tendency, a renal disorder, a urinary tract disorder, or vaginal contamination. In cases of isolated hematuria, dysmorphic erythrocytes indicate that the patient requires further nephrological treatment.
• Leukocytes, which can indicate a urinary tract infection, glomerulonephritis, or interstitial nephritis
• Casts, which can indicate a renal disorder. The diagnostic sensitivity for casts as indicators of renal disorders is low.
• Epithelial cells from the external genitals and the urethra. Except in pregnant individuals, these indicate a mistake in the urine specimen collection method used.
• Cells from the transitional epithelium (urothelium), from the renal pelvis to the ureters. These are indicative of a urinary tract infection, but are also evident in noninfectious urological disorders.
• Renal tubular epithelial cells. These are small, round epithelial cells and are present in disorders such as acute tubular necrosis, acute interstitial nephritis, or kidney transplant rejection.
• Fatty droplets or lipid-laden tubular cells. They are a sign of damage to the glomerular basement membranes, through which lipids can permeate.
• Bacteria which are only clinically significant in spontaneously voided urine at counts of ≥ 10⁵/mL.

Particle counts should be performed within 1 hour of the sample being collected. If this is not possible, the sample can be kept refrigerated for 2–4 hours /4/.

In routine diagnostic testing, particle identification is performed microscopically and qualitatively using urine sediment under a coverslip. Quantitative testing is performed on uncentrifuged urine either using a counting chamber or by automated particle analysis.

12.8.2.4 Standardized urine sediment under a coverslip

Principle of urine sediment test

Urine collected no more than 2 hours previously is briefly shaken, and then 10 mL is decanted into a centrifuge vial. The specimen is then centrifuged for 5 minutes at 400 × g to obtain a 25-fold concentration. The test tube is tipped and emptied, taking care to allow excess material to drip off. The sediment is thoroughly homogenized with the remaining supernatant by drawing it in to and out of a rubber-bulbed Pasteur pipette five times. A small drop of the homogenate is placed on a slide and a cover slip is placed on top. Under the microscope, the distribution density of the cells is first evaluated with the 10 × ocular lens and 10 × objective. If the distribution is homogeneous, a count is performed using the 40 × objective. Microscopy should ideally be performed using a phase contrast microscope with a polarization system /4, 5/.

Centrifugation methods such as the coverslip urine sediment method can never be used for the quantitative assessment of erythrocyte and leukoctyte counts. Variable amounts of cells (on the order of 20–80%) are lost during centrifugation /6/.

12.8.2.5 Chamber counting of uncentrifuged samples

Principle of chamber counting test

A sample from an uncentrifuged, well-mixed spot urine is placed in a counting chamber, and the number of erythrocytes and leukocytes per microliter are determined. If the counting chamber has,for example, a depth of 0.25 mm and squares with an area of 1 mm², then counts are performed in 4 of the 1 mm² squares and the cell count calculated as follows:

One disadvantage of using the counting chamber method is that it is highly time-consuming. Without staining or phase contrast microscopy, only inadequate particle differentiation is possible.

12.8.2.6 Automated urinalysis systems

Urine chemistry analyzers determine chemical constituents of the urine and count and differentiate erythrocytes, leukocytes, epithelial cells and casts flow cytometrically /7/. For particle examination, the cellular DNA is stained with a fluorochrome, and particle size is determined following hydrodynamic focusing of the cells using forward and side scatter.

12.8.2.7 Erythrocyte morphology

Principle of erythrocyte morphology testing

Mid-stream morning urine is examined 1–2 hours following collection /8/. The test needs not to be performed immediately if, following collection, approximately 50 mg of the antiseptic compound thiomersal is added to about 10–20 mL of the urine sample. In urine conserved in this manner, the urinary corpuscular constituents remain stable for 3 to maximally 7 days /9/.

Procedure: 10 mL of urine are centrifuged for 5 minutes at 400 × g (swing bucket rotor), and 9.5 mL of supernatant are discarded. The sediment is resuspended in the remaining 0.5 mL of urine.

The erythrocyte morphology can be assessed as follows:

• With the phase contrast microscope; 1 drop of sediment is placed on a slide, covered with a cover slip and immediately examined microscopically
• With the bright field microscope, following staining of the urine sediment; 1 drop of sediment is put onto the middle of a cover slip, which is then placed on a dye-coated slide. The erythrocytes are examined following 10 minutes of staining time. Slides pre-colored with the dyes new methylene blue N and cresyl violet acetate are well suited and commercially available. The staining of the sediment on the slides, according to Papanicolaou, also supports the morphological assessment /9/.

The morphology of 100 erythrocytes is assessed, and the percentage of isomorphic and dysmorphic forms is determined.

12.8.2.8 Urine cytology

Mid-stream urine from the second voided morning urine is examined, and a sediment is prepared. For the differentiation of leukocytes, a drop of resuspended sediment is placed on a dye-coated slide (see above). For the assessment of the morphology of tubular cells or bladder epithelial cells, these cells are prepared, fixed, and stained according to May-Grünwald-Giemsa or Papanicolaou.

12.8.3 Specimen

Mid-stream morning urine should be collected for the microscopic assessment of cells and casts. It should not be older than 2 hours at the time of the microscopic examination. For the different urine specimens see Section 12.3 – Urinalysis.

The testing for dysmorphic erythrocytes is only considered to be meaningful if the test strips are positive for blood, or as of an erythrocyte count of (6–8) × 10⁶/L of urine, and not in cases of macro hematuria.

12.8.4 Reference interval

Refer to Ref. /4, 10, 11, 12, 13/ and Tab. 12.8-1 – Reference intervals for erythrocytes, leukocytes and casts in urine.

12.8.5 Clinical significance

The use of test strips to analyze urine and the use of microscopy to analyze urine sediment enable the semi quantitative identification of erythrocytes and leukocytes; microscopy can also be used to test for casts and epithelial cells. Patients generally do not need to undergo microscopic urine sediment testing in cases with test strip results being negative for nitrite, normal protein, erythocytes and glucose (< 1 g/L) in the initial evaluation /13/.

For microscopic urine analysis, positivity criteria are defined as the detection of the cellular components erythocytes and/or leukocytes per ≥ 4 per high power field (10 × 40).

12.8.5.1 Hematuria

The microscopic examination is a sensitive way of detecting erythrocytes in urine. The test strip can detect (2.5–5) × 10⁷ erythrocytes per liter (25–50 erythrocytes per μL) with high accuracy corresponding to 3 erythrocytes per high power field (10 × 40). The detection limit of microscopic examination is 1.2 × 10⁷ erythrocytes per liter /14/. Definitions for micro hematuria vary and are ≥ 4 erythrocytes with the standardized urine sediment under cover slip, > 10 erythrocytes/μL, in chamber counting of uncentrifuged specimens and in particle counting instruments > 15/μL /15/. Macro hematuria (visible with the naked eye) and micro hematuria (only visible with the microscope) are distinguished.

Clinically, hematuria is classified into a glomerular and a non-glomerular form /15/.

In glomerular hematuria, the erythrocytes are often dysmorphic and casts are demonstrable in 30% of the cases. In the non-glomerular form, the erythrocytes are normal and casts are not present.

Non-glomerular hematuria can be of urological or non-urological genesis. The absence of proteinuria differentiates urological causes of non glomerular hematuria from other diseases that are associated with hematuria. Frequently causes of non-urological, non glomerular genesis are tubulo-interstitial diseases (vascular diseases, papillary necrosis).

Urological causes of hematuria are divided into painful (infectious disease, nephrolithiasis) and pain-free (urological cancers, benign prostatic hyperplasia). Frequent causes of hematuria are shown in Tab. 12.8-2 – Causes of hematuria, an algorithm for the location of source in Fig. 12.8-1 – Diagnostic procedure for investigating hematuria.

12.8.5.1.1 Prevalence of hematuria

Children

The prevalence of persistent, isolated hematuria in children is 0.4–4% /16/. In one study involving 12,000 schoolchildren, 6% had hematuria (> 5 erythrocytes per high power field) when first tested. Hematuria was more common in girls than in boys. The incidence of hematuria in a second set of samples taken within a week of the first was only half as high. The annual incidence of new cases in 6–12 year-old children was 0.4% /17/. In another study involving 9000 schoolchildren, 4.1% had hematuria, while only 0.5% had the condition for over 6 months. Macro hematuria is rare in childhood.

Adults

In one study /18/, 2.5% of males aged 28–57 had hematuria, while in another study /19/, 5.4% of males aged 18–54 had the condition. The prevalence in postmenopausal females is reported to be up to 13% /20/. Among urological patients, the frequency of hematuria is about 10%. According to the American Urological Association, up to 56% of hematuria cases are associated with a significant disease, while the prevalence of malignant urologic cases among this patient pool is up to 25.8% /14/. The prevalence of hematuria depends on age and sex, and is higher in older individuals and in males. Prospective studies with hematuria patients found the cause to be urinary tract cancer in 37% of cases, while 15% were due to nephrolithiasis, urinary tract obstruction, or chronic urinary retention /21/. Another study /14/ also found neoplasms to be the most common cause of hematuria (in 41.8% of cases). Primary carcinomas of the genitourinary system were found in 22% of cases, of which 9% were in the bladder, 6% in the kidneys, and 6% in the prostate. The benign causes included prostatic hypertrophy (19%), urinary tract infections (26%), nephrolithiasis (13.6%), congenital abnormalities (3.6%), trauma (2%), and unidentified etiology (12%) /22/. No increase in risk of microscopic hematuria with aspirin use of 75–325 mg per day is found by asymptomatic healthy people /23/.

12.8.5.1.2 Macro hematuria

Macro hematuria is present with > 1000 erythrocytes/μL. 1 mL of blood in 1 liter of urine is equivalent to some 25,000 erythrocytes/μL. Depending on the source of the blood, the concentration of blood in the urine, and other factors, the color of the urine can range from light red to pink to tea or cola-brown.

Tea or cola-colored urine tends to be indicative of a glomerular disorder, while red or pink-colored urine indicates bleeding in the urinary tract.

Blood clots indicate the ureters or bladder as the source of the blood. They do not occur with glomerular hematuria, since the glomerular filtrate contains urokinase and t-PA (tissue-type plasminogen activator).

Hematuria at the end of urination indicates the bladder as the source of the blood, hematuria at the start of urination indicates the urethra or prostate, and hematuria throughout urination points to the kidneys, ureters, or diffuse bleeding as the source.

In adults > 40 years an unexplained episode of visible hematuria (macro hematuria) may signify bladder or upper urinary tract cancer and urologic referral is necessary to exclude cancer /36/.

Children

The prevalence of macro hematuria is reported to be 0.13%. The possible causes that need to be considered include renal stone disease, primary glomerulonephritis (GN), systemic disease with GN, hemolytic uremic syndrome, Alport syndrome, and a malignant tumor. The most common glomerulonephritic disorders associated with macro hematuria are IgA nephropathy, acute post infectious GN, and GN associated with Henoch-Schoenlein purpura /24/.

Adults

Macro hematuria is a common symptom of polycystic kidney disease, IgA nephropathy, kidney tumors, kidney stones, prostate disorders, bladder tumors, bladder tuberculosis, bladder stones, hemorrhagic cystitis, hemorrhagic diathesis, and hydronephrosis /25/.

12.8.5.1.3 Pseudo hematuria

Red urine comparable to macro hematuria can be passed following circumstances that include (see also Tab. 12.3-1 – Characteristic appearances of urine):

• Eating foods such as rhubarb, red beets or substances containing aniline dyes
• Use of medications such as phenothiazine, phenazopyridine, or phenindione
• Porphyria, rhabdomyolysis, or hemolysis.

False-positive test strip results can be caused by substances that contain iodine and other oxidants, as well as by peroxidases in cases of bacteriuria /26/.

Hematuria can occur during treatment with the following medications /27/: phenytoin, rifampicin, anticoagulants (including aspirin), NSAIDs such as ibuprofen, and cytostatics such as cyclophosphamide, ifosfamide, and danazol.

12.8.5.1.4 Microhematuria

Nonvisible (microscopic) hematuria or microhematuria is a type of hematuria that can only be identified by a test strip assay, examination of the sediment under a cover slip, or by chamber counting. The prevalence of microhematuria among general medical clinics in outpatients is 5–13% in adults, 1–2% in infants and 4% in school children /28/. Any incidence of microhematuria in the absence of a urinary tract infection or menstrual bleeding should be further investigated. Often microhematuria is an accidental finding.

If the microhematuria was identified by urine test strip, the result must be confirmed by microscopy to rule out pseudo hematuria.

The discovery of micro hematuria without proteinuria is often an incidental finding. About 70% of cases cannot be explained even after a thorough examination of the upper urinary tract with diagnostic imaging.

The most common causes of micro hematuria in the upper urinary tract include:

• In adults, the most common cause of glomerular micro hematuria is IgA nephropathy and thin basement membrane nephropathy (TBMN). Thirty percent of patients with isolated microscopic hematuria have mesangial immunoglobulin A glomerulonephritis and 20–40% of these patients will progress to renal failure without treatment /29/. TBMN is often diagnosed clinically when there is persistent dysmorphic or glomerular hematuria, but minimal proteinuria, normal kidney function, and no other obvious cause /30/.
• In children, urinary tract infections, lithiasis, idiopathic hypercalcuria (in 22% of cases), or hyper uricosuria /31/
• Other common etiologies are chronic glomerulonephritis and the after-effects of acute glomerulonephritis /24/. In addition to urolithiasis, the occurrence of hematuria after 2–3 years is a complication in about one-third of patients following a kidney transplant.

Micro hematuria has two types, and can be:

• Transient in nature, for example due to extreme physical exertion (as seen in long-distance runners), infection, or trauma
• Permanent or recurrent in nature, resulting from kidney stones, malignant tumors in the kidneys and urinary tract, and glomerular or interstitial nephritis.

In one study /32/, patients with clinically confirmed asymptomatic micro hematuria and an average age of 44.2 years were followed for an average of 3.7 years. All of the patients remained asymptomatic during this time. When the initial testing revealed > 9 erythrocytes per high power field, the hematuria was more constant in nature than when 3–9 erythrocytes were detected.

Refer to Tab. 12.8-4 – Diagnosis and definition of microhematuria.

Etiology of micro hematuria

A key question is whether the pathology of micro hematuria results from kidney disease or due to disease of the urinary tract. Investigations of proteinuria or casts in urine will give further information. Both investigations have neither high diagnostic sensivity nor sufficient specificity. For further evaluation most laboratories recommend the determination of dysmorphic erythrocytes.

