Electrolyte and water balance

8.1 Water balance and fluid compartments

Lothar Thomas

8.1.1 Water and electrolyte balance

The term water metabolism describes the balance between the intake and the excretion of water. The cell membrane separates the intracellular fluid compartment (ICF) from the extracellular fluid compartment (ECF). It is freely permeable to water, but not to electrolytes. Based upon the free movement of water between the ICF and the ECF, the osmolality, which is defined as the relationship of electrolytes to free water, is maintained uniform and constant.

Changes in the organism’s water homeostasis are classified as hypo osmolar or hyper osmolar disturbances, depending upon whether an excess or a deficiency of water, relative to the dissolved substances (solutes), is present. Water comprises 55–65% of body weight, depending upon gender, age and body fat, and its distribution between the ICF and the ECF is 2 : 1. In the ECF 80% of the water is found in the interstitial space, and 20% in the intravascular space (circulating blood volume).

Na⁺ is the predominant cation in the ECF, and its concentration in the plasma reflects the osmolality. In the ICF, K⁺ is the most important cation. The different pattern of distribution of the cations is maintained by the energy and oxygen-dependent Na⁺-K⁺ pump of the cell membrane. If there is a deficiency in energy or oxygen, the Na⁺-K⁺ pump becomes insufficient, the ion gradients collapse, mainly intracellularly, and the result is cellular edema /1/.

In order to preserve osmotic equilibrium and volume homeostasis of the fluid compartments, the organism maintains an even balance between the uptake and loss of water and electrolytes. This is regulated by:

• The kidneys, which maintain plasma osmolality within the narrow range of 275–290 mmol/kg through the excretion or the reabsorption of free water solutes. The regulating hormone is arginine vasopressin.
• The thirst mechanism, which is stimulated by intravascular hypovolemia and hyper osmolality. Thirst is the primary defense mechanism against severe fluid loss.

Water and electrolyte homeostasis is regulated by sensors and neurohumoral mechanisms with effector hormones, which function via negative feedback mechanisms (Tab. 8.1-1 – Neurohormonal mechanisms with regulatory effects on volume homeostasis).

8.1.2 Diagnosis of water and electrolyte disorders

Complex processes underlie the electrolyte and water balance disorders. An integrative analysis is, therefore, necessary. It is made up of the medical history, the medical examination and the laboratory test results.

Important biomarkers and their diagnostic value are listed in Tab. 8.1-2 – Examinations for the diagnosis of disorders of the electrolyte and water balance.

Examinations and findings with regard to two clinical issues are shown in an exemplary manner in Tab. 8.1-3 – Cases of disorders in electrolyte and water balance.

8.1.3 Osmotic equilibrium

Total body water is the determining factor with regard to the osmolality of the fluid compartments of the organism.

The maintenance of osmotic homeostasis is regulated by a change in the renal excretion of free, that is to say osmotically unbound water /2/. The antidiuretic hormone (ADH) arginine vasopressin plays a central role in the regulation of free water. Important stimuli of ADH release are:

• Increase in plasma osmolality; even fluctuations of less than 2% are registered by the osmoreceptors of the anterolateral hypothalamus, leading to changes in plasma ADH concentrations. The stimulation of ADH secretion begins at a plasma osmolality of 280 mmol/kg, and is maximal at 290 mmol/kg. ADH acts on the kidneys to increase the reabsorption of water (anti- diuresis). Mediated by the osmoreceptors, an increase in plasma osmolality to ≥ 295 mmol/kg results in the sensation of thirst.
• Changes in the effective arterial blood volume, which are transmitted to the anterolateral hypothalamus by the baroreceptors located in the right atrium.

8.1.3.1 Total body water

Body water derives from dietary intake, retained by the kidneys, and produced by carbohydrate, fat and protein metabolism. Most body water derives from consumption, which is evolutionary programmed by thirst. Urinary water excretion varies to match intake and metabolic generation, less insensible losses. Metabolic water production is directly proportional to energy expenditure and averages 250 to 350 ml per day, a value that can rise after exercise /3/. Daily fluid consumption from all sources in adults in Europe is about 3 liters, even when salt intake is low.

The body water is up to 94% of body weight in the fetus, 75% in the newborn and falls to 60% in children 1 year of age. One transient increase in total body water is between the first and the second years of life, the second one at puberty. Thereafter total body water gradually falls with advancing age to about 60% in males and 50% in females, respectively /2/. Females after puberty have 10% more fat than men, accounting for women’s 10% lower body water.

In temperate climate zones water homeostasis is maintained by the drinking of 1.5 L of water per day. Additionally, the metabolism of foodstuffs and oxidation provide some 600 mL and 300 mL of water, respectively. Regulated water excretion consists of approximately 1 L per day via the kidneys. Daily unregulated (insensible) water loss includes pulmonary loss as humified air (0.3 L), loss in stools (0.1 L) and sweat (0.1 L). Urinary water excretion is regulated by the kidneys, to contain plasma osmolality constant within narrow limits.

8.1.4 Fluid compartments

Total body water is distributed between the extracellular (ECF) and the intracellular fluid compartment (ICF).

Tonicity

The tonicity of a solution, also called effective osmolality, is the concentration of a dissolved substance (e.g., urea) that exerts an osmotic force on the cells in this solution. In hypertonic solution the substance causes a shift of water outside the cell and the cell shrinks to reach a new steady state. In hypotonic solution the cell swells to reach a new steady state. Low-molecular organic substances such as urea, ethanol, methanol or ethylene glycol permeate the cell membrane freely like water, and therefore exert no osmotic force or displacement of water. Non-free permeable substances are Na⁺, K⁺ or glucose.

8.1.4.1 Extracellular fluid compartment (ECF)

The ECF consists of all body water outside the cells and represents 45% of the total body water. The ECF includes plasma, interstitial fluid, lymphatic fluid, fluid of dense tissues such as connective tissue and bone, and trans cellular fluid are included in the ECF /2/.

Plasma

Plasma contains water (93%) and solid constituents (7%), mainly proteins and lipids. The significant cation is Na⁺, while the significant anions are Cl^– and HCO₃^–. Since plasma proteins do not permeate intact endothelium, they are critical with regard to the osmotic pressure at the capillary membranes. Plasma colloid osmotic pressure, up to 80% of which is due to albumin, is 28 mmHg. Plasma comprises 7.5% of the total body water.

Interstitial fluid

Interstitial fluid is formed as a result of filtration across the capillary vessel wall which is highly permeable to water, electrolytes and low-molecular organic substances. Approximately 25–50% of circulating proteins are filtered daily into the interstitium; approximately 80% of the fibrinogen remain intravascular. The level and the direction of plasma capillary flow is determined by Starling’s forces. In certain diseases, for example heart failure or liver cirrhosis, changes in Starling’s forces can lead to the formation of edema. Interstitial fluid comprises 20% of the total body water.

Trans cellular fluid

Trans cellular fluids that are secreted from organs such as saliva, pancreatic juice, bile, and intestinal secretions. The sum of trans cellular fluids is approximately 2.5% of the total body water.

Water in connective tissue and bone

This inaccessible water is bound in the dense connective tissue matrix of bone and cartilage. It comprises 15% of total body water.

8.1.4.2 Intracellular fluid compartment (ICF)

In adults, the ICF contains 55% of the total body water. The ionic polarity between extracellular and intracellular fluid compartment is maintained by the Na⁺-K⁺ ATPase, which pumps intracellular Na⁺ out of cells and extracellular K⁺ into cells in a ratio of 3: 2. The intracellular and extracellular Na⁺ concentrations are 10 mmol/L and 140 mmol/L, respectively. The intracellular magnesium concentration is approximately 13 mmol/L, while its extracellular concentrations are only 1.5 mmol/L. The intracellular concentrations of Cl^– and HCO₃^– are lower than their extracellular levels, while the concentrations of phosphate and sulfate are higher intracellularly /2/.

The tendency of the cells to swell as the intracellular colloidal osmotic pressure rises due to the constant metabolic accumulation of anionic macromolecules is counteracted by the ion pumps of the plasma membrane, which transport equivalent quantities of anions out of the cells.

8.1.4.3 Changes in fluid volume

In order to understand disorders of Na⁺ and water balance, the knowledge of the interrelationships described below is important /3/.

Changes in extracellular fluid volume (ECFV)

The ECFV is directly dependent upon total body Na⁺ since Na⁺ and its anions are restricted to the extracellular fluid and are the most relevant solutes. Expansion or contraction of the ECFV activates regulatory systems with the goal of establishing an equilibrium between intake and excretion of Na⁺. On a constant intake of Na⁺, excretion matches intake. If Na⁺ intake is inadequate low or high, the kidneys perform the task of regulating the extracellular Na⁺ content. The rate of excretion occurs in response to numerous physiological controls (e.g., hormonal paracrine, nervous) which act to prevent large variations in total body Na⁺.

Changes in Na⁺ concentration of the ECFV

The Na⁺ concentration, and thereby the osmolality, of the ECFV is regulated by the circulating blood volume. Volume sensors, localized in the carotid sinus, the atria of the heart and the afferent renal arterioles, respond via negative feedback (Fig. 8.1-1 – Physiology of the water and volume homeostasis in cases of dehydration). The kidney prevents deleterious changes in electrolyte balance, ECFV, and blood pressure. It is widely accepted that small changes in the ECFV, which correlates directly with Na⁺ intake, signals adjustments in the kidney to maintain Na⁺ excretion equal to Na⁺ dietary intake. Approximately 80% of exchangeable Na⁺ are present in intestinal and connective tissues, and about 15% of exchangeable Na⁺ is in plasma /3/.

The Na⁺ concentration in the phases differ in dependence of the ECF compartment /3/:

• The thirst reflex, vasopressin and the kidney, maintain a relatively constant Na⁺ concentration in plasma
• ECF compartments (cartilage) with high concentrations of negative charged glycosaminoglycan attract Na⁺ and generate local osmotic pressure favoring swelling.

The measured marker of the concerted activity of the systems is the plasma osmolality. For the maintenance of normal osmolality, the mechanisms mentioned adapt the intra- and extracellular water volumes to the solutes. In order that the osmolality of the ECFV and, in consequence, the tonicity, are held constant within a narrow range the organism can, if thirst develops, imbibe an unlimited quantity of water or, under water load, excrete 15–20 liters/24 hours of free water.

Changes in total body Na⁺

The ECFV is regulated by the content of body Na⁺ rather than its concentration. Thus, an excess or deficit in total body water alters the osmolality and Na⁺ concentration, but contributes little to the ECFV. In fact, the tonicity of the body fluids may vary independently of ECFV. Thus, either hyponatremia or hypernatremia may occur with increased, decreased, or normal ECFV.

Salt sensitivity

One useful definition of salt sensitivity is a difference in mean arterial pressure that is 10 mmHg or greater when salt balance is altered by a combination of diet or loop diuretics /17/.

8.1.4.3.1 Reduction of the extracellular fluid volume (ECFV)

When salt intake restricts the ECFV a decrease of greater than 5% causes a reversible reduction in renal blood flow. and Na⁺ excretion. This leads to following mechanisms /3/:

• Decrease in glomerular filtration rate (GFR)
• Increase in angiotensin II, aldosterone, norepinephrine epinephrine. The neurohormones contribute to Na⁺ retention
• The neurohormones link the ECFV with cardiovascular and sympathetic effects.

A reduction in the GFR causes the rise in the filtration fraction resulting in:

• Reduced urine volume and high urine osmolality
• Na⁺ excretions of greater than 20 mmol/L, and K⁺ excretions of over 40 mmol/L in the urine
• A disproportionate rise in serum urea relative to creatinine. Normally the ratio of urea-N (mg/dL)/creatinine (mg/dL) = 10. In volume reduction, it is greater than 20 (conversion: urea × 0.357 = urea-N).
• An increase in HCO₃^– in the urine to more than 20 mmol/L due to the exchange of H⁺ for Na⁺
• An increase in serum uric acid because of decreased urinary excretion
• An increase in serum calcium if, for example, hyperparathyroidism or bone metastases are present
• The intensification of hyperkalemia in a poorly controlled diabetic, because less glucose is eliminated renally.

8.1.4.3.2 Increase of the ECFV

When dietary Na⁺ intake increases, urinary Na⁺ excretion increases but does not match intake immediately and thus creates a positive Na⁺ balance. Increasing NaCl intake also increases body water. A volume overload of more than 5% may result in edema, anasarca, pleural effusion and ascites, and occurs frequently in the critically ill. This leads to an increase in body weight, cardiac frequency and arterial blood pressure.

The counter-regulatory mechanisms lead to:

• An increase in urine volume and in Na⁺ excretion (over 20 mmol/L)
• The rise in renal K⁺ excretion (greater than 40 mmol/L), and hypokalemia in spite of suppression of the renin-angiotensin-aldosterone system, contingent upon volume expansion
• Hypouricemia; less uric acid is reabsorbed in the proximal tubule and more is secreted distally.

In chronic kidney disease, the time to reach a steady state after a dietary Na⁺ change is prolonged, making blood pressure more salt sensitive.

8.1.5 Volume homeostasis

Volume homeostasis is determined to a significant degree by the filling of the arterial circulation. The effective arterial blood volume is registered by the baroreceptors in the right atrium of the heart, in the lungs and in the aortic arch. It depends upon the balance between vasoconstrictor and vasodilator substances (Tab. 8.1-1 – Neurohormonal mechanisms with regulatory effects on volume homeostasis).

If the volume homeostasis is disturbed, the result is a change in the water and electrolyte distribution that relies on several factors /2/:

• In consequence of a reduced filling of the arterial circulation (e.g., due to vasodilation or cardiac insufficiency) compensatory vasoconstriction with the retention of Na⁺ and water results. The reasons are activation of the sympathetic nervous system, the renin-angiotensin-aldosterone system, and the increased release of ADH.
• Related to an overfilling of the arterial circulation (e.g., due to volume expansion or tachycardia) vasodilation with augmented water and Na⁺ excretion results. This is induced by atrial natriuretic peptide, by the kallikrein-kinin system, and by endothelial factors such as prostaglandins and NO.

8.1.5.1 Regulators of electrolyte and water homeostasis

Relevant regulators of electrolyte and water homeostasis are:

• The renin-angiotensin-aldosterone system
• The atrial natriuretic peptides
• The ADH thirst mechanism.

Renin-angiotensin-aldosterone system (RAAS)

A change in renin secretion occurs as a function of water and Na⁺ intake. Renin secretion is inhibited by table salt intake and stimulated by water intake. It thereby preserves the blood volume during salt and water loss, which may be modified by perspiration, diarrhea or vomiting. The concentration of aldosterone rises rapidly following a decrease in blood volume or a reduction in renal perfusion.

A similarly powerful stimulator of aldosterone synthesis is a rise in plasma K⁺, the result of which is augmented aldosterone-induced renal K⁺ excretion. In this way the RAAS prevents hyperkalemia with increased K⁺ dietary intake or following K⁺ release due to extreme muscular activity.

In chronic heart failure and liver cirrhosis, hyperaldosteronism with Na⁺ and water retention, as well as volume expansion, occur. In chronic heart failure, the hyperaldosteronism is based upon decreased renal perfusion, and in liver cirrhosis, on reduced hepatogenous metabolism of aldosterone. See also Chapter 31 – Renin-angiotensin-aldosterone system (RAAS).

Natriuretic peptides

This family consists of three structurally similar peptides (i.e., the atrial natriuretic peptide (ANP), the brain natriuretic peptide (BNP), and the C-type natriuretic peptide (CNP)). ANP and BNP are released during the stretching of cardiac muscle cells, while CNP is formed in numerous organs. The primary functions of ANP and BNP are the regulation of blood volume and blood pressure. They are released into the circulation in situations of high volume or elevated pressure. They activate the natriuretic peptide receptor A (NPR-A) in their target organs, namely, the kidneys and peripheral arteries and, as a result, intracellular concentrations of cyclic guanosine mono phosphate are increased. The consequences are natriuresis, diuresis, vasodilation and the decline of blood pressure. The effect on the kidneys is as follows: ANP and BNP cause dilation of the afferent glomerular arterioles and vasoconstriction of the efferent vessels. In this way, the GFR is increased. In the collecting ducts, ANP and BNP lead to reduced reabsorption of Na⁺ and, thereby, the excretion of Na⁺ is augmented. The natriuretic effect of ANP and BNP has two effects.

The Na⁺ excretion is increased as a result of:

• A transient increase of the GFR, coupled with an antagonistic effect on RAAS-mediated Na⁺ reabsorption in the proximal tubule
• Longer-term inhibition of Na⁺ reabsorption in the ascending limb of the loop of Henle and the collecting ducts.

ANP and BNP also suppress the release of renin and endothelin, and this represents an additional regulatory effect with regard to vascular tone.

Fluid consumption

The molecular basis of fluid consumption are:

• The thirst sensors in the brain: neurons in the lamina terminalis centrumventricular organs that are unprotected by the blood-brain barrier. These excitatory neurons drive fluid consumption rapidly as a corrective behavior.
• Extracellular volume depletion (salt loss) stimulates sets of neurons which respond to angiotensin II and aldosterone. Aldosterone-sensitive neurons express 11β-hydroxysteroid dehydrogenase type 2 (11β HSD2), conferring aldosterone selectivity. Neuronal deletion of 11β HSD2 causes persistent activation of mineralocorticoid receptors by glucocorticoids, increasing salt appetite /3/.

Only rarely is the hyponatremia based upon polydipsia, contingent upon an augmented sensation of thirst. In patients with chronic heart failure and hyponatremia, plasma hypoosmolality can be expected to suppress the secretion of arginine vasopressin. This is, however, not the case; on the contrary, persistently elevated concentrations of arginine vasopressin are found (see also Section 8.5 – Osmolality).

8.1.6 Renal regulation of water and sodium excretion

The kidney filters some 150 liters of isotonic glomerular filtrate (GFR) daily. In order to maintain the water equilibrium, the kidney can form maximally diluted as well as maximally concentrated urine.