Most causes of micro hematuria are /36/:

• Glomerulopathies: IgA nephropathy, thin glomerular basement membrane disease
• Inflammatory conditions: urethra, bladder, benign prostatic hypertrophy, renal caculi
• Bladder infestation (Schistosoma haematobium is endemic in Northern Africa)
• Bladder cancer (9.0 cases per 100,000 person-years for men; 2.2 cases per 100,000 person-years for women)
• Renal cancer (6.0 cases per 100,000 person-years for men; 3.1 cases per 100,000 person-years for women).

12.8.5.1.5 Dysmorphic erythrocytes

The morphology of red blood cells is important for the differential diagnosis of isolated hematuria. Dysmorphic erythrocytes appear in the urine as a consequence of glomerulonephritis. These erythrocytes feature characteristic, microscopically recognizable changes in shape (Fig. 12.8-2 – Display of non-glomerular, osmotically modified erythrocytes and glomerular dysmorphic erythrocytes). The acanthocytes are particularly relevant. A fraction of > 10% of the microscopically counted erythrocytes is believed to be indicative of glomerulonephritis, with a diagnostic sensitivity and specificity of > 90%. Most of the dysmorphic erythrocytes shown in Fig. 12.8-2 can also be formed in vitro. This is not as often the case with acanthocytes. These cells have a ring-like form with protuberances. According to one study /33/ in 101 patients with histologically confirmed glomerulonephritis, the fraction of dysmorphic erythrocytes was 68 ± 24%; of these, 16.7 ± 16.3% were acanthocytes. The highest proportion of dysmorphic cells was observed in membranous and mesangio-proliferative glomerulonephritis, the lowest in minimal change nephropathy.

The determination of dysmorphic erythrocytes is of limited value as far as urologists are concerned. Since patients with post glomerular hematuria are more seen by such specialists the diagnostic specificity of dysmorphic erythrocytes is not high enough for glomerular hematuria, too many false positive cases of glomerular hematuria are diagnosed /34/.

12.8.5.1.6 Free hemoglobin, myoglobin

Hemoglobinuria and myoglobinuria can be mistaken for hematuria. The causes are acute intravasal hemolysis or severe muscular damage (e.g., due to trauma, electric shock, seizure).

12.8.5.2 Leukocyturia

If urine is collected according to the conditions described in Section 53.2.4 – Urine collection, a leukocyte count > 10 × 10⁶ per liter (> 10/μl) in males and > 20 × 10⁶ per liter (> 20/μl) in females is to be considered as pathological. Leukocyturia and bacteriuria are characteristic of a urogenital tract infection. The presence of granulocyte casts speaks for an involvement of the kidneys in the pathological process.

The clinical significance of isolated leukocyturia incidentally diagnosed in young men is unclear, while in women it often indicates contamination. A study of 1000 men with asymptomatic leukocyturia showed that all remained free of urogenital tract complaints after an average of 7.6 years /19/. Sterile isolated leukocyturia is seldom a sign of urogenital tuberculosis.

In combination with tubular proteinuria, sterile isolated leukocyturia is suggestive of analgesic nephropathy in adults, and of Kawasaki syndrome in children.

Granulocyturia

The most commonly detected leukocytes in urine are polymorphonuclear granulocytes. In most cases they are associated with an infection of the urogenital tract, but are also found in interstitial nephritis, glomerulonephritis, and aseptic cystitis.

Lymphocyturia

Lymphocyturia is a sign of a chronic inflammatory process in the urogenital tract and can be associated with viral infections and renal graft rejection.

Monocyturia

Appears in urinary tract infections.

Eosinophil granulocytes

They can be a sign of acute interstitial nephritis, but also occur in other renal diseases /4/.

12.8.5.3 Excretion of casts

Casts are formed in the distal tubules and collecting ducts. Cast formation requires favorable conditions in terms of osmolality, pH, electrolyte and protein concentrations. Under these conditions, the Tamm-Horsfall protein secreted by tubular epithelial cells can polymerize to fibrils. Fibrillar precipitates incorporate plasma proteins, lipids, various types of cells, microorganisms, hemoglobin, myoglobin, bilirubin, and crystals. The matrix for all casts is Tamm-Horsfall protein, which from the visual perspective has a very similar refractive index to urine. Hyaline casts, which consist almost entirely of Tamm-Horsfall protein, are therefore barely visible under a high power field microscopy, but are clearly visible using phase contrast microscopy.

Casts can be categorized as:

• Non-cellular casts (e.g., hyaline casts, granulated casts, waxy casts, and fatty casts)
• Cellular casts (e.g., epithelial, erythrocyte, leukocyte, and bacterial casts).

The detection of casts is a specific indicator for renal disorders, but is not very sensitive. While non-cellular casts are normally evident in the urine under circumstances such as strenuous exercise (e.g., long-distance running) this is not the case with cellular casts. The diagnostic significance of the individual cast types is shown in Tab. 12.8-3 – Diagnostic significance of urinary casts.

12.8.6 Comments and problems

Specimen collection

Morning mid-stream urine is recommended for the screening of asymptomatic patients. A first morning, mid-stream sample is recommended for hospital inpatients with nephrological and urological conditions, while a second morning sample is recommended for outpatients. The genitals must be washed prior to specimen collection.

Test strip for hematuria

Erythrocytes, free hemoglobin, myoglobin (≥ 0.5 mg/L), and erythrocyte casts will give a positive reaction.

False-positive results can be caused by oxidants such as cleaning agents and disinfectants that contain chlorine or iodine, as well as by microbial peroxidases.

False-negative results can arise due to reducing agents such as ascorbic acid, particularly when 1 g or more is taken daily. It should be noted that ascorbic acid is frequently used as a stabilizer for other drugs, such as tetracycline.

Test strip for leukocyte detection

The test strip detects leukocyte-bound esterase in the urine, dissolved leukocyte esterase, and the leukocyte esterase from leukocyte casts.

False-positive leukocyte reactions are caused by a high level of tubular esterase activity and by formaldehyde. When compared with quantitative cell counts and the examination of sediment under a cover slip, ostensibly false positive reactions are often seen after leukocytes have been lysed. Lysis can result from a pH > 6, low urine osmolality, and allowing the urine to stand for hours prior to analysis. In hypotonic urine, the leukocyte count will fall to about 25% of its original level within 4 hours.

False-negative leukocyte reactions are caused by a high urine protein concentration (> 5 g/L) which interferes with the reaction in the leukocyte test field. False-negative results have also been reported in patients taking doxycycline, gentamycin, cephalexin, and cephalothin, as well as due to excessive glucose excretion (> 20 g/L) and high concentrations of oxalic acid.

Quantitative cell count

At least 100 of a given type of cell must be counted in order for a reliable result to be obtained. If the cell count is low, the chamber should be filled several times.

Standardized urine sediment under a coverslip

A standardized test procedure is essential for obtaining accurate results. Well homogenized, resuspended sediment and homogeneous cell distribution in the specimen are important. The homogenization must be verified by an initial assessment using the 10 x objective. Should the aggregation of eythrocytes and leukocytes occur despite thorough homogenization, this indicates that the cells did not originate in the kidneys, and therefore points to the urinary tract or prostate /5/.

Detection of casts

Alkaline and hypotonic urine (< 260 mmol/kg) facilitate the rapid dissolution of casts and cells. This can largely be avoided if the patient is appropriately prepared (nothing to eat or drink for 8–10 hours before the first morning mid-stream urine is collected). Screening for casts should be performed using a phase contrast microscope at 100 x magnification, while cast identification should be performed at about 400 × magnification.

Stability

Testing for erythrocytes and leukocytes should be performed within 2 hours of the specimen being collected. Targeted testing for dysmorphic erythrocytes and casts should be performed immediately after the sample is collected, on urine that is no more than one hour old. Dysmorphic erythrocytes reportedly remain stable for at least 3 days in urine that has been preserved using thiomersal /9/.

References

1. Braun JS, Straube W. Die Diagnostik der Mikrohämaturie mit einem neuen Teststreifen. Ein Vergleich mit mikroskopischen Untersuchungsmethoden. Dtsch Med Wschr 1975; 100: 87–9.

2. Gambke B, Kouri T, Kutter D, Nagel D, et al. Multicenter evaluation of the urine analyser Miditron Junior. Scand J Clin Lab Invest 1997; 57: 605–11.

3. Kutter D, Figueiredo G, Klemmer L. Chemical detection of leukocytes in urine by means of a new multiple test strip. J Clin Chem Clin Biochem 1987; 25: 91–4.

4. European Urinalysis Guidelines: Summary. Scand J Clin Lab Invest 2000; 60: 1–9.

5. Thiel G. Urinuntersuchung in der Praxis. Schweiz Rdsch Med 1977; 66: 689–96.

6. Gadeholt H. Quantitative estimation of urinary sediment, with special regard to sources of error. Br Med J 1964; 1: 1547–9.

7. Hoffmann P, Hoffmann C, Ziebig R, Zimmermann M. Evaluation of the iChem velocity urine chemistry analyzer in a hospital routine laboratory. Clin Chem Lab Med 2011; 49: 509–13.

8. Birch DR, Fairly KF, Whitworth JA, et al. Urinary erythrocyte morphology in the diagnosis of glomerular haematuria. Clin Nephrol 1983; 20: 78–84.

9. Roth St, Renner E, Rathert P. Diagnostik der glomerulären Mikrohämaturie. Urologe B 1992; 32: 71–6.

10. Loh EH, Keng VW, Ward PB. Blood cells and red cell morphology in the urine of healthy children. Clin Nephrol 1990; 34: 185–7.

11. Fairley KF, Birch DF. Microscopic urinalysis in glomerulonephritis. Kidney Int 1993; 44 suppl 42: pg. 9 – pg. 12.

12. Thiel G, Bielmann D, Wegmann W, Brunner FP. Erythrozyten im Urin: Erkennung und Bedeutung. Schweiz Med Wschr 1986; 116: 790–7.

13. Chambliss AB, Mason HM, Van TT. Correlation of chemical urinalysis to microscopic urinanalysis and urine culture: implications for reflex urinalysis workflows. JALM 2020; July: 724–31.

14. Copley JB. Isolated asymptomatic hematuria in the adult. Amer J Med Sci 1986; 29: 102–11.

15. Cohen RA, Brown RS. Microscopic hematuria. N Engl J Med 2003; 348: 2330–8.

16. Diven SC, Travis LB. A practical primary care approach to hematuria in children. Pediatr Nephrol 2000; 14: 65–72

17. Dodge WF, West EF, Smith EH, Bruce H III. Proteinuria and hematuria in school children: epidemiology and early natural history. J Pediatr 1976; 88: 327–47.

18. Ritchie CD, Bevan EA, Collier SJ. Importance of occult hematuria found at screening. BMJ 1986; 292: 681–3.

19. Froom P, Gross M, Froom J, Caine Y, Margaliot S, Benbassat J. Factors associated with microhematuria in asymptomatic young men. Clin Chem 1986; 32: 2013–5.

20. Ahmed Z. Asymptomatic urinary abnormalities. Hematuria and proteinuria. Med Clin North Am 1997; 81: 641–52.

21. Bolenz C, Schröppel B, Eisenhardt A, Schmitz-Dräger DJ, Grimm MO. The investigation of hematuria. Dtsch Arztebl Int 2018; 115: 801–7.

22. Carter WC, Rous SN. Gross hematuria in 110 adult urologic hospital patients. Urology 1981; 18: 342–4.

23. Jeong CW, Lee S, Byun SS, Lee DH, Lee SE. No increase in risk of microscopic hematuria with aspirin use by asymptomatic healthy people. JAMA Intern Med 2013; 173: 1145–6.

24. Gillat DA, O’Reilly PH. Hematuria analyzed: a prospective study. J R Soc Med 1987; 80: 559–62.

25. Froom P, Ribak J, Benbassat J. Significance of macrohematuria in young adults. Br Med J 1984; 288: 20–22.

26. Lam MH. False “hematuria” due to bacteriuria. Arch Pathol Lab Med 1995; 119: 717–21.

27. Bryden AAG, Paul AB, Kyriakides C. Investigation of haematuria. Br J Hosp Med 1995; 54: 455–8.

28. Cohen RH,Brown RS. Microscopic hematuria. N Engl J Med 2003; 348: 2330–8.

29. Kincaid-Smith P, Fairley K. The investigation of hematuria. Semin Nephrol 2005; 25: 127–35.

30. Packham DK, Perkovic V, Savige J, Broome MRA. Hematuria in thin basement membrane nephropathy. Semin Nephrol 2005; 25: 146–8.

31. Stapleton FB. Hematuria associated wit hypercalciuria and hyperuricosuria: a practical approach. Pediatr Nephrol 1994; 8: 756–61.

32. Rüttimann S, Dreifuss M, Di Gallo A, Huser B, Dubach UC. Asymptomatische Hämaturie: Verlaufsbeobachtung bei 39 Patienten. Schweiz Med Wschr 1990; 120: 1461–5.

33. Fünfstück R, Klinzing S, Stein G, Schultz W, Risler T, Rösch R, et al Dysmorphe Erythrozyten im Urin bei unterschiedlichen Glomerulonephritisformen. Nieren- und Hochdruckkrankheiten 1995; 24: 554–8.

34. Offringa M, Benbassat J. The value of urinary red cell shape in the diagnosis of glomerular and post-glomerular haematuria. A metaanalysis. Postgrad Med J 1992; 68: 648–54.

35. Barocas D, Boorjian S, Alvarez RD, Downs T, Gross CP, Hamilton BD, Kobashi K, et al. Microhematuria: AUA/SUFU guideline. J Urol 2020; 204 (4): 778–86.

36. Ingelfinger JR. Hematuria in adults. N Engl J Med 2021; 385 (2): 153–63.

37. Oyaert M, Maghari S, Speeckaert M, Delanghe J. Improving clinical performance of urine sediment analysis by implementation of intelligent verification criteria. Clin Chem Lab Med 2022; 60 (11): 1772–9.

12.9 Urinary proteins

Lothar Thomas

12.9.1 Proteinuria

The distinction between primary kidney disease and systemic disease is based on the origin and locus of the disease process. In primary kidney disease the process arises and is confined to the kidney, whereas in systemic disease the kidney is compromised by a specific systemic process, for example diabetes mellitus and hypertension. In chronic kidney disease (CKD) proteinuria, especially albuminuria, is a marker of kidney disease /1/.