Conceptually, renal water regulation is subdivided into the following steps /5/:

• In the proximal tubule two thirds of the GFR is reabsorbed isotonically. In situations of arterial volume depletion, an increase to 80% is possible.
• In the descending limb of the loop of Henle water is reabsorbed, while salts remain in the tubule and the osmolality can rise to 1,200 mmol/kg.
• The ascending limb of the loop of Henle and the distal tubule are relatively impermeable to water. Only salts are reabsorbed, and this is why these parts of the nephron are also called diluting segments. The osmolality of the tubular fluid can fall to below 50 mmol/kg.
• Water reabsorption is modulated by the antidiuretic hormone arginine vasopressin in the collecting ducts. If the tonicity in the ECF is inappropriately low, the secretion of vasopressin is inhibited and diluted urine is excreted. If the tonicity is high, the secretion of vasopressin is increased, the permeability to water of the collecting ducts is augmented, and the excreted urine is more concentrated. Because water reabsorption is a function of the secretion of arginine vasopressin, urine osmolality can fluctuate from less than 100 up to 1200 mmol/kg.

Refer to Fig. 8.1-2 – Renal-tubular handling of water and electrolytes.

8.1.6.1 Determinants of maximal concentration and dilution of the urine

The delivery of GFR to the diluting segments is dependent upon the volume and the composition of the GFR, and on the function of the proximal tubule. Thus, decreased formation of free water results from:

• A reduction of the GFR due to volume depletion, heart failure, the nephrotic syndrome and liver cirrhosis
• The reduction of salt transport in the diluting segments. In this way minimal osmolality is not achieved and the formation of free water is limited; this is the case in interstitial kidney disease or during treatment with loop diuretics and thiazides
• The maintenance of a corticomedullary concentration gradient in the renal interstitium. The concentration gradient begins with isotonicity at the corticomedullary junction and increases to up to 1,200 mmol/kg at the papilla. This interstitial osmotic gradient is responsible for the reabsorption of water from the collecting ducts into the blood. The development of the gradient is dependent upon effect of vasopressin on the collecting ducts. The development of a normal interstitial osmotic gradient is impaired in interstitial kidney disease, during treatment with loop diuretics, in deficient protein nutrition (reduced urea accumulation), in osmotic diuresis, and in all situations of increased urine flow.

The kidney regulates water excretion as a response to changes in osmolality and in the effective arterial blood volume. Due to the previously described regulatory mechanisms, urine volume can vary from 0.5–20 liter/24 hours. Nonetheless, neither gastrointestinal nor insensible water loss are recognized by the kidneys. Even with maximal renal reabsorption of water, daily renal and extrarenal water loss is of the order of 1 liter per day. The kidneys cannot prevent dehydration and ECF hypertonicity by themselves. Ultimate protection is provided by the thirst mechanism.

Disturbances of the acid-base balance influence the excretion of Na⁺ and K⁺. Thus, metabolic acidosis leads to natriuresis due to the inhibition of Na⁺ reabsorption in the proximal and distal tubules. The natriuresis induces an ECF volume restriction, and this leads to secondary hyperaldosteronism.

8.1.6.2 Glomerulotubular balance (GTB)

The role of the kidney in maintaining Na⁺ homeostasis is determined by the relationship between the amount of Na⁺ filtered and the quantity reabsorbed into the peritubular capillaries, which is thereby returned to the systemic circulation /6/. For any alteration in GFR, a proportional change in Na⁺ reabsorption occurs, so that a fractional reabsorption (GFR × Na⁺ serum concentration) remains constant. This is owing to the property of the kidneys termed GTB, which maintains a close coupling between GFR and tubular Na⁺ reabsorption. By this coupling wide variations in distal tubular Na⁺ delivery are kept within reasonably narrow limits.

A disturbance in the GTB, with an alteration of water and Na⁺ excretion, can be contingent upon:

• A change in kidney hemodynamics, for (e.g., in congestive heart failure) or in aortic stenosis above the renal arteries
• Intraluminal factors such as elevated concentrations of solutes like glucose, amino acids, bicarbonate and chloride
• Chronic renal insufficiency with a reduction in the number of functional nephrons. If the GFR falls, the Na⁺ excretion rate of individual nephrons increases. This adaptive mechanism is dependent upon ANP and BNP. But in spite of the elevated concentrations of these peptides, the extracellular fluid volume increases and furthermore, exogenous administration of ANP does not lead to augmented Na⁺ excretion /7/.

Patients with a GFR below 20 [mL × min^–1 × (1.73 m²)^–1] have only a low remaining capacity to concentrate urine; on the other hand, they remain sufficiently capable of diluting urine. As chronic renal insufficiency progresses, significant ECF volume expansion develops, with hypertension as a consequence.

8.1.6.3 Free water clearance

The formation and excretion of free water, also called free water clearance, is an important variable in the understanding of renal water regulation /5/.

Urine volume for the excretion of solutes is subdivided conceptually into two compartments:

• One compartment which is necessary to excrete solutes isoosmotically to the serum
• A second compartment which describes the quantity of excreted or reabsorbed electrolyte-free water.

Since urea, the substance that is excreted in the urine in the greatest quantity, does not contribute to tonicity, the sum of the Na⁺ and the K⁺ concentrations is determined as the measured variable of solutes in the urine. Electrolyte-free water and electrolyte-free urine is the remaining volume after the volume that is required for the generation of an isotonic (to plasma) volume has been subtracted. Positive and negative values represent, respectively, the excretion and the reabsorption of free water.

Sample calculation: excretion of 300 mmol/L Na⁺ + K⁺ in 2 liters of urine. Serum osmolality 280 mmol/kg. Excretion of 300/280 = 1.07 liters of isotonic (to serum) urine and 0.93 liters of electrolyte-free urine (free water). Refer to Fig. 8.1-3 – Formation of electrolyte-free water by the kidneys.

8.1.6.4 Acute renal failure (ARF)

Specific changes occur in ARF, depending upon whether pre renal, renal or post renal failure is the reason.

Pre renal acute renal failure

Diagnostic biomarkers:

• Refer to Tab. 8.1-4 – Examinations for differentiating pre renal from renal acute renal failure
• Refer to Section 8.3 – Chloride).

The oliguria that results is associated with a urine that is very highly concentrated and has a very low Na⁺ content /6/.

Renal acute renal failure

The renal form of ARF includes diverse entities. The clinical course is divided into four phases, in which different disorders of water and Na⁺ balance occur:

• Initiating phase: the initiating phase is dependent upon the extent of renal impairment. It can last, for example, for days if there is acute tubular necrosis due to damage caused by antibiotics, or only for hours in ischemic tubular necrosis. The GFR is normal, and the biochemical findings in urine correspond to those obtained in pre renal acute renal failure. The urinary Na⁺ excretion is below 20 mmol/L, the fractional Na⁺ excretion is below 1%.
• Oliguric phase: if, due to the damage, the kidneys are no longer capable of concentrating the urine, a urinary Na⁺ excretion of greater than 40 mmol/L ensues and the fractional Na⁺ excretion is over 2%. A decrease in the GFR follows, and oliguria develops. The urine osmolality corresponds to that of the plasma; it is seldom above 320 mmol/kg.
• Diuretic phase: the daily urine volume is doubled, the quantity increasing to 2–3 liters. The urine is similar to the plasma filtrate with Na⁺ and urea excretions are comparable to plasma. The latter does not decrease at the onset of the diuretic phase.
• Recovery phase: the initial recovery of GFR to about 60% of normal within days or weeks, is followed by a lower rate of recovery to full normal values over several months. Tubular function and concentrating ability are present if the fractional Na⁺ excretion is below 1%. The urine osmolality rises to greater than 600 mmol/kg, and the Na⁺ excretion in the urine is below 20 mmol/L.

Post renal acute renal failure

In bilateral obstruction of the ureters a hemodynamic disturbance develops, causing a reduction in the tubular flow rate with enhanced resorption of Na⁺ and water. The result is reduced urine volume with elevated osmolality and a urinary Na⁺ excretion below 20 mmol/L, thus, a clinical picture similar to that or pre renal acute renal failure.

8.1.7 Potassium balance

Potassium is the most abundant intracellular cation. Total body potassium is 3500 mmol, corresponding to 50 mmol/kg body weight. 90% of the potassium is found in the intracellular fluid compartment (ICF), 2% in the extracellular fluid compartment (ECF), and 8% in bone and cartilage. The ECF concentration is regulated by the K⁺ exchange between the ECF and the ICF (internal potassium homeostasis). The loss of only 1% of total body K⁺ leads to a marked disturbance of the K⁺ balance, with massive physiological changes.

The daily potassium requirement is 40–50 mmol (1.6–2.0 g; 1 mmol = 40 mg). However, the daily intake of potassium varies considerably. Thus, elderly persons often ingest too little potassium, while persons who eat a lot of fruits and vegetables have a daily intake of 200–250 mmol (8–10 g). The urban population ingests some 62.5 mmol (2.5 g) per day. The minimal requirement is estimated at 40–50 mmol (1.6–2.0 g) /9/. Prospective population studies have shown that stroke-associated mortality increases with low potassium intake /9/, and that an increase in potassium intake exerts an anti-hypertensive effect. This is based upon augmented natriuresis, improved sensitivity of the baroreceptors, direct vasodilation and reduced cardiac responsiveness to noradrenaline and angiotensin-II /10/.

The regulation of potassium that is ingested with the diet (external potassium balance) is carried out to an extent of 80% by renal excretion of K⁺, to 15% via the gastrointestinal tract and to 5% via perspiration.

8.1.7.1 Renal potassium homeostasis

Renal K⁺ excretion is dependent upon /11/:

• Potassium intake with the diet and the serum concentration of K⁺
• The delivery of Na⁺ and water to the distal tubules and the content of non absorbable anions
• The acid-base status
• The mineralocorticoid concentration.

Delivery of Na⁺ and water

In Fig. 8.1-4 – Renal-tubular K⁺ treatment, renal management of K⁺ is shown. In healthy fasting individuals the renal response to acute potassium loading is prompt, occurring within 3 hours. The K⁺ ions are secreted via the distal renal tubules and the collecting ducts. The secretion occurs via a direct stimulation of the Na⁺-K⁺-ATPase.

Variables relevant to the urinary K⁺ excretion are /12/:

• The Na⁺ content in the distal tubules. Through increased Na⁺ delivery, the shift of K⁺ to extracellular is augmented due to the increasing electronegativity of the tubular cells, if more Na⁺ is reabsorbed.
• Chloride anions which accompany Na⁺ during tubular reabsorption. The distal tubules are relatively impermeable to bicarbonate, phosphate, and sulfate. Elevated concentrations of these anions increase tubular electronegativity and, in consequence, K⁺ secretion.
• The tubular flow of water /13/: increased water delivery to the distal tubules augments the K⁺ excretion. It is also responsible for the loss of K⁺ if renal tubular sodium chloride reabsorption caused by diuretics such as thiazide and furosemide in Henle’s loop and the initial portion of the distal tubule is inhibited (Fig. 8.1-5 – Mechanism of thiazide diuretics) /14/. In contrast, in situations of more marked proximal K⁺ reabsorption, sodium chloride and bicarbonate are delivered to a lesser degree to the distal tubule, thereby resulting in decreased K⁺ secretion in exchange for Na⁺. Hence, in situations like pre renal acute renal failure, volume depletion, and congestive heart failure, which cause increased proximal tubular sodium chloride reabsorption via a reduction in the GFR, a tendency for hyperkalemia follows. Medication-dependent causes which lead to hyperkalemia are presented in
• Fig. 8.1-6 – The effect of medicines on the tubule cell of the distal nephron
• Fig. 8.1-7 – Mechanism of the aldosterone-dependent K⁺ secretion and its suppression.

Acidosis and alkalosis

In systemic acidosis the intracellular H⁺ content is increased and buffered within the cell. In order to preserve electroneutrality, K⁺ is shifted from the intracellular to the extracellular fluid compartment. In cases with decreased distal tubular K⁺ , the K⁺ shift is reduced. Thus, diminished tubular K⁺ contributes to the development of hyperkalemia in systemic acidosis.

In systemic alkalosis K⁺ is shifted from the tubular urine into the tubular cells and in this way, K⁺ secretion is enhanced. This effect, combined with increased water loading of the distal tubules in metabolic alkalosis, elevates the excretion of K⁺ and the tendency for hypokalemia.

Mineralocorticoids

Aldosterone is the essential mineralocorticoid responsible for potassium homeostasis. K⁺ act on the distal tubule and the collecting ducts and stimulate the reabsorption of Na⁺ and the secretion of K⁺. The effect of K⁺ is based upon an increase in the number of Na⁺ channels in the tubular luminal cell membrane and in the Na⁺-K⁺-ATPase of the basolateral membrane (Fig. 8.1-7 – Mechanism of the aldosterone-dependent K⁺ secretion and its suppression).

Adrenal secretion of aldosterone is stimulated by the renin-angiotensin-aldosterone system. Renin is increasingly synthesized in volume depletion and with reduced perfusion of the renal juxtaglomerular apparatus. The secretion of aldosterone is also directly stimulated by an elevation in intracellular K⁺ and is suppressed by certain medications /13/.

8.1.7.2 Extrarenal potassium homeostasis

The maintenance of the extracellular K⁺ concentrations depends, apart from renal excretion, on the following factors /11/:

• Acid-base balance
• Insulin secretion
• Mineralocorticoids
• The sympathetic nervous system.

Acid-base balance

Augmented loading caused by inorganic acids such as HCl, NH₄Cl or organic acids like lactate or ketone bodies lead to the shift of K⁺ from the intracellular to the extracellular fluid compartment, and to hyperkalemia. The situation is reversed in systemic alkalosis.

Insulin

Insulin increases the cellular uptake of K⁺ and a high K⁺ loading of the organism stimulates the secretion of insulin. The effect is, however, minimal, as long as the K⁺ loading does not coincide with a glucose load. The effect of insulin on the Na⁺-K⁺-ATPase is believed to be based upon a cyclic AMP-independent mechanism.

Insulin-stimulated K⁺ cellular uptake is independent of glucose uptake. This independent effect has to be taken into consideration in the treatment of hyperkalemia with glucose and insulin. Glucose should not be administered alone, because if the secretion of insulin is suppressed, the increased osmolality that results from the glucose administration causes a K⁺ efflux of the cells, and the hyperkalemia increases further.

Mineralocorticoids

Aldosterone also acts on non-renal tissue and can affect K⁺ homeostasis in this way. Thus, through activation of the Na⁺-K⁺-ATPase of the colon mucous membrane, the stool content of K⁺ is increased, while that of Na⁺ is reduced. The concentrations of K⁺ in the saliva and the perspiration are also increased. These mechanisms play an important role with regard to K⁺ excretion in chronic renal insufficiency.

Sympathetic nervous system

The β-adrenergic stimulation shifts K⁺ from the ECF to the ICF. Thereby, renal K⁺ excretion is decreased. β-receptor blockers have an opposing effect. The effect of β-adrenergic stimulation is independent of changes in the concentrations of aldosterone and insulin.

The effect of β-adrenergic stimulation is based, in particular, upon activation of the β₂-receptors. β₂-agonists bind to these receptors and activate, via cyclic AMP, the Na⁺-K⁺-ATPase. α-adrenergic substances mediate a contrary effect. Thus, hyperkalemia that is caused by extreme physical effort is augmented by β-blockade with propranolol, while α-blockade with phentolamine results in an attenuated elevation of K⁺.

References

1. Martin PY, Schrier RW. Renal sodium excretion and edematous disorders. In: Dluhy RG (ed). Clinical disorders of fluid and electrolyte metabolism. Endocr Metab Clin North Am 1995; 24: 459–79.

2. Knepper MA, Kwon T.H. Nielsen S. Molecular physiology of water balance. N Engl J Med 2015; 372: 1349–58.

3. Ellison DH, Welling P. Insights into salt handling and blood pressure. N Engl J Med 2021; 385: 1981–93.

4. Khaw KT, Barrett-Konner E. Dietary potassium and stroke associated mortality: a twelve year prospective study. N Engl J Med 1987; 316: 235–40.

5. Fried LF, Palevsky PM. Hyponatriemia and hypernatriemia. Med Clin North Am 1997; 81: 585–609.

6. Méndez RE, Brenner BM. Glomerulotubular balance and the regulation of sodium excretion by intrarenal hemodynamics. In: Seldin DW, Giebisch G (eds). The regulation of sodium and chloride balance. New York: Raven, 1990: 105–31.

7. Shemin D, Dworkin LD. Sodium balance in renal failure. Current Op Nephrol Hypertens 1997; 6: 128–32.

8. Subramanian S, Ziedalski TM. Oliguria, volume overload, Na balance, and diuretics. Crit Care Clin 2005; 21: 291–303.

9. Cohn JN, Kowey PR, Whelton PK, Prisant LM. New guidelines for potassium replacement in clinical practice. Arch Intern Med 2000; 160: 2429–36.

10. Krishna GG, Kapoor SC. Potassium depletion exacerbates essential hypertension. Ann Intern Med 1991; 115: 77–83.

11. Clark BA, Brown RS. Potassium homeostasis and hyperkalemic syndromes. Endocrinol Metab North Am 1995; 24: 573–91.

12. Rasteger A, DeFronzo RA. Disorders of potassium metabolism associated with renal disease. In: Schrier RW, Gottschalk CW (eds). Diseases of the kidney. Boston; Little, Brown 1993: 2649.

13. Krapf R. Drei gefährliche Elektrolytentgleisungen: Hyponatriämie, Hyperkaliämie und Hypomagnesiämie. Schweiz Med Wschr 1993; 123: 739–48.

14. Perazella MA. Drug-induced hyperkalemia: old culprits and new offenders. Am J Med 2000; 109: 307–14.

15. Krapf R. Iatrogene Hyperkaliämie. Schweiz Med Wschr 1996; 126: 626–31.

16. Halperin ML, Bohn D. Clinical approach to disorders of salt and water balance. Emphasis on integrative physiology. Crit Care Clin 2002; 18: 249–72.

17. Elijovich F, Weinberger MH, Anderson CA, Appel LJ, Bursztyn M, Cook NR, et al. Salt sensitivity of blood pressure: a scientific statement form the American Heart Association. Hypertension 2016; 68 (3): e7–e46.

8.2 Sodium

Lothar Thomas

Sodium is the most abundant cation (Na⁺) of the extracellular fluid. It is critical for the maintenance of tonicity, also known as the effective osmolality, and thereby for water distribution between the extracellular fluid compartment (ECF) and the intracellular fluid compartment (ICF). Hyponatremia and hypernatremia occur if the ratio of the amounts of water and sodium in the ECF is shifted in favor of one or the other.