Albuminuria

Increased urinary albumin excretion is a clinical symptom indicating disturbance of the intrinsic permeability of the capillary wall. Albumin and most other plasma proteins are retained in the blood stream. The amount of plasma albumin that escapes with the glomerular filtrate depends on the selective permeability of the intricate structure of the podocytes in the glomerular filtration barrier. The slit membrane and podocytes of the glomerular filtration barrier can be damaged. The podocyte-expressed genes, when mutated and the cytoskeletal genes (ACTN4, IFN2) when mutated cause albuminuria. Multiple acquired diseases (diabetic nephropathy, minimal change disease, focal and segmental glomerulosclerosis,membranous nephropathy, hypertensive kidney disease, human immunodeficiency virus-associated nephropathy, and lupus nephritis) cause dysfunction of podocytes and albuminuria in addition /86/. Albuminuria and proteinuria express the severity of kidney disease not only because they are indicators of the severity of injury but also because albuminuria itself strongly associates with progression of kidney disease /43/.

CKD is classified based on cause, GFR category, and albuminuria category. The extent of kidney damage and glomerular permeability can be assessed using quantitative measurement of albumin or protein excretion. The pattern of the excreted plasma proteins allows differentiation into selective glomerular, selective tubular or non-selective proteinuria. Urinary albumin measurements may be useful in early risk profiling and prevention of cardiovascular disease in the population at large. A 24-hour urine collection is no longer necessary for the quantitative assessment of proteinuria, and test strip methods are insufficiently sensitive and specific for its detection /2, 3/.

Proteinuria is defined as a loss of:

• Albumin /2/; generally proteinuria reflects albuminuria. Albuminuria is the earliest marker of glomerular disease, including diabetic glomerulosclerosis, where it generally appears before the reduction in GFR. Albumin is the predominant protein in the majority of proteinuric kidney diseases.
• Total protein /3/; in pathological states, total urinary protein is made up of low levels of physiological proteins (e.g., uromodulin), albumin (the predominant protein in most disease states), non-albumin proteins (comprising a wide range of proteins, of varying molecular weights) and specific pathologic proteins (e.g., immunoglobulin light chains).

The 24-hour urine collection is the gold standard for the quantification of proteinuria, but is associated with considerable collection errors and is burdensome for the patients. Therefore, the spot measurement is recommended, which has a good overall correlation with the 24-hour total proteinuria. In the spot urine sample the total protein/creatinine ratio or the albumin/creatinine ratio should be measured. If the excretion of total protein and albumin in the 24-hour urine is compared with the total protein/creatinine ratio, and the albumin/creatinine ratio, significant proteinuria can be excluded with acceptable degree of reliability. Further proteinurias with important diagnostic thresholds (> 0,5 g total protein/day or > 1 g/day) are correctly identified /4, 5/. Proteinuria > 1 g/day is frequently an indication for renal biopsy.

12.9.1.1 Diagnostic approach

The KDIGO /1/ recommends using the following measurements for initial testing of proteinuria (in descending order of preference; in all cases an early morning urine sample is preferred):

• Albumin/creatinine ratio (ACR)
• Total protein/creatinine ratio (PCR)
• Reagent strip analysis for total protein with automated reading
• Reagent strip analysis for total protein with manual reading.

Further recommendations are:

• Confirm reagent strip positive albuminuria and proteinuria by quantitative laboratory measurement and express as a ratio to creatinine whenever possible
• Confirm ACR of ≥ 30 mg/g (≥ 3 mg/mmol) on a random untimed urine with a subsequent early morning urine sample
• If a more accurate estimate of albuminuria or total proteinuria is required, measure albumin excretion rate or total protein excretion rate in a timed urine sample
• If significant non-albumin proteinuria is suspected, use assays for specific urine proteins (e.g., α₁-microglobulin, monoclonal heavy or light chains). Specific marker proteins may help to localize the damage, or the protein pattern in urine may be analyzed with sodium dodecylsulfate (SDS) polyacrylamide gel electrophoresis (SDS-PAGE).

In order to decide how proteinuria should be detected and measured, the following must be kept in mind /6/:

• Albumin represents only a small fraction of the total protein excreted physiologically. As proteinuria develops, the albumin fraction increases more and more; roughly calculated, twice the albumin value is equivalent to the total protein concentration, if that concentration is ≤ 1.0 g/L
• For the diagnosis of renal disease, in primary and secondary prevention of cardiovascular events and in hypertensive patients, the ACR and not the PCR is the diagnostic criterion
• Mild tubulo-interstitial proteinuria and the excretion of free light chains are only diagnosed with the total protein determination.

12.9.1.2 Classification of proteinuria

Pre renal proteinuria

Increased amounts of systemically produced low-molecular weight proteins such as free immunoglobulin light chains, hemoglobin, or myoglobin enter the primary urine as a result of systemic disease processes which do not initially cause any renal damage. If the tubular reabsorption capacity is exceeded, the proteins are excreted in the final urine, resulting in so-called overflow proteinuria.

Renal proteinuria

Renal proteinurias can be glomerular, tubular, or mixed (glomerular/tubular). Glomerular proteinurias are divided into selective and nonselective forms. In the former, medium-sized proteins (41–100 kDa) are excreted while in the latter, high-molecular weight proteins are also excreted (41–400 kDa). The mixed glomerular/tubular proteinurias are caused by:

• A combination of glomerular and tubular damage
• Glomerular damage only and, in consequence, overload of the tubular reabsorption capacity for low molecular weight proteins.

Post renal proteinuria

In this form of proteinuria, proteins of all sizes are evident, but there is a significant proportion of very large proteins with a molecular weight over 400 kDa that are unable to pass through even a severely damaged glomerular basement membrane. These proteins enter the urine via bleeding or exudates in the lower urinary tract after passage through the tubules.

12.9.1.3 Differentiation of proteinuria

The composition of the excreted proteins depends on whether the proteinuria is renal, pre renal, or post renal, and depending on the type and location of any renal damage. Therefore, based on the pattern of urine protein excretion, it is possible within certain limits to deduce the type and location of renal proteinuria, or to judge whether a disorder is pre renal or post renal. The amount of characteristic marker proteins that is excreted correlates with the activity of the disease; the exception to this is minimal change nephropathy.

Certain marker proteins are characteristic of the following localizations of kidney damage:

• Isolated proteinuria of medium molecular weight proteins (albumin, transferrin) filtered by the glomeruli is indicative of glomerular injury
• Isolated proteinuria of the low molecular weight proteins (α₁-microglobulin and β₂-microglobulin), which are normally reabsorbed by the tubuli, points to tubulo-interstitial disease
• The presence of high molecular weight proteins such as IgG is diagnostically relevant in renal diseases with severe glomerular filtration impairment
• The presence of α₂-macroglobulin, which is admixed from the urinary tract into the urine, is suggestive of post renal proteinuria
• Elevated excretion of neutrophil gelatinase-associated lipocalin (NGAL) is indicative of acute kidney damage.

The excreted quantity of typical marker proteins correlates with the disease activity.

12.9.2 Indication

Test strips

• Screening of asymptomatic patients for proteinuria
• Patient self-monitoring for proteinuria.

Albumin

Diagnosis and management of chronic kidney disease (e.g., in patients with):

• Diabetes mellitus types 1 and 2
• Metabolic syndrome
• Hypertension, edema, obesity
• Cardiovascular disease.

Total protein

• Quantification and monitoring of known proteinuria
• Classification of nephropathy by the type and severity of disease
• In comparison to test strip analysis: detection of Bence-Jones proteinuria
• Plausibility control regarding the individual immunochemical protein determinations: ruling out erroneous measurements due to antigen excess or due to alterations in immunoreactivity.

Determination of the protein excretion pattern

Information for location of renal pathologic-anatomic abnormalities (e.g., detection and differentiation between pre renal, renal (glomerular, tubular and mixed forms) and post renal proteinuria). The examination is performed either with SDS-polyacrylamide gel electrophoresis or a pattern of the marker proteins is determined:

• Low molecular weight proteins (α₁-microglobulin, β₂-microglobulin, free light chains)
• Medium molecular weight proteins (albumin, transferrin)
• Proteins of high molecular weight (IgG, α₂-macroglobulin).

Individual proteins with regard to specific questions

• NGAL in suspicion of acute renal failure
• Free light chains in suspicion and monitoring of multiple myeloma
• Myoglobin in crush kidney (see also Tab. 12.1-5 – Diagnostically significant laboratory tests in acute renal failure).

12.9.3 Method of determination

12.9.3.1 Test strips for the semi quantitative determination of total protein

Principle: the protein error of certain pH indicators is used for a colorimetric detection of proteins. The acid-buffered reaction zone on the test strip contains the indicator tetra bromphenol blue, which can release hydrogen ions. If the reagent strip is immersed in protein-free solutions, no hydrogen ions are released and the zone remains yellow. In protein-containing solutions, the indicator donates hydrogen ions to the proteins, and the color changes to greenish-blue. Of the diagnostically relevant urine proteins, only albumin and transferrin are good hydrogen ion acceptors. The test strips are therefore primarily sensitive to these proteins. Test strips have a diagnostic sensitivity of 90–95% for albuminuria of > 200 mg/L. This is lower for immunoglobulins, mucoproteins and low molecular weight proteins, and is very low for free light chains.

12.9.3.2 Test strips for the estimation of the albumin/creatinine ration (ACR)

The following tests are commercially available /7/:

Test 1: the test measures the albumin and creatinine in the sample. In the albumin test field, the albumin binds to a sulfonaphthyl dye and causes a color reaction. In a separate test field, the creatinine forms a copper-creatinine complex. This complex has peroxidase activity and catalyzes a reaction between diisopropylbenzene dihydroperoxide and 3,3’, 5,5’-tetramethylbenzidine. The reactions in both test fields result in colors that can be measured using a reflectometer. The test gives the following results:

• Albumin in concentrations of 10, 30, 80 and 150 mg/L
• Creatinine in concentrations of 10, 50, 100 and 200 mg/dL (0.9; 4.4; 8.8; 17.7 and 26.5 mmol/L)
• The albumin/creatinine ratio (< 30, 30–300, and > 300 mg/g).

Test 2: this test only measures albumin. Albumin in the sample passes over a guide pad onto a conjugate pad. On the conjugate pad, the albumin binds with specific gold-labeled antibodies then flows into the detection field. Here, a chemical reaction takes place and the color is produced. The color intensity is compared with standard color blocks representing albumin concentrations of 0, 20, 50 and 100 mg/L.

12.9.3.3 Quantitative determination of total protein

The Biuret assay after protein precipitation and the turbidimetric measurement following precipitation of the proteins with benzethonium chloride or trichloroacetic acid are the most frequently used assays. All of the tests are more or less equally effective at detecting albumin, IgG, and hemoglobin. However, the turbidimetric methods are less sensitive when used to detect glycoproteins such as α₁-microglobulin and Tamm-Horsfall protein than is the biuret assay.

12.9.3.4 Selective determination of urinary proteins

The determination of albumin, α₁-microglobulin, transferrin, β₂-microglobulin, IgG and α₂-macroglobulin is performed with immunonephelometric and immunoturbidimetric methodology (see Chapter 52 – Selected analytical laboratory techniques).

Quantitative determination of albumin, transferrin, α₁-microglobulin, and IgG

Immunonephelometric and immunoturbidimetric assays are used. The CRM 470 reference material is used as reference to calibrate the assays.

Principle of the immunonephelometric assay: the protein in the sample binds to a specific antibody, triggering an antigen-antibody complex. The increase in light scattering is measured with a nephelometer and converted into a concentration.

Principle of the immunoturbidimetric assay: the protein in the sample and the same protein bound to latex particles compete for a specific monoclonal antibody, which aggregates the latex particles. The resultant number of aggregates is measured spectrophotometrically and is inversely proportional to the concentration of the protein in the sample.

12.9.3.5 Determination of the protein excretion pattern

The sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis (SDS-PAGE) is used /8/.

Principle: the urine is treated with SDS. The binding of the SDS to the proteins in the urine generates mixed micelles with a molecular radius that is proportional to the logarithm of the molecular mass, and the proteins acquire an excess negative charge.

Electrophoretic separation takes place in polyacrylamide gradient gels in an SDS containing alkaline buffer medium whereby the protein zones are compressed on the front of a discontinuous system with running and following buffers. The proteins can therefore migrate as sharp bands toward the anode and are slowed and separated according to molecular weight by the molecular sieving effect of the polyacrylamide gel. The distance migrated is inversely proportional to the logarithm of the molecular weight. The polyacrylamide gradient is selected to ensure that albumin will migrate about halfway across the gel plate.

The separated proteins will then be located as follows:

• On the anodic side of albumin will be the low-molecular weight (tubular) urine proteins
• On the cathodic side will be the high-molecular weight (glomerular) proteins.

The proteins are fixed and stained with Coomassie Blue, and the relative ratios between albumin and the high- and low-molecular weight proteins are then determined.

There are characteristic protein patterns that are indicative of location of pathologic-anatomic abnormalities in the kidney. In addition the test indicates pre renal and post renal proteins such as free light chains and hemoglobin monomers. All diagnostically significant urine proteins are detected qualitatively using SDS-PAGE and are classified according to molecular weight.

12.9.4 Specimen

First spontaneous voided morning urine. Determined are the albumin/creatinine ratio (ACR) or the total protein/creatinine ratio (PCR).

The relation to creatinine is a reference to the elimination of the volume rate of the urine excretion. Conversion: 1 g of creatinine represents 8.85 mmol.

The initial determination of the ACR in a urine sample collected during the course of the day (random urine), and, in the case of a pathological finding, additionally in a first morning urine sample, is proposed /9/.

A 24-hour urine collection (no additive), entire quantity to be brought to the laboratory. If this is not possible, remove 10 mL following repeated swiveling of the collection container and send it to the laboratory.

12.9.5 Reference interval

Total protein (adults)

Test strips /6/: Negative reaction at:

• < 300 mg/L
• < 500 mg/24 h (urine volume ~ 1.5 L)
• < 50 mg/mmol creatinine (443 mg/g creatinine).

Biuret method /10/:

• < 100 mg/L
• < 150 mg/24 h (urine volume ~ 1.5 L)
• < 25 mg/mmol creatinine (220 mg/g creatinine).

Turbidimetric test /11/:

• < 50 mg/L
• < 75 mg/24 (urine volume ~ 1.5 L)
• < 12.5 mg/mmol creatinine (110 mg/g creatinine).