A frequent cause is an increase or decrease in total body water with no change in the electrolyte. Since sodium salts make up the major portion of the solutes in the ECF, hyponatremia and hypernatremia are often associated with hypo osmolality or hyperosmolality. Dysnatremia is also a frequent finding in patients who are not critically ill. Thus in outpatients, the prevalence of hypernatremia is up to 1%, and that of hyponatremia up to 6%. The latter is associated with elevated morbidity and mortality.

8.2.1 Indication

• Disorders of fluid and electrolyte balance
• Critically ill patients
• Intra- and post-operatively
• Persistent diarrhea or vomiting
• Intake of diuretics
• Deviations of other serum electrolytes from the reference range
• Poly uric polydipsic syndrome and disorders of the thirst sensation
• Disturbances of acid-base balance
• Kidney disease, hypertension, edema
• Certain endocrine diseases(e.g., hypothyroidism, hyper mineralocorticoid syndromes, mineralocorticoid deficiency syndromes)
• High salt intake.

8.2.2 Method of determination

Direct and indirect potentiometric methods are used for the determination of Na⁺. With the direct methods the measurement is performed in undiluted blood, plasma or serum, while with the indirect potentiometric (ISE) methods and flame photometry, the samples are diluted. ISE is adapted to clinical chemistry analyzers, direct potentiometric measurement is used by blood-gas analyzers.

Flame photometer measures the concentration of the substance, while potentiometric methods determine ion activity.

Flame photometry

Principle: the emitted light intensity is measured when Na⁺ or K⁺ are excited in the flame. The light intensity is directly proportional to the number of atoms, and these in turn are directly proportional to the level of the corresponding ions in the sample. The flame photometer consists of a nebulizer, a burner and a photocell. The previously diluted sample is vaporized in the instrument and dispersed into the burner, whose flame is fed by propane or acetylene gas and compressed air /1/.

In the hot, non-luminous flame, Na⁺ emits a characteristic spectrum. Via a monochromator, the sodium-typical wavelength to be measured is selected and directed onto a photomultiplier. This creates an electric signal which, following intensification, guides the reading unit. The light emission as displayed by this unit is directly proportional to the Na⁺ concentration of the sample.

Some flame photometers function according to an internal standard principle. A lithium-containing solution is used for the dilution of calibrators, controls and patient samples; the flame emission is then measured using separate photocells for the wavelengths typical for lithium and sodium. Lithium is considered to be the internal standard and the voltage of the lithium photocell is considered as the reference voltage for the Na⁺ photocell. If the constant reference voltage changes, (e.g., due to instrument-dependent instability) the voltage of the Na⁺ photocell is compensated by this amount, and thus also the impact of potential instabilities on the analysis result.

Ion-selective electrode (ISE)

The ISE technology is employed for the determination of electrolytes (Na⁺, K⁺, Cl^–, Ca²⁺, Li⁺, Mg²⁺), of pH and of metabolites like glucose, urea, uric acid and lactate /2, 3, 4/.

Principle: the ISE measurement is a potentiometric method. The ISE comprises an ion-selective membrane in a measuring cell, which selectively permits the ion to be measured to shift into the inner electrolyte of the measurement electrode. If the ion activity of the sample is greater than that of the inner electrolyte, a positive membrane potential is generated. This is recorded by an electrode which is submerged in the inner electrolyte and measured against the constant potential of the outer reference electrode. Polymeric membranes, selected according to the ionophore that they contain, are employed. Ionophores are synthetic molecules that act as ion exchangers (e.g., for Na⁺ ETH 157, 227 or 2120 are used).

The measurement takes place within the ISE cell. An example, is illustrated in Fig. 8.2-1 – ISE unit for the determination of, for example, sodium. The cells contain the ISE electrode, which is in contact with the external reference electrode via the sample and the bridging solution.

Ion-selective electrode: the electrode consists of a flow cell in which the sample is separated from the inner electrolyte by a membrane. The inner electrode, which is made of, for example, Ag/AgCl, is submerged in the inner electrolyte.

External reference electrode: the external reference electrode is submerged in the bridging solution and is composed of an inner element, using usually Hg/HgCl₂, a filling solution, usually concentrated KCl, and a device that provides a liquid junction.

Enzymatic spectrometric determination

Principle: Na⁺ activates the enzyme β-galactosidase (EC 3.2.1.23), which catalyzes the conversion of o-nitrophenyl-β-D-galactopyranoside (ONPG) to galactose and o-nitrophenol. The o-nitrophenol is measured kinetically at 405 nm, according to the following reaction:

Because the K_m of Na⁺ for β-galactosidase is 0.1 mol/L, and since a linear relationship between the Na⁺ concentration and kinetic activity exists only at low concentrations, the sample must be pre diluted. A further possibility is, with a cryptand such as Cryptofix 221, to trap a constant fraction of the Na⁺ and thereby to obtain a good test signal for the clinically important measurement range /5/.

8.2.3 Specimen

Serum, plasma (lithium and ammonium-heparinate): 1 mL

8.2.4 Reference range

Refer to Tab. 8.2-1 – Reference intervals for sodium.

8.2.5 Clinical significance

8.2.5.1 Basic principles for interpreting the serum sodium concentration

Na⁺ is the most important electrolyte in the extracellular fluid (ECF) and K⁺ in the intracellular fluid (ICF). This asymmetrical distribution of electrolytes across the cell membrane requires the active exchange of both cations through Na⁺-K⁺-ATPase. Since the cell membrane is freely permeable to water, bodily fluids are in osmotic equilibrium.

The volume of the ECF is determined by the total body Na⁺. The concentrations of Na⁺ in the serum and in the interstitial fluid are identical. Na⁺ and its anions are responsible for more than 95% of the plasma osmotic activity. Because Na⁺ is the primary electrolyte in the ECF, it determines, based upon the flow of free water across the cell membrane, not only the osmolality of the ECF but also that of the ICF.

The distribution of water between the ICF and ECF is normally constant, with fluctuations of merely 1–2%. Acute changes in plasma Na⁺ that are not accompanied by qualitatively similar alterations in intracellular K⁺ (e.g., hyponatremia) lead to the movement of water from the ECF into the ICF. The result is cellular edema.

The organism regulates plasma Na⁺ concentrations by adjusting the water content of the ECF and by keeping total body sodium and the concentration of Na⁺ in the plasma constant within a narrow range. This takes place through drinking and by the renal excretion of free water. The intraindividual variation of serum Na⁺ level is 0.7% (e.g., approximately 1 mmol/L). It is therefore important, in order to recognize a disturbance of free water excretion, to be able to reliably detect a decrease in serum Na⁺ level of 3–4 mmol/L.

Changes in total body sodium and body water often occur together and are the most frequent cause of isotonic disturbances in Na⁺ and water balance /8/.

The ratio of serum Na⁺ to total body water is reflected in the osmolality. As a rule, changes in plasma Na⁺ concentrations go hand in hand with a comparable change in osmolality. There are, however, exceptions. While hypo osmolality is always associated with hyponatremia, hyponatremia can be associated with elevated, normal or low plasma osmolality.

The combined assessment of serum Na⁺ level and osmolality allows statements to be made regarding the distribution of water between the ECF and the ICF /9/:

• If both behave in the same manner, no increase or decrease in solutes other than Na⁺ is present. The Na⁺ concentration provides information about the distribution of water.
• The concentration of Na⁺ also determines the water distribution in the presence of freely permeable substances such as ethanol, ethylene glycol and urea. These substances can be distributed freely in the total body water, and they can elevate intracellular and extracellular osmolality. The result is a hypotonic status with a relative excess of water. In this case serum osmolality is elevated in hyponatremia.
• Osmolality alone provides information concerning the distribution of water if solutes such as glucose and mannitol, which are distributed only in the ESC, are present. Water is shifted from the ICF to the ECF. In consequence, hyponatremia is the outcome, in spite of the fact that total body Na⁺, K⁺ and water content remains unchanged. In this case serum osmolality is increased.
• Neither the Na⁺ concentration nor the osmolality provides information about the distribution of water. This is the case in situations of combined excess of a substance that is distributed only in the total body water, such as urea, and of another substance, the distribution of which is limited to the ECF, such as glucose. A situation such as this occurs in diabetics with hyperglycemia, in end stage renal failure.

8.2.5.2 Disorders of sodium and water balance with normal serum sodium

All abnormalities of the extracellular fluid volume (ECFV) that are not associated with a change in serum osmolality are included in this group of disorders. They are also known as isotonic disorders. No fluid shift between the ICF and the ECF occurs, and Na⁺ levels in the serum remain within the reference interval. The serum Na⁺ concentration is tightly controlled by thirst and arginine vasopressin.

A distinction is made between /10/:

• Isoosmolar (isotonic) dehydration. The ECFV is decreased, due to the isotonic loss of water and Na⁺. Isotonic losses occur in vomiting, diarrhea, poly uric renal failure, with enteral fistula and with the sequestration of fluid in the third space (e.g., peritonitis, pancreatitis). Clinical signs of dehydration are: orthostatic hypotension, tachycardia, thirst, dry skin and dry mucous membranes, deficient filling of the neck veins, low urine volume, and a high urinary urea/creatinine ratio.
• Isoosmolar (isotonic) hyper hydration. The ECFV is increased by the isotonic excess of Na⁺ and water. Isotonic edema is present, since the renal excretion of Na⁺ is disturbed due to cardiac insufficiency, the malabsorption syndrome, the nephrotic syndrome, decompensated liver cirrhosis, or renal insufficiency. The serum level of Na⁺ is normal. Clinical symptoms are weight increase and edema.

8.2.5.3 Hyponatremia

Hyponatremia is generally defined as Na⁺ concentration below 135 (136) mmol/L. Hyponatremia is classified as follows:

• Mild; 135 (136)–126 mmol/L
• Moderate; 125–121 mmol/L
• Severe; below 121 mmol/L.

In hospitalized patients, the prevalence of hyponatremia is 20% with a threshold value of ≤ 136 mmol/L, 6–10% with a threshold value of ≤ 135 mmol/L, 1–4% with a threshold value of ≤ 130 mmol/L, and about 3% at a threshold of ≤ 125 mmol/L /11, 12/.

It is important to establish if hyponatremia is acute or chronic (see also Tab. 8.1-3– Cases of disorders in electrolyte and water balance).

• Acute hyponatremia develops within 48 hours and is associated with neurological manifestations due to brain swelling, elevated intracellular pressure, and cerebral hypoxia. The symptoms may be mild (headache, nausea, vomiting) or severe (confusion, cramps, coma). The symptoms are the result of a water shift from the hypotonic fluid of the ECF into the more hypertonic brain cells.
• Chronic hyponatremia develops slowly (longer than 48 hours). Due to adaptive mechanisms, the effects on the brain are more moderate. It is less urgent to clarify chronic hyponatremia, but a thorough clinical and laboratory evaluation is necessary.

Hyponatremia occurs if the normal ratio of electrolytes to total body water is disturbed due to a parallel decrease in plasma Na⁺ concentrations and osmolality. In hyponatremia there is often no Na⁺ deficiency in the organism but rather, primarily, a relative water excess due to decreased renal excretion, polydipsia or the infusion of Na⁺-poor solutions. Normally the capacity of the kidneys to excrete water is greater than the possible water intake.

Hyponatremia is generally dependent upon /13/:

• Intrarenal factors that lead to a reduction in the diluting capacity in Henle’s loop, resulting in augmented free water excretion (see Fig. 8.1-2 – Renal-tubular handling of water and electrolytes).
• Due to osmotically-independent release of arginine vasopressin (AVP). Normally the secretion of AVP is very low with an osmolality of 280 mmol/L or less, and water diuresis ceases. There are, in consequence, potent stimulators which assure a continuing basic secretion of AVP and anti diuresis. In spite of the fact that serum osmolality is the most important stimulus to AVP release, this can also occur non-osmotically via the baroreceptor stimulation.

8.2.5.4 Differentiation of hyponatremia

Decreased osmolality of the ECFV is always associated with hyponatremia. Hyponatremia is, however, also seen in situations of normal or elevated osmolality of the ECFV /14/. Therefore, from the pathophysiological point of view, hyponatremia is classified according to the dependence on serum osmolality as normo-osmolar (normotonic), hyper osmolar (hypertonic) and hypo osmolar (hypotonic) forms (Fig. 8.2-2 – Differential diagnosis of hyponatremia).

From the clinical perspective the hyponatremia results from dilution or loss of Na⁺.

8.2.5.4.1 Dilutional hyponatremia

Dilutional hyponatremia, the most common form of hyponatremia, is a hypotonic disorder and caused by water retention. Total body water is in excess in comparison with the total body Na⁺, which can be low, normal, or elevated. The excessive renal water retention is the most important cause. Dilutional hyponatremia can be classified according to the state of ECFV in hyper volemic and euvolemic disorders.

Hyper volemic hyponatremia

This form occurs if free water and Na⁺ are present in excess, but the free water is more markedly increased than the Na⁺ level and, in consequence, edema develops. The three primary causes are chronic cardiac insufficiency, liver cirrhosis, and kidney disease. Acute oliguric renal failure and chronic renal insufficiency, particularly chronic glomerulonephritis, lead to hypotonic hyper hydration. The Na⁺ excretion in random urine samples are above 20 mmol/L.

Euvolemic hyponatremia

Euvolemic hyponatremia is characterized by normal or nearly normal total body Na⁺ content and increased total body water, without symptoms of volume depletion or hypervolemia (edema, ascites). Often, a persistent or intermittent increase in AVP, in response to volume stimuli or osmotic stimuli that normally suppress the secretion of AVP, occurs. The following may be present /15, 16/:

• An excess of free water due to exogenous intake, like that which occurs in the TURP syndrome following absorption of hypotonic irrigation fluids during transurethral prostatectomy. Other causes are glucocorticoid deficiency, severe hypothyroidism, thiazide diuresis-induced hyponatremia and polydipsia. Na⁺ excretion in random urine samples is above 20 mmol/L.
• Increased secretion of AVP with retention of free water. Most frequently, the syndrome of inadequate ADH secretion (SIADH or Schwartz-Bartter syndrome) is present. Urine osmolality is higher than that of the serum, and the urine Na⁺ excretions are above 20 mmol/L.

8.2.5.4.2 Pseudo hyponatremia

Hyponatremia may occur in the presence of normal plasma osmolality (pseudo hyponatremia). Na⁺ is measured by ion-selective-electrodes using either indirect potentiometry (ISE) on diluted samples on automated chemistry analyzers or by direct potentiometry on undiluted samples on blood-gas analyzers. During a hospital stay a patient may have several Na+ determinations measured by either of these methods and results may be assessed interchangeably, because discrepancies between direct and indirect potentiometry exist /45/. The causes can be:

• Severe hyperlipidemia (turbid serum). For every mg of triglycerides/dL the concentration of Na⁺ decreases by 0.002 mmol/L /16/.
• Hyperproteinemia, i.e. if multiple myeloma or Waldenström’s macroglobulinemia are known to exist. For every gram of total protein per liter above 80 g/L, the concentration of Na⁺ decreases by 0.25 mmol/L /17/.
• High concentrations of solutes like glucose /45/, mannitol, sorbitol, glycerin, maltose, glycine and contrast media, which shift water from the intracellular fluid into the extracellular fluid. This is the case in transurethral resection of the prostate (TURP) and in endoscopic extirpation of the uterus, where 1.5% glycine (2006 mmol/L) is employed as a rinse solution in a volume of 3–5 liters. This is partially reabsorbed and can lead to hyponatremia and cerebral edema in the presence of normal osmolality, in the early phase (2 hours) following the start of the operation. Osmolality is normal, while the osmotic gap is large.
• In a study /45/ increasing glucose levels caused a difference of of Na+ concentration between direct and indirect Na⁺ measurement. A glucose concentration of 131.2 mmol/L caused a difference in Na⁺ concentration by 2.15% between direct and indirect potentiometric measurements.

8.2.5.4.3 Hyponatremia due to loss of Na⁺

These forms of hyponatremia are hypovolemic, with an absolute water deficiency, but even so a relative excess of water in comparison with Na⁺ /14/.

Pathophysiologically, the following can be present /15, 18/:

• Extrarenal water loss, as is usually the case in vomiting, burns, edema and diarrhea. Additionally, third space loss in ascites or ileus plays a role. The drinking of a large amount of water following extreme sweating and the administration of plasma expanders following bleeding likewise lead to hypotonic dehydration /16/. Na⁺ excretion in random urine samples is below 20 mmol/L.
• Renal Na⁺ and water loss. Following the exclusion of diuretic ingestion, the following less common diseases should be taken into consideration: Addison’s disease, hypophyseal anterior lobe insufficiency (mineralocorticoid deficiency), chronic kidney disease (renal salt loss), metabolic alkalosis and renal-tubular acidosis. The cerebral salt wasting syndrome should also be considered. The Na⁺ concentrations in random samples are above 20 mmol/L.

8.2.5.4.4 Hyper osmolar (hypertonic) hyponatremia

The accumulation of solutes in the ECF shifts water from the intracellular to the extracellular fluid compartment, where it dilutes the Na⁺ concentration. The most common cause is hyperglycemia. For every increase of the glucose level of 100 mg/dL (5.6 mmol/L), serum Na⁺ concentration falls by 1.6 mmol/L. In acute renal insufficiency, high urea values also cause hyponatremia.

8.2.5.4.5 Hospital-acquired hyponatremia

Hyponatremia is commonly acquired in hospitals. It results from the administration of hypotonic solutions, particularly to children, in special situations (e.g., post-operatively or in gastroenteritis). The cause is water retention which is contingent upon increased secretion of AVP. Situations which disrupt the close coordination between plasma osmolality and AVP secretion are non-osmotic stimuli such as stress, nausea, pain, vomiting, and intravascular volume depletion. The accumulation of free water can lead to cerebral edema which, often in the early phase and especially in children and in patients who cannot communicate, is not recognized clinically.

The rate of the Na⁺ decrease is as critical as is its absolute level. Thus, cerebral edema developed in an 8-week-old infant with a decrease in serum Na⁺ of 6 mmol/L within 9 hours and, likewise, in a 13-month-old baby with a decrease from 137 mmol/L to 120 mmol/L within 12 hours.

8.2.5.4.6 Differential diagnostic examinations in hyponatremia

If hyponatremia is diagnosed, the findings from the following evaluations are relevant to the differential diagnosis.