Method-independent /12/:

• Below 23 mg/mmol creatinine (200 mg/g creatinine)
• Pregnant women: below 300 mg/L /13/.

Total protein excretion (children) /14, 15, 16/

Age	mg/24 h	mg/m2/24 h
Premature infants	14–60	88–377
Full-term neonates	15–68	68–309
2–12 months	17–85	48–244
13 months – 3 yrs	20–121	37–233
4–9 yrs	26–194	31–234
10–16 yrs	29–238	22–181

Values are the 2.5th and 97.5th percentiles.

Total protein in children (random urine)

• 6 months to 2 years /17/: Below 56 mg/mmol creatinine (500 mg/g creatinine)
• Over 2 years: ≤ 23 mg/mmol creatinine (200 mg/g creatinine) after rest and ≤ 28 mg/g creatinine (250 mg/g creatinine) after orthostasis /18/.

Albumin

Test strips (semi-quantitative):

• Test strips for total protein: Negative reaction with albumin excretion of ≤ 200 mg/L /19/
• Albumin-specific test strips: Negative reaction with albumin concentrations < 10 mg/L /7/.

Quantitative according to the Kidney Disease Improving Global Outcomes (KDIGO) classification of chronic kidney disease /1/:

• ACR ≤ 10 mg albumin/g creatinine (1.0 mg albumin/mmol creatinine); young adults
• Normal to mildly elevated < 30 mg albumin/24 h.

The reference values of the American Diabetes Association and the National Institute for Health and Clinical Excellence (NICE) are shown in Tab. 12.9-1 – Definition of albuminuria according to the American Diabetes Association and NICE /20, 21/.

Transferrin /22/: 0.2–1.2 mg/L

Immunoglobulin G (IgG)

• First morning urine: ≤ 0.7 mg IgG/mmol creatinine (6 mg IgG/g creatinine) /23/
• Second morning specimen (random urine): ≤ 1.0 mg IgG /mmol creatinine (9 mg IgG/g creatinine) /24/.

Values are the 95th percentiles.

α₁-microglobulin (α₁-M)

• First morning specimen ≤ 1.75 mg α₁-M/mmol creatinine (14 mg α₁-M/g creatinine) /24/
• Second morning specimen (random urine): ≤ 2.0 mg α₁-M/mmol creatinine (17 mg α₁-M/g creatinine) /25/.

Values are the 95th percentiles.

β₂-microglobulin (β₂-M) /25/

• ≤ 0.2 mg/L
• ≤ 0.023 mg β₂-M/mmol creatinine (0.2 mg β₂-M/g creatinine)
• Clearance: 0.03–0.12 mL/min.

α₂-macroglobulin (α₂-M) /26/

≤ 0.79 mg α₂-M/mmol creatinine (7.0 mg α₂-M/g creatinine)

Neutrophil gelatinase-associated lipocalin (NGAL)

Adults ≤ 150 μg/L, children ≤ 135 μg/L /27/

12.9.6 Clinical significance

Total protein excretion in healthy individuals is ≤ 150 mg/24 h. The total protein is composed of filtered plasma proteins (60%) and non-albumin proteins (40%) mainly tubular Tamm-Horsfall protein. Albumin is the major filtered plasma protein, making up 20% of the total protein excreted; this is, as a rule, up to 20–30 mg/24 h or 14–21 μg/min. in healthy individuals. Glomerular proteinuria is the most common form and involved with a prevalence of 90% in elevated protein excretion.

12.9.6.1 Definition of albuminuria

• The National Institute for Health and Clinical Excellence (NICE) CKD Guideline /21/ defines albuminuria as ≥ 30 mg/mmol creatinine (266 mg/g creatinine), corresponding to an excretion of ≥ 300 mg/24 h
• According to the KDIGO classification of chronic kidney disease /1/, a value ≥ 3.4 mg/mmol creatinine (30 mg/g creatinine) is a marker of kidney damage
• According to the American Diabetes Association /20/ and the National Institute for Health and Clinical Excellence CKD Guidelines /21/, albuminuria is classified as per the criteria listed in Tab. 12.9-1 – Definition of albuminuria according to the American Diabetes Association and NICE.

12.9.6.2 Definition of proteinuria

The definition of proteinuria varies:

• Depending upon the laboratory, pathological proteinuria starts with the excretion of 150–300 mg/24 h
• According to the UK CKD guidelines /28/, proteinuria is defined as a protein/creatinine ratio ≥ 45 mg/mmol creatinine (400 mg/g creatinine), but if hematuria is not present this should not be a reason to take more extensive measures, unless the value of 100 mg/mmol creatinine (885 mg/g creatinine) is exceeded
• The National Institute for Health and Clinical Excellence CKD Guideline /21/ defines proteinuria ≥ 50 mg/mmol creatinine (443 mg/g creatinine)
• According to the National Kidney Foundation /12/ proteinuria is defined as a protein/creatinine ratio ≥ 23 mg/mmol creatinine (200 mg/g creatinine).

In preeclampsia, proteinuria is ≥ 300 mg/24 h /13/.

In adults, proteinuria ≥ 3 g/24 h is defined to be nephrotic. In children, the following evaluations apply regarding 12-hour or 24-hour urine:

• < 4 mg/m²/h = normal
• 4–40 mg/m²/h = abnormal
• > 40 mg/m²/h = nephrotic.

12.9.6.3 Epidemiology of proteinuria

Proteinuria is a frequently occurring symptom, particularly in adolescents. When Japanese schoolchildren aged 6–15 years were screened using test strips for total protein and albumin, 4.3% showed elevated levels of total protein excretion, while increased albumin excretion was evident in 2.1% /17/.

However, most proteinurias are functional in nature and are only transient or intermittent (Tab. 12.9-2 – Functional, transient and isolated proteinuria). Studies involving test strip assays performed on children in the U.S. showed that up to 10% of the children tested had proteinuria when tested once. The prevalence fell to about 2.5% for cases where two of four urine samples collected at set time intervals were positive, while only 1 in 1000 children exhibited proteinuria in all four samples /29/. In only 10% of the children with proteinuria did the condition persist for 6–12 months /30/. The prevalence of proteinuria is age-dependent, gradually increasing with age, and reaching a maximum in adolescence.

12.9.6.4 Significance and treatment of proteinuria

If proteinuria is present, examinations to answer the following questions are necessary /31/:

• Is the proteinuria evident consistently?
• How much albumin or total protein is excreted?
• Is the protein pattern glomerular, tubular, mixed, pre renal or post renal?
• Can the cause of the renal injury be recognized or localized?
• If glomerular damage is present, is it progressive?

12.9.6.5 Etiology of proteinuria

From the pathophysiological point of view, proteinuria is an important factor in the genesis of kidney disease. The type and severity of the existing kidney disease is often unrelated to the clinical symptoms. Commonly, patients with proteinuria are also asymptomatic. A classification and the etiology of proteinuria is shown in Tab. 12.9-3 – Etiology of proteinuria.

In patients with total protein of < 1 g/24 h, without pathological laboratory findings or clinical symptoms (isolated proteinuria), separate daytime and night time urine collections should be performed to rule out orthostatic proteinuria.

Nephrotic proteinuria [albumin > 2,200 mg/24 h, ACR > 2,200 mg/g creatinine (> 220 mg/mmol creatinine)] or [total protein > 3,500 mg/24 h, PCR > 3,500 mg/g creatinine (> 350 mg/mmol creatinine)] are caused by glomerulonephritis and indicate a high progression rate in chronic kidney disease /3/.

12.9.6.6 Albuminuria

The National Kidney Foundation of the USA recommends the indications mentioned in Tab. 12.9-4 – Recommendations of the National Kidney Foundation for albuminuria as a clinical marker of kidney damage for the determination of urinary albumin /32/.

In kidney disease there is a positive relationship between decreasing GFR, increasing proteinuria, and the consequences. Risks which must be prognostically estimated are: progression of a chronic kidney disease (CKD), end stage renal failure, acute kidney injury, cardiovascular mortality, and overall mortality. Because of increasing evidence demonstrating the importance of proteinuria as a predictor of adverse outcomes albumin excretion, measured as ACR, was integrated by KDIGO into the staging of CKD in addition to the GFR /1/. Three albumin (A) categories were created (Tab. 12.9-5 – Categories of albumin excretion in chronic kidney disease).

The relationship of albumin excretion (abscissa) to the GFR (ordinate) is shown in a diagram (Fig. 12.9-1 – Assessment of the course of the GFR and of albumin excretion in type 1 diabetes mellitus). The risk increases progressively in both directions, downwards the GFR categories and from left to right the albumin categories.

For patients with reduced GFR and albuminuria, the prognosis regarding cardiovascular events and overall mortality is more worse than for those with GFR reduction without albuminuria. The GFR and albuminuria are independent risk factors for cardiovascular disease and mortality /33/. Cardiovascular disease is 10–20-fold more frequent in patients with end stage renal failure than in age-matched control groups. Albuminuria usually reflects injury to the vascular endothelium and associated diseases.

It is generally accepted that incipient glomerular disease is recognized years before GFR impairment by detection with albumin tests with a sensitivity of ≤ 5 mg/L. Tab. 12.9-6 – Albuminuria as a marker of nephropathy and systemic diseases shows albuminuria-associated diseases.

Albumin excretion is an important criterion for the . The heritability of albuminuria ranges from 0.17 to 0.20 in the general population, 0.20 in families with diabetes, and 0.12 to 0.49 in families with hypertension, depending on ethnicity. Genom wide scans for albuminuria have mostly yielded regions with suggestive evidence for linkage. Evidence of linkage for urine albumin/creatinine ratio to 20q12 was isolated. It has been suggested that the genes controlling urinary albumin excretion are the same in relatives both with and without diabetes.

Refer to the following figure and section:

• Fig. 12.9-2 – Relative risk of overall mortality, cardiovascular mortality, progression and end stage chronic renal disease, as a function of the albumin/creatinine ratio and the glomerular filtration rate
• Section 3.1.11.3 – Diabetic nephropathy

Albuminuria, defined as urine albumin/creatinine ratio (ACR) ≥ 30 mg/g creatinine is a diagnostic component of CKD. In a study /9/, overall, 43.5% of the adults had increased ACR (≥ 30 mg/g creatinine) in a first morning urine. This proportion was higher among individuals ≥ 50 years old (48.9%), males (53.3%), participants with diagnosed diabetes (56.3%), and hypertension (51.5%), and eGFR < 60 [mL × min^–1 × (1.72 m²)^–1] (56,9%).

12.9.6.7 Proteinuria and secondary prevention of renal disorders

In CKD, the severity of total protein excretion is related to the degree of renal function loss, and there is also a connection between proteinuria and the progression of nephropathy. In secondary prevention, the reduction of proteinuria is used as a measure of the effectiveness of nephroprotective therapy. The following nephroprotective therapies have been indicated:

• In patients with nondiabetic nephropathy and proteinuria ≥ 3 g/24 h who are under treatment with drugs that inhibit the renin-angiotensin system, proteinuria can be reduced and the rate of renal failure slowed down (Renal Efficiency in Nephrology Study /34/).
• The use of ACE inhibitors or angiotensin II type 1 receptor antagonists to block the renin-angiotensin system leads to a reduction of proteinuria in chronic nephropathies. However, this therapy will only benefit those patients in whom the proteinuria declines by a significant amount (30–60% /35/) within the first 8–12 weeks after the treatment was initiated. The antiproteinuric and nephroprotective effects of the ACE inhibitors stem not only from the fact that these drugs reduce blood pressure, but also from their reduction of the proteinuria itself, which in turn also exerts a nephroprotective effect /35/.
• The relationship between proteinuria and the progression of nephropathy also applies to patients with diabetes mellitus. Baseline protein excretion is also a strong predictor of renal failure in patients with type 2 diabetes mellitus. Thus, it was shown in a study /36/ that a two-fold increase in proteinuria also doubles the risk of renal failure. The risk of renal failure was reduced by 50% by administering the appropriate therapy; thus halving the level of proteinuria within 12 months. Treatment with the angiotensin receptor blocker irbesartan allowed proteinuria to be reduced by 41% within one year and had a nephroprotective effect of 36%.
• In patients with focal segmental glomerular sclerosis (FSGS), specifically with primary FSGS, which most frequently results in end-stage renal failure. These patients had a proteinuria of 4.7 (0.2–98.3) g/24 h /37/. Under immunosuppresive therapy, the reduction in proteinuria was a valuable surrogate marker both for the prediction of future renal failure and for the rate of relapse from complete to partial remission in FSGS patients. Therapy was at its most effective when the proteinuria had declined by over 50% to a low-point of less than 3.5 g/24 h /37/.

In addition to the level of proteinuria, the underlying renal disorder and the proteinuria pattern are also important for secondary prevention. For example, patients with minimal change glomuleronephritis have severe proteinuria without any resultant chronic deterioration of renal function. This is because not every protein molecule causes the same degree of damage to the tubule cells. In interstitial nephritis, the selectivity index is a predictive factor.

12.9.6.8 Marker proteins

Marker proteins represent a characteristic size category for each form of proteinuria, and are present in the urine under normal conditions in concentrations that are easy to measure using immunonephelometric or immunoturbidimetric methods. Identifying marker protein levels makes it possible to classify proteinuria according to the various types. This testing also allows conclusions to be drawn regarding the location of any renal damage, but not regarding etiology.

Marker proteins can be determined qualitatively by sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE) or quantitatively with immunoassays. In SDS-PAGE assays, the marker proteins are identified by determining their relative mobility compared with a standard. This results in characteristic protein patterns that make it possible to classify the proteinuria according to the various possible types, and to draw conclusions regarding the location of the renal damage (Tab. 12.9-7 – Proteinuria types of SDS-PAGE).

Based upon the typical molecular weight category of the respective protein form, the following are differentiated:

• Non-selective versus selective proteinuria, by the quantitative determination of IgG and albumin or transferrin, or the implementation of SDS-PAGE. Increased excretion of albumin, transferrin and IgG is a sign of glomerular damage. The increase in the excretion of these proteins is consistent with the hypothesis that glomerular selectivity decreases progressively with the increase in albuminuria.
• Tubular proteinuria through the quantitative determination of α₁-microglobulin, β₂-microglobulin or the implementation of SDS-PAGE
• Pre renal proteinuria by determining the free light chains κ and λ, myoglobin and hemoglobin, or the implementation of SDS-PAGE
• Post renal proteinuria through the determination of α₂-macroglobulin and albumin or the implementation of SDS-PAGE. Renal and post renal forms of hematuria can be differentiated based upon the concentration relationships of both of these proteins.

12.9.6.8.1 Concentration ratios of the urinary proteins

The concentration ratios of the urinary proteins provide an indication as to the mechanism of the proteinuria (Tab. 12.9-8 – Concentration ratios of urinary proteins and their predictive value). It must, however, be noted that common pathological mechanisms can underlie chronic nephropathies with highly diverse etiologies and can cause their progression, irrespective of the original etiology. Thus many etiologies lead to the same pathogenesis. This often has the following course: systemic hypertension, augmented glomerular filtration pressure, increased protein ultrafiltration, glomerular and tubular protein overload, chronic inflammation and, finally, structural kidney damage. In many of these cases the proteinuria pattern is similar and provides no information regarding the etiology of the disease /38/.

12.9.6.8.2 Selectivity index

The concept of proteinuria selectivity is useful for predicting the success of steroid therapy for nephrotic syndrome. The selectivity index (SI) is determined:

A proteinuria is considered highly selective with an SI < 0.10, moderately selective for an SI > 0.11 to ≤ 0.20 and nonselective for an SI > 0.21 /39/.

12.9.6.8.3 α₁-microglobulinuria

α₁-microglobulin (α₁-M) is produced in the liver and by lymphocytes, and has a molecular weight of 30–33 kDa. It is a glycoprotein that is present in the free form in serum to the extent of 50%, up to 40% is bound to IgA, and up to under 10% is bound to albumin. The serum concentration is 10–20 mg/L. Only the free form is filtered and 99.8% is reabsorbed in the proximal tubule. Under normal glomerular filtration conditions and with normal serum creatinine levels, urine concentrations > 10 mg/L or an elevated α₁-M/creatinine ratio indicate proximal tubular dysfunction. In the case of glomerular disorders and elevated serum creatinine, this is not necessarily the case. This is because under these conditions, the serum level of α₁-M increases as a result of a reduced GFR. The load on the remaining intact nephrones from α₁-M increases, resulting in an elevated α₁-M concentration in the ultra filtrate. This results in a saturation of α₁-M reabsorption capacity of proximal tubular renal cells, leading to an increase in α₁-M concentration in the end urine without there being any tubular dysfunction . It is therefore important to distinguish these overflow proteinuria from that due to tubular dysfunction. This can be accomplished using the urine α₁-M/albumin ratio (which is measured in mg/mg). A value < 0.1 is indicative of glomerular proteinuria, while a value ≥ 0.1 indicates a mixed proteinuria /40/.Mixed proteinuria is the result of a combination of glomerular and tubular damage. The diseases and factors that can cause tubular proteinuria are listed in Tab. 12.9-9 – Glomerular and tubulo-interstitial proteinuria.

The various forms of proteinuria can be distinguished from one another by determining the excretion of α₁-M in relation to the excretion of albumin using a diagnostic diagram /41/ (Fig. 12.9-3 – Differentiation of proteinuria forms). This makes it possible to distinguish between primary and secondary glomerulopathy, as well as tubulo-interstitial nephropathy.

12.9.6.8.4 IgG excretion

The glomerular filtration system has a charge-sensitive permeability which is further enhanced by pores used for mechanical size selectivity. In this way, the passage of proteins through the glomerular barrier depends on charge and pore size.

A reduced anionic charge in the filtration system results in an increased excretion of negatively-charged proteins such as albumin without macromolecular proteins such as IgG appearing in the urine, since their molecular weight prevents them from being filtered out. A prime example of impaired charge selectivity is minimal change glomerulopathy. If the pore size in the filtration system is pathologically enlarged, larger molecules can also enter the urine.

By determining the urine albumin and IgG levels, selective (charge-dependent) proteinuria, characterized by a high proportion of albumin, can be distinguished from nonselective proteinuria, which is characterized by pathological changes in charge and pore size, as well as by a high proportion of IgG. The IgG/albumin ratio (in mg/mg) can be used as a measure of selectivity. A ratio ≤ 0.03 is indicative of selective proteinuria, while a ratio > 0.03 indicates a nonselective proteinuria /40/.

In glomerular disorders, not only increased amounts of albumin and IgG are excreted, but also low-molecular weight proteins such as α₁-M, resulting in the development of a mixed proteinuria pattern. However, neither an α₁-M/albumin ratio > 1 nor an α₁-M excretion rate > 100 mg/g of creatinine are ever observed in cases of glomerular proteinuria /42/. Diseases associated with glomerular proteinuria are listed in Tab. 12.9-9 – Glomerular and tubulo-interstitial proteinuria.

12.9.6.8.5 Neutrophil gelatinase-associated lipocalin (NGAL)

Human NGAL, also known as lipocalin 2, is expressed in a variety of tissues. NGAL consists of a single disulfide-bridged polypeptide with molecular weight of 25 kDa. The majority of the NGAL molecules exist as monomers, dimers and trimers, as well as in a complex with neutrophil gelatinase. The monomeric NGAL form is secreted by injured kidney tubule epithelial cells, whereas the dimeric form is the predominant form secreted by neutrophils. Siderophores are the major ligands for NGAL. Iron containing NGAL binds to surface receptors such as megalin, is internalized and releases its iron to the labile iron pool of the cell. Plasma NGAL originates from injured kidneys and from extrarenal organs. The urine NGAL is derived predominantly from epithelial cells of the distal nephron, although a fraction may come from the systemic pool escaping reabsorption due proximal tubular damage /43/.

NGAL is a biomarker for acute renal failure (ARF) and is elevated in serum and random urine samples within 2–6 hours of the onset of ARF. Following cardiac surgery and renal transplantation, and in the critically ill, values > 150 μg/L are to be considered as high risk predictors of ARF (area under curve 0.88). The level of NGAL correlates with the severity of the ARF. Values of ≤ 100 μg/L rule out ARF with high accuracy. In the range of 100–150 μg/L, the risk of ARF is moderate, patients with ARF risk factors (older individuals, chronic kidney disease, cardiovascular disease) and with values in this range should be considered to be at high risk for ARF.

In a systematic review /44/ the utility of NGAL to predict the occurence of AKI in septic patients was investigated. For plasma NGAL the pooled sensitivity and specificity were 83% and 57%, respectively. The pooled positive and negative likelihood ratios were 3.10 and 0.24, respectively. The pooled diagnostic odds ratio was 14.72 (95% CI: 6.55–33.10).

12.9.6.8.6 Marker proteins in pre renal proteinuria

Pre renal proteinurias involve increased amounts of systemic, low-molecular weight proteins entering the primary urine. These proteins come from the systemic pool escaping due to exceeding the tubular reabsorption capacity or due to proximal tubular damage, and the proteins are excreted in the final urine. Clinically important pre renal proteinurias are free light chain excretion (associated with multiple myeloma), myoglobinuria (associated with rhabdomyolysis), or hemoglobinuria (associated with a transfusion-related condition). Hemoglobin and myoglobin are detected using a test strip, while light chain excretion is identified by quantitative immunochemistry assays for free κ light chains and free λ light chains in the urine (refer to Section 22.3.4 – Immunosubtraction and capillary zone electrophoresis (CZE)).

12.9.6.8.7 Marker proteins in renal proteinuria

Renal proteinuria occurs as glomerular, mixed glomerular-tubular, and tubular forms. The determination of the marker proteins albumin, IgG and α₁-microglobulin or the SDS-PAGE permit the differentiation of the three types of proteinuria. Glomerular proteinuria is subdivided into selective (disturbances of anion filtration) and non-selective (structural damage to the molecular sieve) forms (Tab. 12.9-10 – Clinical assessment of urine protein marker analysis).

12.9.6.8.8 Marker proteins in post renal proteinuria

It is possible to distinguish between renal and post renal hematuria by testing for urine marker proteins. In the case of post renal (urological) bleeding, the protein pattern in the urine is comparable to that in serum, while in the case of renal (glomerular) hematuria, a typical glomerular protein pattern (over 80% albumin) is evident. The marker proteins for post renal proteinuria is albumin and α₂-macroglobulin (α₂-M). α₂-M is a high-molecular weight protein that is not filtered through the glomeruli, even if the glomerular filtration system is injured. Renal hematuria can be distinguished from post renal hematuria by evaluating the excretion rate of α₂-M in relation to the excretion of albumin by means of the urine α₂-M/albumin ratio (measured in mg/mg). A ratio < 0.02 is indicative of renal hematuria, while a ratio ≥ 0.02 indicates post renal hematuria. However, it is not possible to differentiate between renal and post renal hematuria if the urine albumin concentration is less than 100 mg/L /42/. The causes of post renal proteinuria are listed in Tab. 12.9-11 – Post-renal proteinuria.

12.9.6.8.9 Medication-dependent proteinuria

Pharmaceuticals, environmental pollutants and radio-opaque substances that are excreted via the kidneys can cause tubulopathy and/or glomerulopathy. Nephrotoxic drugs and X-ray contrast media are frequent causes of iatrogenic nephropathy.

Tubular and glomerular proteinuria are early signs of toxic nephropathy and can serve as indicators for dose reduction or for nephroprotective measures (Tab. 12.9-12 – Toxic nephropathies: urine protein findings and clinical evaluation).

12.9.6.8.10 Marker proteins in unilateral kidney

Individuals with only a single kidney, a normal GFR and normal blood pressure have, in comparison with healthy individuals, higher 24-hour total protein excretion, but no selective proteinuria.

12.9.7 Comments and problems

Specimen

24-hour urine collection /40/: the volume of urine excreted in 24 hours varies considerably both in individual patients and between patients. One problem is the procedure used to collect the urine, which is rarely reliable. On average, the amount of urine excreted in 24 hours is ~ 1.5 liters, while the creatinine excretion for an individual weighing 70 kg is about 1.5–2.5 g. This means that the creatinine concentration in a 24 hour urine sample is on average about 100 mg/dL (8.84 mmol/L). Creatinine concentrations < 30 mg/dL (2.65 mmol/L) are indicative of polyuria, artificial dilution of the urine, or an individual with low body weight.

First morning urine /40/: first morning urine provides relatively reliable sample material as it is collected after the patient has been inactive for about 8 hours. The urine is concentrated, and the urine volume rate is 20 –50 mL/h as long as no liquids have been taken after 10:00 p.m. on the evening before.

Second morning urine /40/: the sample should be collected 2–4 hours after the first morning void. The quality of this type of sample will be better if one 200 mL glass of water is taken after 10:00 p.m. Early morning physical exercise is one of the major interfering factors. This causes dehydration while at the same time inducing benign exertional proteinuria.

Protein/creatinine ratio /40/: the ratio has gained general acceptance for correction of diuresis. The excretion of creatinine over time is relatively constant and distorting effects caused by diuresis and anti diuresis can be eliminated by reference to a specifically defined amount of excreted creatinine. The amount of creatinine excreted by a healthy individual is about 30 mg/kg of body weight per 24 hours. The ratio is higher in females than in males, and can frequently decline with age.

Factors affecting proteinuria results: stress, physical exertion, and fever before sample collection can cause proteinuria. Therefore, pathological protein results from spot urine samples should be confirmed by more than two independent consecutive tests.

Method of determination of total protein

Test strip: due to the high degree of affinity between the indicators and amino groups, test strips exhibit a high degree of sensitivity for albumin and transferrin when compared with other urine proteins. Other plasma proteins and mucoproteins from the urinary tract will only cause a positive result if they are present in high concentrations. Bence-Jones protein will be detected only occasionally or not at all. False-negative results occur at pH levels of < 4 and > 8. False-positive results may be caused by drugs, as well as by disinfectants and cleaners that contain quaternary ammonium bases.

Biuret reaction: this is an acceptable method if the proteins are first precipitated using trichloroacetic acid and then dissolved in the biuret reagent. A better precipitant is Tsuchiya’s reagent: 6 mol/L HCl and 1.7% (w/v) of tungstophosphoric acid in 80% (v/v) ethanol. Results within the reference interval are not precise. Protein fragments and free light chains are also detected.

Turbidimetric total protein assays: the concentrations of protein fragments, tubular proteins, glycoproteins, and Tamm-Horsfall protein are too low or are not detected. Contrast media, uric acid, detergents, and polyethylene glycol can mimic excessively high results.

Method of determination of albumin

A fresh midstream collection for urine albumin measurement is preferred.

Albumin-specific test strip assays: test strips do not meet the requirements for the early detection of diabetic nephropathy or of albuminuria as a cardiovascular risk factor if the albuminuria is still at the lower end of the established range, since their diagnostic specificity is only 70–80% /44/.

Quantitative albumin measurement: urine albumin is routinely measured by immunonephelometry or immunoturbidimetry, as well as by test strip assay. However, only immunoreactive albumin is detected. Measurement of urine albumin is not standardized due to the lack of a reference system, which include both a reference measurement procedure and certified reference materials /45/. In a study that evaluated the state of agreement among quantitative immunoassay measurement procedures from in-vitro diagnostic manufacturers results from urine albumin samples had total coefficients of variations of 5.2% to 8.1% and the effect of sample-specific influences were below 10% for most measurement procedures. The median difference range for routine measurement procedures versus comparator LC-MS/MS procedure was approximately 40%. Mean biases ranged from –35% to +34% for concentrations near 15 mg/L and –15% to +18% for concentrations near 30 mg/L /46/. The results of this study demonstrate that fixed decision thresholds cannot be effectively utilized due to lack of agreement among routine measurement procedures /45/.

Albumin levels in urine can vary by 20–30%, and can vary even more in diabetics /47/. Diagnostic conclusions should therefore not be based on only one test result.

Albumin-to- creatine ratio (ACR): the ACR of a random urine sample can be used in patients with kidney disease to rule in or rule out abnormal 24 h losses of albumin. An ACR cutoff of 30 mg/mmol showed a similar good performance in being able to rule in or rule out significant albuminuria (≥ 300 mg/day) /85/. Conversion: 1 mg albumin/g creatinine = 0.113 mg/mmol.

Stability

Total protein: urine can be kept after preservation with 0.1% sodium azide for up to 7 days at room temperature and for up to 30 days in a refrigerator, but must not be frozen /48/.

SDS-PAGE: urine can be kept after the addition of sodium azide (to a final concentration of 0.1% in the urine) for up to 45 days at room temperature and for more than 6 months at 4 °C. Specimens can be frozen after the addition of 50 g/L saccharose /49/.