Na⁺ excretion in urine

Useful for the differentiation between hyper- and euvolemic hyponatremia.

• With euvolemia and Na⁺ excretion of greater than 20 mmol/L, water intoxication within the framework of the SIADH is usually present
• If hypervolemia and hyponatremia are present, renal insufficiency (urine Na⁺ over 20 mmol/L) must be differentiated from hepatic insufficiency (urine Na⁺ below 20 mmol/L).
• If hyponatremia and hypovolemia (exsiccosis patient) are found, then either extrarenal Na⁺ loss (urine Na⁺ below 10 mmol/L) or renal loss (urine Na⁺ greater than 20 mmol/L) are present.

Serum and urine osmolality

These investigations are used to determine whether the water excretion is reduced or increased. Polydipsia can be distinguished from pseudo hyponatremia. In the former case, the urine osmolality is lower than the serum osmolality.

Lipids and total protein

Confirmation of pseudo hyponatremia. Clinical significant pseudo hyponatremia is a consideration only with triglyceride levels greater than 1,500 mg/dL (17 mmol/L).

Calculation of serum osmolality: if hyperglycemia is present, in order to confirm this as a cause of hyper osmolality (see Section 8.5 – Osmolality). If the measured serum osmolality is higher than the calculated value by more than 10 mmol/kg, then there are additional solutes, such as urea or ethanol, present in the ECF.

Determination of the free water clearance

The amount of actual excess free water (i.e., water free from solutes) is determined. See also Section 8.5.5.3 – Osmolality in the urine.

Further examinations

• AVP in suspected SIADH
• Creatinine: suspected renal insufficiency
• Cortisol: suspected Addison’s disease
• TSH: suspected hypothyroidism
• ALT, cholinesterase: suspected hepatopathy.

Diseases and disorders with hyponatremia are listed in Tab. 8.2-2 – Diseases and causes that can lead to hyponatremia.

8.2.5.5 Hypernatremia

The term hypernatremia refers to hyper osmolar (hypertonic) disorders of electrolyte and water balance. They almost always result from a deficiency in water relative to total body sodium. The thirst mechanism is the ultimate defense for the prevention of hypernatremia, which usually develops through insensible loss of water. When dietary salt intake increases, urinary sodium excretion increases, but it does not match intake immediately and thus generates a positive Na⁺ balance until excretion again equals intake /44/.

The effect on the thirst mechanism is diminished in persons with hypodipsia, in spite of the stimuli provoked by hyper osmolality and decreased arterial blood pressure. If the free water deficiency is mild and AVP secretion remains unchanged, renal regulation can compensate for the insufficient water intake and hypernatremia does not occur /11/.

8.2.5.5.1 Diseases and conditions with hypernatremia

The water deficiency can be due to (Fig. 8.2-3 – Differential diagnosis of hypernatremia):

• Loss of free water. This is the case in renal concentrating disorders caused by diuretics, osmotic diuresis or renal water loss due to decreased AVP secretion /20/.
• Loss of hypotonic fluid. Although insensible water loss in critically ill patients is immediately compensated for by the breathing of humidified air, the following possibilities for the loss of hypotonic body fluids are present: nasogastric aspiration, glucosuria, loop diuretics, high urea excretion during catabolism, parenteral nutrition, corticosteroid therapy. In spite of the free water deficiency the patients can, due to an increase in isotonic fluid, become volume overloaded /21/.
• Accidental intake of Na⁺. Thus, patients in intensive care or during operations often receive large quantities of hypertonic salt solutions for the maintenance of an adequate circulatory system function. Hypertonic NaCl or NaHCO₃ solutions are also administered within the framework of reanimation, or to compensate for metabolic acidosis. Furthermore, Na⁺-rich penicillin solutions can be responsible /10/. Volume overload and edema do not rule out the loss of free water /24/.

The incidence of hypernatremia is 0.3–1% in in-patients. While 20–40% already manifest this at admission, most acquire the hypernatremia during their in-patient stay. The typical patient admitted with hypernatremia is old, often arrives from an old age home and has an infectious disease. Alternatively small children, who as yet do not have access to water, are involved /15/.

In patients who acquire hypernatremia in the hospital, this is due primarily to iatrogenic causes related to insufficient water intake, for (e.g., in intubated patients, cases with reduced mental status, inadequate infusion of free water).

Clinically relevant are serum Na⁺ levels above 150 mmol/L. Due to ECF hyper osmolality, these lead to cellular dehydration and shrinkage with neurological symptoms like restlessness, excitability, muscle tremor, hyperreflexia; the late symptoms are cramps and coma. This is particularly the case in patients with Na⁺ concentrations greater than 160 mmol/L. Mortality due to hypernatremia is 40–55%. However, not the hypernatremia per se but rather the underlying disease is often the reason for the high mortality rate /15/.

8.2.5.5.2 Diagnostic examinations in hypernatremia

Additionally, the following examinations are required for the clarification of hypernatremia (Fig. 8.2-3 – Differential diagnosis of hypernatremia) /10/:

• Urine volume; criteria for differentiation are volume below 1,000 mL/24 h and above 2,500 mL/24 h
• Serum osmolality; decision criterion is the upper reference range value
• Urine osmolality; decision criterion is the value of 700 mmol/kg.

Severe hypernatremia with a level of greater than 155 mmol/L are mainly due to extrarenal fluid loss. Na⁺ excretions in the urine are below 20 mmol/L in these cases and urine osmolality is above 700 mmol/kg.

The constellation of findings occurring in hypernatremia are listed in Fig. 8.2-3, and the diseases and causes that go hand in hand with hypernatremia are shown in Tab. 8.2-3 – Diseases and causes that can lead to hypernatremia.

8.2.5.5.3 Therapy of hypernatremia

A 5% isotonic dextrose solution in distilled water is infused. In a person weighing 60 kg, some 200 mL of free water (0.7% of 30 liters) have to be infused for a Na⁺ increase of 1 mmol/L (0.7%), in order to compensate for the hypernatremia /21/.

8.2.6 Comments and problems

Pre-analytical effects /2/

The NCCLS recommends the following pre-analytical conditions for the determination of Na⁺ and K⁺:

• The tourniquet should not stop blood flow in the veins for more than a minute before the blood is drawn
• The blood should be drawn using a lithium heparin containing tube
• The samples should be stored at room temperature (20–25 °C) until the measurements are performed
• The separation of plasma and blood cells should be carried out within 60 minutes of sampling.

Serum, plasma, whole blood /2/

For the Na⁺ determination, no significant differences between the three specimens are seen if the determination is performed via direct ISE measurement using the same analysis system. If differences are seen they are attributed primarily to changes in the liquid-junction potential due to erythrocytes and can be minimized with the use of properly designed systems and electrolyte solutions.

Method of determination /2/

Basic methodologies in the measurement of Na⁺ are:

• Flame photometry (FP); measures the concentration of Na⁺. FP is the reference method according to a recommendation of the International Federation of Clinical Chemistry.
• Potentiometric determination with ISE. The activity of the respective electrolyte ion relative to the pure solvent (e.g., plasma water) is measured. In order to avoid confusion for the clinician, the ion activity is converted in terms of substance concentration (i.e., mmol/L). For this purpose, the manufacturers use either calibrators that are adapted to flame photometric values (supplementary assigned values) or the analyzers perform a conversion using algorithms. The determination using ISE is performed either on the undiluted sample (direct ISE) or on a previously diluted sample (indirect ISE). With indirect ISE the sample is diluted in the analysis system such that the activity of the ion to be measured corresponds very closely to that of the calibrator. Under these conditions the measured ion activity of the electrolyte is proportional to its concentration.

Differences in the measured values between direct ISE, indirect ISE and flame photometry are based upon the volume displacement effect and the binding of the electrolytes to organic and inorganic ligands.

Solvent displacement effect /2/

The difference in the Na⁺, K⁺ and Cl^– results between the direct and indirect methods is mainly due to the electrolyte exclusion effect. This effect is caused by the solvent displacement effect of lipids and proteins in the plasma. The electrolytes Na⁺, K⁺ and Cl^– are exclusively in the water phase of plasma. Because the volume fraction of plasma water is 93% indirect ISE and the flame photometer produce results that are 7% lower than the direct ISE method. In one study /22/, 16% more samples with protein concentrations of over 80 g/L indicated pseudo hyponatremia with indirect ISE measurements, if direct ISE was taken as the reference. The difference in the Na⁺ level between direct and indirect ISE measurements in hyperproteinemia is believed to be calculable according to the following equations /23/:

Difference (mmol/L) = 0.0196 × total protein (g/L) – 5.9528
Difference (%) = [0.0849 total protein (g/L) – 4.1199] × 100

Ligand binding

The concentration of Na⁺ in undiluted samples is calculated from the potential of its relative molal activity. The molal activity derived concentration of Na⁺ is, however, higher by 1.5% in comparison with free Na⁺ since Na⁺ is also bound to ligands (bicarbonate and proteins).

Comparison of direct and indirect methods

Direct and indirect ISE measurements, and flame photometry, are compared with one another using correction factors.