Marker proteins: albumin, α₁-M and IgG do not exhibit any significant reduction in concentration in untreated urine kept at room temperature and at 4 °C for 7 days or less /48/. Freezing at –20 °C for 6 months leads to a median decrease of 47% in the concentration of IgG, of 14% in the level of α₁-M, and 5% in the albumin concentration /48/.

Albumin: Albumin can remain stable in urine for up to 8 weeks when stored under refrigerated conditions at 4 °C /45/. According to another study, the albumin concentration declines after one year of frozen storage at –20 °C by 9–28%, depending on the initial concentration. The greatest changes are observed at concentrations ranging from 10–20 mg/L, while the smallest changes are seen at concentrations of 200–500 mg/L /50/.

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12.10 Renal stones

Christian Thomas, Lothar Thomas

12.10.1 Urolithiasis

Renal stones occur in the renal pelvis, the ureter and the bladder. The prevalence of upper renal stone disease in the Western world ranges between 5% and 12%. It is estimated that 12% of men and 5% of women will develop stone disease by the age of 70 years. The recurrence rate without medical treatment is 10% at 1 year, 35% at 5 years and 50% at 10 years /1/. The highest risk for a first renal colic is in the age of 25–50 years. In the USA, the estimated annual incidences for men range from 100 to 300 per 100,000 /2/. Some 50% of the renal stones pass spontaneously, while one third of the patients require clinical treatment. The 10-year recurrence rate is 25% following the first renal colic and 75% with repeated colics /3/. The prevalence of urolithiasis is low in children; in the USA it is, depending upon the region, of the order of 1 in 1000 to 1 in 7600 hospital admissions.

In the Western world 75–80% of stones are composed of calcium oxalate, 5% of uric acid and 5% of calcium phosphate. In developing countries, the urinary stones are composed mainly of phosphates and urates. Life style factors have an influence on urolithiasis (Tab. 12.10-1 – Factors that increase the risk of renal stones). Renal stone diathesis increases with the prevalence of obesity, metabolic syndrome and diabetes mellitus. Apart from these patients, who are included in the group of idiopathic stone formers, other diseases such as hyperparathyroidism, cystinuria, hyper oxaluria and hypo citraturia are unequivocally associated with stone diathesis. Around 80% of stone formers have an idiopathic genesis and 20% have a family history of stones, probably indicative of a genetic predisposition.

The presence of stones can lead to renal tract obstruction with macro hematuria, infection, and loss of renal function.

In patients with renal stones the identification of risk factors and stone analysis for successful prophylactic management of stone disease are important.

Lithogenous substances are calcium, oxalate, uric acid, phosphate and cystine. The lithogenous effect is increased by acid urine, and a reduced urine volume.

Antilithogenous substances are citrate and magnesium.

12.10.1.1 Intratubular nephrocalcinosis

Crystal formation and crystal retention are the two key mechanisms in stone formers which determine the development of intratubular nephrocalcinosis. Calcium oxalate and calcium phosphate crystals frequently form in the tubular fluid as a result of supersaturation. Crystal formation in the distal nephron can be considered a renal mechanism to excrete an increased amount of waste per unit of volume. To ensure safe tubular crystal passage, the healthy kidney /4/:

• Presents a non-crystal binding epithelium
• Is able to keep crystal nucleation, growth and aggregation under control via urinary micro- and macromolecular constituents such as citrate, magnesium and proteins
• Triggers a reduction in antidiuretic hormone-stimulated water permeability at high intracellular calcium concentrations. The increase in calcium level in the distal tubule leads to a decrease in arginine vasopressin secretion via the calcium-sensitive receptors and thus to an increased urine volume, which counteracts the over saturation.
• The rate and extent of crystal formation and aggregation is a cell biological process in which a particular tubular epithelial phenotype is responsible for firm crystal adhesion. Under stress conditions the luminal surfaces of dedifferentiated or regenerating cells express multiple crystal-binding molecules, such as sialic acid containing proteins, phospholipids, hyaluran, phosphatidylserin, nucleolin-related protein, annexin II and osteopontin. The molecules are not present at the luminal membrane of intact tubular epithelia. Nucleation and crystal formation can, however, also primarily take place in the interstitium. The presence of crystals in the interstitium is termed interstitial nephrocalcinosis.

12.10.1.2 Clinical symptoms and diagnosis

The passage of a stone causes renal colic which is associated with pain, nausea and vomiting. The pain begins suddenly and can last from a few minutes to many hours. Hematuria is also often present. In children, the dramatic appearance of flank pain, as in adults, is rather unusual; hematuria occurs in up to 90% of the cases and pre-school children often have, in addition, a urinary tract infection /5/.

A preliminary rough clinical-radiological identification, if at all possible, of the stone or its fragments should be confirmed /6/. Important clues as to the etiology can be derived from the stone analysis. However, laboratory stone analysis is requested less frequently because of the advent of endoscopic stone removal and extra corporal shock-wave lithotripsy. Material that is worthy of being analyzed is not always obtained /7/. An overview of the composition and frequency of different kidney stones in the industrialized Western countries is shown in Tab. 12.10-2 – Composition and frequency of renal stones.

The stone analysis does not replace the identification of risk factors that are associated with the formation of renal stones. Diagnostic laboratory examinations for the assessment of the metabolic status of the stone former include /7/:

• To recognize, with serum and urine tests, risk factors that favor the development of renal stones
• To verify the effectiveness of the therapy.

Not uncommonly, conclusions concerning the stone formers metabolic disorders can be drawn from the composition of the renal stones:

• With the demonstration of a cystine stone
• Calcium phosphate stones can be attributable to hypercalcuria or renal tubular acidosis, owing to primary hyperparathyroidism
• Calcium oxalate stones are indicative of primary hyper oxaluria, elevated dietary intake of oxalate precursors, or gastrointestinal disorders
• Pure ammonium urate stones are rare in Western countries, but they are common in children in developing nations. Since the stones form in acidic urine, they occur in Western countries more frequently in the bladder in patients with laxative abuse. Due to enhanced intestinal loss of K⁺, laxative abuse causes increased renal excretion of NH₃⁺ (refer to Section 8.8.4 – Disturbances of chloride excretion).

12.10.2 Examination of stones, lithogenic and antilithogenic substances

In patients with complaints of urolithiasis, the following investigations should be performed at initial presentation /7, 8/:

• An intravenous urogram to detect any anatomical abnormality of the renal tract, and any intercurrent urinary obstruction. Radiological examinations can give some hint regarding the presence and type of renal stone (Tab. 12.10-3 – X-ray appearance of renal stones).
• Serum creatinine, uric acid, calcium, total protein or albumin and electrolytes
• Urine status with regard to hematuria, leukocyturia, proteinuria, pH, osmolality or specific gravity and bacteriuria, possibly urine culture.

In patients with urolithiasis, the examinations listed in Ref. /9/ and Tab. 12.10-4 – Further investigation in addition to the basic program in renal stone formers are recommended in addition to the basic diagnostics (Tab. 12.10-4 – Further investigation in addition to the basic program in renal stone formers). The reference intervals of the urine tests are provided in Tab. 12.10-5 – Reference intervals of the solutes in the urine.

In principle, the examination of the stone patient on his usual diet is meaningful since it corresponds to the metabolic situation in which the urolithiasis has developed. Following the diagnosis of urolithiasis, the majority of patients maintain a certain diet which, however, is often insufficient for the prevention of the further development of stones. A dietary history is, in any event, important for the assessment of the urine findings.

A further step in patients with recurrent renal stones or who are at elevated risk (family history) is the clinical examination for metabolic diseases such as metabolic syndrome, diabetes mellitus, gout, osteopathy, inflammatory intestinal disease, chronic urinary tract infection and nephrocalcinosis /10/.

12.10.2.1 Analysis of stones

The chemical analyses of stones can provide information on the stone composition. The technique is facilitated by the use of test kits, however, they suffer from false-positive and false-negative results. This method is therefore best replaced by infrared spectroscopy, thermoanalysis and especially X-ray diffraction, which offer semi quantitative results /12/. As an alternative microscopy can be performed on thin sections of the concretion, which allows localization of stone components into the centre or periphery of the stone.

12.10.2.2 Analysis of lithogenic and antilithogenic substances

Urinary oxalic acid

Principle: the determination is performed with a broad spectrum of assays, all of which provide different values. Enzymatic methods, gas chromatographic procedures, ion chromatography and HPLC supported methods are employed.

Urinary citric acid

Principle: by the use of citrate lyase, citrate is transformed to oxalo acetate and acetate. In the presence of malate dehydrogenase and lactate dehydrogenase, oxalo acetate and its decarboxylation product pyruvate, are transformed by NADH₂ to malate and lactate. The quantity of NADH₂ consumed is measured spectrophotometrically /11/.

Qualitative test for urinary cystine

Principle: the reduction of cystine to cysteine by the use of cyanide; demonstration of SH-groups after adding sodium nitoprusside. Confirmation of a positive result by chromatography /12/.

Uric acid in urine refer to section 5.4, phosphate in urine refer to Section 6.3.

Estimation of additional covariates

The following covariates may significantly influence the excretion of lithogenic substances in stone formers /13/

• Urinary sodium, as a proxy of salt intake
• Urinary ammonium and citrate; are related to dietary acid load
• Urinary sulfate, as a proxy of animal protein intake influencing calcium excretion independently of acid load
• Urinary pH
• Urinary volume
• Estimated GFR
• Family history

12.10.3 Specimen

Serum: 5 mL

24-hour urine collection (2.5–3 liter container):

• No additives for urate excretion
• No additives for electrolyte excretion
• For the excretion of calcium, oxalate, magnesium, citrate, cystine, creatinine; requirement: 60 mL of HCl, 2 mol/L.

Random urine: 10 mL

12.10.4 Reference intervals

Reference intervals and abnormal excretions associated with nephrolithiasis are shown in Tab. 12.10-5 – Reference intervals and abnormal excretions associated with nephrolithiasis.

12.10.5 Clinical significance

Stone formation is the result of an imbalance between the solubility of the soluta and their crystallization. The urinary concentration of soluta is, therefore, the decisive factor. Studies in serum are only complementary and serve to clarify an underlying disease. Some stone formers do not have concentrations of lithogenic substances that represent a risk, but they do have low 24-hour urine volumes or significant pH changes.

At initial presentation /3, 7/:

• Patients with no previous stone history who show no abnormality of lithogenic substances are not investigated further
• Patients with recurrent nephrolithiasis and those who have a renal stone for the first time but, based upon their medical history, are at risk for recurrent stones need long-term patient management.

Urinary supersaturation is a state that the concentration of solutes (e.g., lithogenic substances) in urine exceeds the saturation point, causing spontaneous precipitation of solutes, with the inadequacy of stone inhibitors (e.g., antilithogenic substances). The crystals will aggregate and adhere to the renal tubular epithelium with the presence of tubular adhesion molecules. In addition urinary proteins such as osteocalcin promote crystal aggregation. Some glycoproteins such as prothrombin fragment-1 and osteopontin exert stone-inhibiting property /10/.

12.10.5.1 Stone types

The most common type of urinary stone found in humans worldwide is calcium oxalate (Tab. 12.10-6 – Renal stones).

Calcium containing stones

These stones constitute the largest portion of renal stones and often contain oxalate. In serum the determination of calcium, phosphate and parathyroid hormone is important, in 24-hour urine the determination of calcium and oxalic acid.

Uric acid stones

For the less common pure uric acid stones, the determination of uric acid in the 24-hour urine collection is recommended.

Infection stones

So-called infection stones contain magnesium-ammonium phosphate. Patients complain about bouts of fever with flank pain. With imaging techniques large, low density stones are seen which require immediate removal and intensive antibiotic treatment.

Cystine stones

To rule out cystinuria, a qualitative test should be performed in every case of nephrolithiasis.

Drug-induced stones

With primary, difficult-to-identify stones, medication-induced cause must be considered. Stones may occur following treatment with triamterene, indinavir, salicylate, antibiotics, or due to oxypurinol, a metabolic product of allopurinol.

12.10.5.2 Hypercalcuria

Hypercalcuria is the most frequent pathological finding in patients with renal stones and can contribute to the formation of calcium oxalate and calcium phosphate stones (see also Section 6.2.2 – Urinary calcium excretion).

The mechanisms by which hypercalcuria may contribute to stone formation are /14/:

• High urinary calcium concentration promotes crystallization of stone-forming calcium salts by increasing the ionic activity of calcium and in turn the saturation of calcium salts
• Hypercalcuria attenuates urinary inhibitor activity against calcium salt nucleation and growth. Many of the known inhibitors are negatively charged (citrate and glycosaminoglycans), calcium can bind these inhibitors and inactivate their inhibitory activity in urine.

There are three important pathogenic mechanisms for the formation of calcium containing renal stones (Fig. 12.10-1 – Mechanisms that cause hypercalcuria):

• Absorptive hypercalciuria
• Renal hypercalciuria
• Resorptive hypercalciuria.

12.10.5.2.1 Absorptive hypercalcuria

For a given calcium load, patients with absorptive hypercalcuria absorb greater proportions of calcium than normal individuals /13, 14/. The calcium concentration in blood increases slightly, raises the glomerularly filtered load of calcium and suppresses parathyroid hormone (PTH) secretion. Thus, the combination of increased filtered calcium load and the decreased tubular reabsorption of calcium from PTH suppression contributes to hypercalcuria. Absorptive hypercalcuria is the cause of calcium containing renal stones in up to 50% of cases, but it rarely appears in a pure form. Hypercalcuria is frequently accompanied by other metabolic derangements including bone disease, fasting hypercalcuria, and chronic acidosis These individuals often have a hypercalcuria of > 250 mg/24 h (6.2 mmol/24 h), which results from augmented intestinal Ca²⁺ absorption and PTH suppression.

A genetic and an acquired form of absorptive hypercalcuria are distinguished.

Genetic form of absorptive hypercalcuria

The major hormonal determinant of intestinal calcium absorption is vitamin D status. It is supposed that end organ sensitivity for 1,25(OH)₂D is elevated due to a genetically determined increase in the number of vitamin D receptors in the small intestine /15/. The results, leading to hypercalcuria are: an increase in intestinal vitamin D receptor number, increased 1,25(OH)₂D₃ binding and an elevation in vitamin D dependent calcium-binding protein, resulting in enhanced intestinal calcium absorption /14/.

Acquired form of absorptive hypercalcuria

It is assumed that excessive dietary intake of animal protein leads to an increased membrane arachidonic acid content and an increase in the synthesis of prostaglandins. These alterations promote changes at the kidney, bone, and intestine that translate into increased urinary calcium excretion /14, 16/.

12.10.5.2.2 Renal hypercalcuria

There is a defect in the renal tubular reabsorptive mechanism for calcium /14/. Due to the continuous loss of calcium in the urine, secondary hyperparathyroidism enhances formation of 1,25(OH)₂D and an increase in intestinal calcium absorption. Thiazide therapy reduces the renal tubule calcium leak and in turn corrects the fasting hypercalcuria, suppresses PTH secretion, and reduces calcitriol and intestinal calcium absorption /14/.

12.10.5.2.3 Resorptive hypercalcuria

Resorptive hypercalcuria is the result of an adenoma of the parathyroid glands with the development of primary hyperparathyroidism /14/. The excessive PTH synthesis leads not only to increased bone resorption of calcium, but also to renal synthesis of 1,25(OH)₂D₃. Thereby, elevated intestinal calcium absorption also occurs.

12.10.5.2.4 Hypercalcuria and hypernatruria

Hypercalcuria can be attributable to increased salt intake /17/. If the sodium excretion is > 100 mmol/24 h, then every further rise by 100 mmol leads to an increase in calcium excretion by 1.25 mmol/24 h (50 mg/24 h). Sodium excretion is therefore an important indicator for the assessment of the dietary influence on calcium excretion, since sodium restriction reduces the hypercalcuria.

12.10.5.2.5 Diagnostic interpretation of hypercalcuria

Resorptive hypercalcuria

If hypercalciuria is accompanied by hypercalcemia or by a serum calcium level that is near the upper reference interval value along with hypophosphatemia, primary hyperparathyroidism is suspected /18/. See also Section 6.4 – Parathyroid hormone. Elevated PTH values confirm the tentative diagnosis. If elevated serum calcium or hypernatruria cannot explain the hypercalcuria, absorptive or renal hypercalcuria must be taken into consideration.

Absorptive hypercalcuria

The findings are fasting hypercalcuria, plasma calcium near the upper reference interval, PTH low-normal, positive calcium absorption test (Tab. 12.10-7 – Calcium/creatinine ratio in healthy individuals and patients with absorptive and resorptive hypercalcuria in the Ca absorption test).

Renal hypercalcuria

The findings are fasting hypercalcuria, plasma calcium low-normal, PTH elevated. The administration of thiazide diuretics corrects fasting hypercalcuria and suppresses PTH secretion.

12.10.5.3 Hyperoxaluria

Oxalate, dicarbolic acid (HOOC-COOH) is a highly insoluble end product of metabolism in humans. It is excreted, particularly in the forms of its calcium salt, almost entirely by the kidneys. Oxalate has the tendency to crystallize in the renal tubules and the earliest symptoms among those affected are Randal’s plaques.They act as an anchor for calcium oxalate crystals and are considered to be a predisposing factor of renal stone formation and nephrocalcinosis. Progressive renal damage is caused by a combination of tubular toxicity from oxalate, intratubular and interstitial deposits of calcium oxalate, obstruction by renal stones and superimposed infection /19/. Hyperoxaluria is subdivided into primary and secondary forms.

12.10.5.3.1 Primary hyperoxaluria (PH)

PH has a prevalence of 1–3 cases per 1 million in the general population. In Europe, the incidence is 1 case per 120,000 live births per year and is involved in 1–2% of the end stage cases of chronic renal disease in children. The PH types I and II are autosomal recessive inherited diseases. Each type is caused by an enzyme defect and affects different cellular organelles /19, 20/.

The PH type 1 (OMIM 604285) is a rare progressive genetic autosomal recessive disease of glyoxalate metabolism. The metabolic defect in PH1 results from a deficiency of the liver-specific peroxisomal enzyme alanine-glyoxalate aminotransferase (AGT), which converts the oxalate precursor glyoxalate to glycine (Fig. 12.10-2 – Metabolic pathways of oxalate formation in the liver). The enzyme deficiency results in the accumulation of glyoxalate and excessive production of oxalate and glycolate. Mutations in AGXT, the gene encoding AGT, result in PH type 1 with absent or deficient AGT. At least 178 mutations have been described. Gly170Arg and c.33dupC occur across populations at a frequency of 30% and 11%, respectively. Patients with PH1, commonly present in childhood with kidney stones, nephrocalcinosis, end-stage liver disease or systemic oxalosis with deposition of oxalate in tissues such as bone, retina, heart, and skin /42/.

PH 2 (OMIM 260000) is attributable to a deficiency of glyoxylate reductase/hydroxy pyruvate reductase (GRHPR). The enzyme is localized mainly in the cytosol of hepatocytes and catalyzes the reduction of glyoxalate to glycolate and hydroxy pyruvate to D-glycerate (Fig. 12.10-2). In GRHPR deficiency, lactate dehydrogenase metabolizes accumulated glyoxalate to oxalic acid. A total of 30 mutations have been identified in the GRHPR gene, in which the c.103delG and c.403_–404+2delAAGT mutations are relatively common.

PH type 3 (OMIM 613616) results from a defect in the liver-specific enzyme 4-hydroxy-2-oxoglutarate aldolase (HOGA), which plays a role in the metabolism of hydroxyproline. A minimum of 19 mutations have been described in HOGA1.

Clinical symptoms

Clinical symptoms of PH can occur at any age, the median age is 5.5 years. The spectrum of the primary symptoms ranges from childhood nephrocalcinosis to nephrolithiasis in adulthood /19/. At the time of diagnosis, 20–50% of the patients have advanced chronic renal disease. Oxalate crystals are deposited preferentially in the kidneys, in the blood vessels, and in bone. The most common subtype is PH type 1, particularly in childhood.

Absorption mechanism of oxalate

Plasma oxalate is derived from endogenous oxalate produced by the liver and oxalate and its precursors absorbed in the intestine. In the intestine dietary derived oxalate promotes oxalate absorption through SLC26A3 (functions as a chloride/bicarbonate exchanger)-mediated and paracellular absorption. SLC26A6 mediated oxalate secretion limits the net absorption of oxalate. Intestinal oxalate-degrading bacteria decompose oxalic acid to reduce the absorption of oxalic acid; calcium ions combine with oxalate to form insoluble calcium oxalate, which is not absorbed by the intestine and is excreted with feces, thus limiting the absorption of oxalate. Fatty acids reduce calcium combined with oxalate by binding to calcium, thereby promoting oxalate absorption /20/.

Laboratory findings

Kidney stones consist of more than 95% calcium oxalate monohydrate (whewellite) have a non homogenous appearance and are of pale color. Children have elevated creatinine and metabolic acidosis. The following excretions are considered to be pathologic in PH type 1 /20, 21/:

• Oxalic acid of ≥ 0.56 mmol × 24 h^–1 or ≥ 50 mg × 24 h^–1. Oxalic acid tends to be higher in PH type 1 with 2.14 ± 1.29 mmol × 24 h^–1 than in PH type 2 with 1.46 ± 0.49 mmol × 24 h^–1
• Glycolate of ≥ 0.93 mmol × 24 h^–1 or ≥ 70 mg × 24 h^–1
• An elevated glycolate value in PH type 1 is helpful in the differentiation of PH type 1 and PH type 2.

Oxalate in renal insufficiency: plasma concentrations in end stage renal disease (ESRD) are 20–60 μmol/L (1.8–5.4 mg/L), while in PH patients with ESRD they are ≥ 60–100 μmol/L (5.4–9.0 mg/L) /19/.

Calcium excretion: low in PH type 1 and PH type 2, variable in PH type 3. In uncategorized cases liver biopsy maybe performed to test the activities of AGT and GRHPR.

Definitive diagnosis of PH: requires genetic testing. Some 80% of patients have the PH type 1. If no AGT or GRHPR deficiencies are found on liver biopsy, PH 1 and PH 2 can be excluded.

12.10.5.3.2 Secondary hyperoxaluria

Secondary hyperoxaluria is the most common cause of renal stone formation and can be due to:

• Elevated oxalic acid load
• Increased intestinal absorption of oxalic acid
• Chronic renal disease
• Exaggerated vitamin C supplementation.

Elevated oxalic acid load

The elevated intake of oxalic acid-rich foods such as rhubarb, beet root, spinach, almonds, tofu, tea, and chocolate can lead to mild hyperoxaluria of up to 60 mg × 24 h^–1 or 0.67 mmol × 24 h^–1. The absorption of oxalic acid is dependent upon the calcium content of the food. If oxalate is ingested simultaneously with foods rich in calcium then much of oxalate will be complexed to calcium in the gut and intestinal absorption is reduced. Precursors of oxalate metabolism such as ascorbic acid, fructose, xylose and hydroxyproline can also lead to augmented synthesis of oxalic acid. Plasma oxalate levels are in the range of 1–5 μmol/L (90–450 μmol/L) and are supersaturated at concentrations of ≥ 30 μmol/L (2.7 mg/L) /21, 22/.

Enteric hyper oxaluria

The daily dietary oxalate intake is 50–200 mg, but may range to 1000 mg. Intestinal absorption ranges between 5% to 15%. Oxalate is absorbed throughout the length of the small intestine, and the colon also participates in its absorption. The major portion of the oxalate that is ingested with food is absorbed within 4–8 hours /22/.

Oxalate degrading bacteria play a role in the control of the transport of oxalate. In this regard, the colon is colonized with the anaerobic Oxalobacter formigenes. It is believed that deficient colonization with this bacterium may be a cause of elevated oxalate absorption and thereby a risk factor for renal stones.

Moderate to severe enteric hyper oxaluria occurs in bowel disease. The hyper oxaluria in bowel disease is not from increased endogenous production but rather intestinal over absorption. In the setting of malabsorption with steatorrhea, dietary calcium is bound by free fatty acids in the intestinal lumen. There is less calcium available to bind oxalate, resulting in increased amounts of oxalate free for absorption. In contrast to normal physiology, the colon appears to be the major site of oxalate absorption in enteric hyper oxaluria. Overall, enteric hyper oxaluria refers to a state in which disease or resection of the small bowel leads to malabsorption of fat and bile acids. It is most frequently seen in Crohn’s disease or patients with jejunal-ileal bypass /22/.

Renal oxaluria

Oxalate is not protein-bound, so it is freely filtered at the glomerulus. Net tubular secretion of oxalate with fractional excretions as high as 200–300% have been described. Serum oxalate levels are in the range of 1–5 μmol/L, but in urine the concentration is about 100-fold higher, mainly due to water reabsorption by the nephron. Urine is generally supersaturated with respect to calcium oxalate and the risk of crystallization is only in the urinary tract unless serum levels rise above 30 μmol/L. At the physiological pH of 7.0, oxalate forms insoluble crystals with calcium because in aqueous solution oxalic acid is only soluble up to a concentration of 56 μmol/L (5 mg/L). Generally, however, 0.44 mmol (40 mg) are excreted with 1.5 liters of urine in 24 hours /21, 22/.

In supersaturation, the calcium oxalate crystals adhere to the tubular cells. Some of these migrate into the interstitium and cause inflammation with subsequent interstitial fibrosis, which can progressively lead to nephrocalcinosis and the formation of renal stones /20/. Secondary complications are the obstruction of tubuli and collecting ducts, as well as urinary tract infection.

With impaired kidney function and a decline in the GFR below 30–40 [mL × min^–1 × (1.73 m²)^–1], a rise in serum oxalate concentration, which can slightly exceed the supersaturation concentration (30 mmol/L), occurs. The result is systemic deposition of calcium oxalate crystals in the kidneys and in extrarenal tissue such as myocardium, vessel walls, bone, the central nervous system and the eyes.

Supplementation with ascorbic acid : vitamin C is excreted in urine both in its unmetabolized form and as oxalate. Ascorbic acid intake of > 1 g daily increases the risk of kidney stones with a relative risk (RR) of 2.23 (1.28–3.88). With ingestion of less than 7 times weekly, the RR is 1.66 (0.99–2.79) /23/.

Laboratory findings

In all patients with recurrent calcium oxalate stone disease, a 24 h urine oxalate excretion should be determined. If the excretion is greater than 1.0 mmol × 24 h^–1 glycolate and L-glyceric acid should be determined. These marker metabolites would distinguish PH type 1 from PH type II, but if negative, cannot exclude a diagnosis of pH /20/.

In oxalate stone formers with secondary hyper oxaluria the 24 h urine oxalate excretion is 0.55–0.8 mmol × 24 h^–1 or 50–72 mg × 24 h^–1; upper reference interval ≤ 0.50 mmol × 24 h^–1 or 45 mg × 24 h^–1.

12.10.5.4 Hyper uricosuria

Uric acid urolithiasis can present as pure uric acid stones or mixed uric acid/calcium stones. The incidence of uric acid stones shows wide regional differences. In Great Britain and the USA the incidence is 5–10%, in Germany it is 25%, and in the Middle East it is 30% in both adults and children /24/. Some 5% of kidney stone carriers have mixed uric acid/calcium stones /25/. A small proportion of patients with uric acid containing stones has hyper uricosuria, but the majority have normo uricosuria. The common element in these patients is low urinary pH promoting uric acid precipitation /24/. For hyper uricosuria, see also Section 5.4 – Uric acid.

Uric acid (C₅H₄N₄O₃) promotes the formation of salt crystals.

Factors related to uric acid crystallization are persistent acid urine, hyper uricosuria and small urine volume /24/:

• Persistent acidic urine is the principal determinant of uric acid crystallization. Uric acid is a weak acid with a current urine pK of 5.35. At this pH, half of the uric acid is in its sparingly soluble non-dissociated, form. With a solubility of undissociated uric acid of 97 mg/L, to exceed solubility at urinary pH’s of 4.5, 5.5, and 6.5 would require total uric acid (undissociated and urate) concentrations of approximately 110, 200, and 1100 mg/L respectively /24/. This shows that increased solubilization of uric acid in urine is only possible with an increase in urinary pH. In the majority of patients with uric acid stones the urine pH is abnormally low because of increased renal H⁺ secretion, decreased NH₄⁺ excretion, or a combination of both.
• Hyper uricosuria is due to elevated synthesis, increased renal excretion or a combination of both. Only a small proportion of patients with uric acid stones has increased uric acid excretion. If hyper uricosuria is defined as the excretion of > 750 mg/24 h (4.5 mmol/24 h) in women and > 800 mg/24 h (4.8 mmol/24 h) in men, then approximately one-third of normal patients and one-third of calcium oxalate stone formers have hyper uricosuria. However, only 15% of the patients excrete more than 1000 mg/24 h (5.9 mmol/24 h) of uric acid. Of all stone formers undergoing a metabolic evaluation, hyper uricosuria was the only metabolic abnormality in 10% and was found in combination with other metabolic abnormalities in 40% of patients /25/.
• Small urine volume. In dry regions, oliguria is an independent risk factor for urate nephrolithiasis /24/.

12.10.5.4.1 Etiology of uric acid stone diathesis

The etiological mechanisms of uric acid stone formation are various; they include congenital, acquired and idiopathic causes. The latter are the most common. Idiopathic genesis refers to etiology that is neither congenital, nor can it be ascribed to secondary causes. Uric acid stones occur in an augmented manner in the metabolic syndrome, in type 2 diabetes mellitus and in obesity /24, 25/.

Congenital causes

Monogenic disorders of uric acid metabolism: a defect in the enzyme hypoxanthine guanine phospho ribosyl transferase (Lesh-Nyhan syndrome), phospho ribosyl pyrophosphate synthetase over activity, and glucose-6-phosphatase deficiency lead to serum uric acid levels of > 10 mg/dL (595 μmol/L) and urinary uric acid excretion greater than 1000 mg/24 h (5.9 mmol/24 h).

Primary gout: the incidence of uric acid stones is directly proportional to the extent of hyper uricosuria. Primary gout is associated with a low urinary pH and with reduced fractional excretion of urate.

Adenine phospho ribosyl transferase deficiency: about 1 out of 1000 stone carriers has a deficiency of the enzyme adenine phospho ribosyl transferase, which plays a role in purine metabolism (see Section 5.4 – Uric acid). Adenine is oxidized exclusively to 2,8-dihydroxyadenine, excreted in the urine, where it leads to the formation of stones. The diagnosis of the autosomal recessive disease is made with the finding of the enzyme deficiency in red blood cells /7/.

Secondary causes

Diarrheal states: chronic diarrheal diseases, such as ulcerous colitis, M. Crohn, and ileostomy favor the formation of urate stones due to small urine volumes and the formation of acidic urine. The acidic urine is due the loss of HCO₃^– in the stool coupled with defects in urinary NH₄⁺ excretion (see Section 8.8.4 – Disturbances of chloride excretion) /24/.

Oliguria

Excessive sweating and exercise, particularly in arid regions, are important causes of the development of urate stones in the Near East /24/.

High animal protein diet

The excessive intake of animal protein raises the organism’s uric acid load, lowers the pH and reduces urinary citrate concentrations. The cause of hyper uricosuria is elevated purine intake in 70% of the patients. In most patients, however, urinary pH is higher than 5.5 due to adaptive increases in NH₄⁺ excretion /24/.

Purine overproduction

Myeloproliferative diseases, hemolytic anemia and strong diet restriction lead to elevated uric acid production. The cause of hyper uricosuria is raised endogenous production in 30% of the patients.

Uricosuric drugs

Uricosuric drugs can lead to transient hyper uricosuria (e.g., probenecid, high dose salicylic acid, radio contrast agents).

12.10.5.4.2 Idiopathic uric acid nephrolithiasis

The two essential factors in the diathesis of idiopathic uric acid nephrolithiasis is persistent low urinary pH and the fractional excretion of urate is reduced. The causes of inappropriately low pH, are defective NH₄⁺ formation and elevated net acid excretion. The acid-base balance and the effective buffering of the H⁺ in the kidneys is normally maintained by the H₂CO₃–HCO₃^– and the NH₃ –NH₄⁺ buffer systems. The latter system, with a pKa of 9.2, has a high buffering capacity for H⁺ (see also Fig. 8.8-4 – H+ secretion into the cortical collecting tubule). Remaining H⁺ are buffered by titratable acids. In this way, the urinary pH is held constant within a certain range, especially under acid load. If the NH₄⁺ is low in urine, the buffering under acid load must be carried out exclusively by the titratable acids.

An inappropriately acidic urine, attributable to decreased NH₄⁺ formation, is seen in obesity, metabolic syndrome and insulin resistance /21/. In these patients, inappropriately acidic urine is formed, e.g., due to a meat-rich diet. The intake of sulfur-containing amino acids affects the tubular secretion of H₂SO₄. The H⁺ that is released is titrated by HCO₃^–. However, at a urinary pH of ≤ 6, no more HCO₃^– is available for titration and the titration must be carried out exclusively by NH₄⁺.

12.10.5.5 Hypocitraturia

Urinary citrate inhibits calcium stone formation by complexing calcium in a soluble form and by effects on urinary crystals to prevent growth to stones. Citrate is a tricarboxylic acid with pKa of 2.3, 4.3 and 5.6. In urine citrate exists predominantly in its trivalent form. Since the last pKa is 5.6 the abundance of divalent species increases as the pH falls. Hypocitraturia contributes to calcium nephrolithiasis in 20–60% of stone formers. However, hypocitraturia is only infrequently the sole cause of calcium stones, except in certain unusual causes with renal tubular acidosis /26/. Causes of the high prevalence of hypocitraturia in Thai people are believed to be a low citrate intake and an increased urinary citrate reabsorption from chronic metabolic acidosis, which maybe caused by endemic incomplete renal tubular acidosis /27/.

Citrate is filtered freely in the kidney and 65–90% is reabsorbed as a divalent anion mainly in the proximal tubule. The remainder is found in the final urine.

Changes in acid-base homeostasis are the predominant determinant of proximal tubule reabsorption and urinary excretion of citrate (Fig. 12.10-3 – Transport of citrate in the proximal tubule of the kidney) /26/.

Factors are:

• Tubular reabsorption of citrate increases in intracellular tubular acidosis and hypocitraturia can ensue
• Decrease in tubular pH cause increase in the proportion of divalent citrate and tubular citrate reabsorption will increase. For example the divalent citrate increases 3-fold as the luminal pH decreases from pH 7.4 to pH of 6.9 /26/.

Causes of hypocitraturia because of acidosis are /17/:

• Conditions that lead to increased reabsorption of citrate (chronic systemic acidosis, increased tubular acid load). This is the case in chronic diarrhea, in infections or with high animal protein intake. Causes of metabolic acidosis with severe hypocitraturia (< 100 mg/24 h; 0.52 mmol/24 h) are chronic diarrhea, gastrectomy, and ulcerative colitis.
• Hypokalemia and potassium deficiency. In potassium deficiency the intracellular pH declines and in spite of systemic alkalosis renal tubular H⁺ secretion increases. Potassium deficiency enhances citrate transport in the proximal tubule.
• Type 1 renal tubular acidosis (see Tab. 8.8-5 – Renal-tubular acidoses).

Causes of hypocitraturia in the absence of acidosis are /17/:

• Thiazide medication; thiazides lower the excretion of citrate, especially in potassium deficiency
• Excessive physical work, chronic kidney disease and high salt intake.

Potassium citrate is administered in order to raise urinary citrate concentrations. This is metabolized to HCO₃^–; the urinary alkaline load is thereby increased and tubular citrate reabsorption is reduced.

12.10.5.6 Hypomagnesuria

Urinary magnesium is an inhibitor of calcium salt formation. Low urinary concentrations promote the formation of calcium stones.

12.10.5.7 Low urinary pH

The excessive intake of acid with the food, due to an animal protein diet, can lead to high uric acid excretion, citrate and sulfate and can lower the pH of the 24-hour urine from 5.5 to 5.0. Constant acidic pH values below 6.0 in the daily urinary pH profile are an indicator of persistently acidic urine and they promote the formation of uric acid and calcium oxalate stones, and the crystallization of both /17/.

12.10.5.8 High urinary pH and hyper calcuria

Hyper phosphaturia could be caused by several etiologies, including phosphate intake, gastrointestinal absorption and the rate of phosphate reabsorption which is dependent on the abundance of phosphate co transporters, hormone regulation and dietary factors /28/. The prevalence of calcium phosphate stones is less common than calcium oxalate stones and seem preferentially likely in women /29/.

The following are distinguished based on the mineral form:

• Carbonate apatite Ca₁₀(PO₄CO₃OH)₆(OH)₂. It crystallizes at a pH of ≥ 6.8 and occurs particularly in association with urinary tract infections and as a constituent of calcium oxalate stones.
• Brushite (CaHPO₄ × 2 H₂O), which is formed at the narrow urinary pH range of 6.5–6.8 with hypercalcuria of > 8.0 mmol/24h and phosphate excretions of > 35 mmol/24 h.

Calcium phosphate stone formation plug the inner medullary collecting ducts with apatite crystals. Formation of initial calcium phosphate deposits can lead to a vicious cycle of inner medullary collecting ducts cell injury, rising pH and more calcium phosphate deposition /28/. The patients have high calcium phosphate supersaturation due to high urine pH coupled with hyper calcuria. Hypercalcuria is genetic in nature or it is due to primary hyperparathyroidism. The elevated urinary pH is believed to be genetic in nature or to be based upon distal renal tubular acidosis (type 1 RTA). The latter is congenital or acquired. See also Section 8.8 – Renal electrolyte excretion).

It is believed that along with supersaturation of the urine, the fragmentation of calcium oxalate stones with shock wave lithotripsy is also responsible for the increase in calcium phosphate stones. It is possible that this favors the formation of calcium phosphate stones through renal injury /28/.

12.10.5.9 Small urine volume

The 24-hour urine volume should be 1.5–2 L. Since the insensible water loss in adults amounts to approximately 1 liter per day, daily fluid intake must be 2–3 liters. If less than 2 liters are ingested daily the urine volume will be below 1 liter and supersaturation of the urine with solutes can occur with the danger of crystal formation /17/.

12.10.5.10 Cystinuria

The worldwide incidence of cystinuria is variable. In Great Britain, the screening of newborns estimated an incidence of 1 : 2000; in Australia of 1 : 4000 and in the USA of 1 : 15,000 /18/. Cystinuria is characterized by a renal and intestinal transport defect in dibasic amino acids (cystine, lysine, ornithine and arginine). The physiological fractional excretion of cystine (C₃H₆O₂NS) is 0.4%. The tubular reabsorption of cystine occurs to an extent of 80% via a low affinity, high capacity Na⁺-independent transport system, while the remaining 20% is effected through a high affinity system which also transports other dibasic amino acids /12/.

Three types of cystinuria are distinguished /31/:

• The homozygous type with excretion of cystine and high levels of aminoaciduria. Because of the absence of the neutral and basic amino acid transport protein rBAT, a transport activator or co-transporter of cystine, renal tubular reabsorption is decreased. The homozygous type is clinically symptomatic (i.e., a stone carrier) in 90% of the cases. Homozygous variants in the gene SLC3A1 are responsible for the homozygous type.
• Heterozygotes of type I show normal aminoaciduria, whereas heterozygotes of type II and type III show high and moderate levels of cystine and other dibasic amino acid excretion, respectively.

Cystine stones account for 1–2% of all kidney stones in adults with nephrolithiasis and 6–8% in children. The peak age of onset of stone disease is during the third decade of life. Patients present with symptoms related to urolithiasis. Cystine stones should be suspected in patients with /31/:

• History of renal stones in childhood
• Recurrent episodes of nephrolithiasis
• Strong family history of stone disease
• Amber colored kidney stones.

Cystine is almost insoluble at pH 5–7. Normal cystine excretion is below 20 mg/24 h (85 μmol/24 h). Due to its high pK value, the upper limit of solubility of cystine at physiological pH of the urine is about 300 mg/L (1,250 μmol/L) or 400 mg (1,660 μmol/24 h) /12, 30/. Supersaturation of the urine leads to the formation of renal stones. The solubility of cystine increases with rising pH; at a urinary pH of 8 it is almost 3 fold higher than at the physiological pH. The therapeutic objective is the alkalization of the urine.

By definition a urinary cystine excretion above 250 mg/24 h (1,000 μmol/24 h) in adults and above 75 mg/24 h (312 μmol/24 h) in children is abnormal. The excretion of > 250 mg (1,000 μmol) cystine/g creatinine or > 400 mg (1,660 μmol/24 h) is indicative of homozygous cystinuria. The final diagnosis is made based upon the analysis of the stone. Patients with homozygous cystinuria may have stones with calcium oxalate as the major component and cystine as the minor component. Heterozygous individuals, who excrete less cystine than homozygous patients, do not necessarily have a lower incidence of stone formation /30/.

Laboratory findings: on microscopic analysis of fresh urine, hexagonal cystine crystals are typical of cystinuria. These crystals are seen in only a minority of cystinuric patients /31/. The following tests are used for the chemical detection of cystinuria:

• The cyanide nitroprusside method, a quick test for qualitative detection
• The nickel/dithionite test for qualitative and semi-quantitative detection /30/.

12.10.6 Comments and problems

24-hour urine collection

For the determination of calcium, oxalate, magnesium, citrate, electrolytes, and cystine, the urine has to be acidified by HCl so that the pH is below 2. For the determination of urate the collection is to be made without additives /7/.

Excretion of calcium and oxalate

The urinary pH must be below pH 6.5 during the collection because otherwise calcium oxalate crystals that are not readily soluble will precipitate /7/.

Uric acid excretion

Uric acid precipitates in acidic urine. The urine collection should, therefore, not be performed with the addition of acid /7/. Before the uric acid determination, the urine has to be alkalized (pH 8–9) in the laboratory with the addition of NaOH to re-dissolve any uric acid that had precipitated in acid urine or due to low temperature.

Cystine excretion

The urine collection should be at a pH above 7.5 to improve the solubility and avoid false low concentrations due to precipitation at physiological pH. 2 g of sodium bicarbonate are added to the container before starting the collection to alkalize the urine /7/.

Cystine rapid test

False positive reactions with the sodium nitroprusside test in acetonuria and homocystinuria /32/.

pH value

The determination has to be performed on fresh fasting morning urine /17/.

Influence factors

Diuretics alter the excretion of calcium. Thiazides lead to reduced excretion and loop diuretics lead to increased excretion /33, 34/. Magnesium excretion reacts in the contrary manner. The excretion of uric acid decreases, as does that of oxalic acid, under thiazide treatment /35/.

The oral administration of phosphate salts leads to increased urinary excretion of phosphate /36/. The result is an increase in the excretion of pyrophosphates as suppressors of the growth of calcium oxalate crystals /35/ and a decrease in hyper calcuria /37/.

Vitamin C enhances the formation of oxalic acid and its excretion with a urinary pH of above 7 /38/.

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