In accordance with the NCCLS /2/ this takes place using normal plasma specimens. These specimens are defined as having mass concentration of plasma water 0.93 ± 0.01 kg/L, total CO₂ 24 ± 2 mmol/L, total protein 63–79 g/L, albumin 35–50 g/L, cholesterol 150–250 mg/dL (3.9–6.5 mmol/L), triglycerides 50–150 mg/dL (0.57–1.71 mmol/L), pH 7.35–7.45.

Interference factors /2/

Changes in the concentrations of analytes beyond the reference interval lead to a difference in the direct ISE and indirect ISE measured values. Only direct ISE, that is to say, measurement on undiluted samples, reflects the pathophysiological status of Na⁺ in the plasma water.

The comparison of capillary and venous blood via direct ISE measurement shows a good correlation but, nonetheless, with a value for capillary blood that is, on average, 1.7 mmol/L lower.

There may be a poor correlation between capillary blood, estimated with a direct ISE measurement using a point-of-care testing (POCT) analyzer, and venous plasma, determined in a central laboratory using indirect ISE. Thus, the mean difference was only 0.6 mmol/L, but the variation in the 2.5th and 97.5th interval of all values was, nonetheless, 10.6 mmol/L /24/.

Hyperlipidemia, hyperproteinemia: with flame photometry and indirect ISE, too low Na⁺ values are obtained.

Hemolysis /2/: the Na⁺ concentration in erythrocytes is one-tenth that of plasma. For example, hemolysis resulting in plasma hemoglobin of 500 mg/dL would decrease a 140-mmol/L plasma Na⁺ concentration by only 0.4%. Na⁺ concentrations are markedly effected only by severe hemolysis (> 1 g Hb /dL).

References

1. Hermann R, Onkelinx C. Quantities and units in clinical chemistry: nebulizer and flame properties in flame emission and absorption spectrometry. J Clin Chem Clin Biochem 1985; 23: 365–71.

2. NCCLS. Standardization of sodium and potassium ion selective electrode systems to the flame photometric reference method; approved standard. NCCLS Document C29-A, Vol 15 No 1. Villanova: NCCLS, 1995.

3. Burnett RW, Covington AK, Fogh-Andersen N, Külp­mann WR, Lewenstam A, Maas AHJ, et al. Use of ion selective electrodes for blood-electrolyte analysis. Recommendations for nomenclature, definitions and conventions. JIFCC 1997; 9: 16–22.

4. Lewenstam A. Design and pitfalls of ion selective electrodes. Scand J Clin Lab Invest 1994; 54, Suppl 217: 11–19.

5. Berry MN, Mazzachi RD, Peake MJ. Enzymatic determination of sodium in serum. Clin Chem 1988; 34: 2295–8.

6. Payne RB, Levell MJ. Redefinition of the normal range for sodium. Clin Chem 1968; 14: 172–8.

7. Soldin SJ, Brugnara C, Wong EC. Pediatric reference ranges. Washington: AACC Press, 2003: 170.

8. Lin M, Liu DJ, Lim IT. Disorders of water imbalance. Emerg Med Clin N Am 2005; 23: 749–70.

9. Kapsner CO, Tzamaloukas AH. Unterstanding serum electrolytes. Postgrad Med 1991; 90: 151–61.

10. Sterns RH. Disorders of plasma sodium – causes, consequences, and correction. N Engl J Med 2015; 55–65.

11. DeLuca L, Klein L, Udelson JE, Orlandi C, Sardella G, Fedele F, et al. Hyponatremia in patients with heart failure. Am J Cardiol 2005; 96, suppl: 19L–23L.

12. Clayton JA, Le Jeune IR, Hall IP. Severe hyponatriaemia in medical patients: aetiology, assessment and outcome. QJM 2006; 99: 505–11.

13. Anderson RJ, Chung HM, Kluge R, et al. Hyponatremia: A prospective analysis of its epidemiology and the pathogenetic role of vasopressin. Ann Intern Med 1985; 102: 164–8.

14. Ellison DH, Felker GM. Diuretic treatment in heart failure. N Engl J Med 201/; 377: 1964–75.

15. Fried LF, Palevsky PM. Hyponatriemia and hypernatriemia. Med Clin North Am 1997; 81: 585–609.

16. Decaux G, Musch W, Soupart A. Hyponatriemia in the intensive care: From diagnosis to treatment. Acta Clinica Belgica 2000; 55: 68–78.

17. Sterns RH, Ocdol H, Schrier RW, Narins RG. Hypo­Natremia: Pathophysiology, diagnosis, and therapy. In: Narins RG (ed). Clinical disorders of fluid and electrolyte metabolism. New York; McGraw Hill 1995: 615–883.

18. Passare G, Viitanen M, Törring O, Winblad B, Fastbom J. Sodium and potassium disturbances in the elderly. Clin Drug Invest 2004; 24: 535–44.

19. Long CA, Marin P, Bayer AJ, Shetty HGM, Pathy MJS. Hypernatremia in an adult in-patient population. Postgrad Med J 1991; 67: 643–5.

20. Androgue H, Madias NE. Hypernatremia. NEJM 2000; 342: 1493–8.

21. Anonymous. Hypernatremia in the intensive care unit: Instant quality – just give water. Crit Care Med 1999; 27: 1041–2.

22. Lang T, Prinsloo P, Broughton AF, Lawson N, Marenah CB. Effect of low protein concentration on serum sodium measurement: pseudohypernatraemia and pseudonormonatraemia. Ann Clin Biochem 2002; 39: 66–7.

23. Jones BJ, Twomey PJ. Relationship of the absolute difference between direct and indirect ion selective electrode measurement of serum sodium and total protein concentration. J Clin Pathol 2008; 61: 645–7.

24. Loughrey CM, Hanna EV, McDonnell M, Archbold GP. Sodium measurement: effects of differing sampling and analytical methods. Ann Clin Biochem 2006; 43: 488–93.

25. Schrier RW, Bertl T. Disorders of water metabolism. In: Schrier RW (ed). Renal and electrolyte disorders, 2nd ed. Boston: Little, Brown, 1980: 1–64.

26. Hsu JL, Chiu JS, Lu KC, Chau T, Lin SH. Biochemical and etiological characteristics of acute hyponatremia in the emergency department. J Emergency Med 2005; 29: 369–74.

27. Klein L, O’Connor CM, Leimberger JD, Gattis-Stough W, Pina IL, Felker M, et al. Lower serum sodium is associated with increased short-term mortality in hospitalized patients with worsening heart failure. Circulation 2005; 111: 2454–60.

28. Lee WH, Packer M. Prognostic importance of serum sodium concentration and its modification by converting enzyme inhibition in patients with severe chronic heart failure. Circulation 1986; 73: 257–67.

29. Rich MW, Beckham V, Wittenberg C, Leven CL, Freedland KE, Caney RM. A multidisciplinary intervention to prevent the readmission of elderly patients with congestive heart failure. N Engl J Med 1995; 333: 1190–5.

30. Gheorgiade M, Adams KF, O’Connor CM. Improvement of hponatremia during hospitalization for worsenening of heart failure is associated with improved outcomes: Insights from the Acute and Therapeutic Impact of Vasopressin Antagonist in Chronic Heart Failure (ACTIV in CHF). J AM Coll Cardiol 2005; 45: suppl 145A.

31. Clayton JA, Rodgers S, Blakely J, Avery A, Hall IP. Thiazide diuretic prescription and electrolyte abnormalities in primary care. Br J Clin Phamacol 2006; 61: 87–95.

32. Ring T, Frische S, Nielsen S. Clinical review: renal tubular acidosis – a physicochemical approach. Crit Care 2005; 9: 573–80.

33. Oliver JA, Verna EC. Afferent mechanisms of sodium retention in cirrhosis and hepatorenal syndrome. Kidney Int 2010; 77: 669–80.

34. Biccins SW, Rodriguez HJ, Bacchetti P, Bass NM, Roberts JP, Terrault NA. Serum sodium predicts mortality in patients listed for liver transplantation. Hepatology 2005; 41: 32–9.

35. Machek P, Jirka T, Moissl U, Chamney P, Wabel P. Guided optimization of fluid status in haemodialysis patients. Nephrol Dial Transplant 2010; 25: 538–44.

36. Wizemann V, Rode C, Chamney PW, et al. Fluid overload and malnutrition assessed with bioimpedance spectroscopy are strong predictors of mortality in hemodialysis patients. Nephrol Dial Transplant Plus 2008; 1, suppl 2: ii16–ii17.

37. Howard RL, Bichet DG, Schrier RW. Pathogenesis of hypernatremic and polyuric states. In: Seldin DW, Giebisch G (eds). Clinical disturbances of water metabolism. New York: Raven, 1993: 189–209.

38. Siegel AJ, d’Hemecourt P, Adner MM, Shirey T, Brown JL, Lewandrowski KB. Exertional dysnatremia in collapsed marathon runners. Am J Clin Pathol 2009; 132: 336–40.

39. Darmon M, Timsit JF, Francais A, Nguile-Makao M, Adrie C, Cohen Y, et al. Association between hypernatraemia acquired in the ICU and mortality: a cohort study. Nephrol Dial Transpl 2010; 25: 2510–5.

40. Konetzny G, Bucher HU, Arlettaz R. Prevention of hypernatraemic dehydration in breastfed newborn infants by daily weighing. Eur J Pediatrics 2008; 10.1007/s00431-008-0841-8.

41. Palmer BF,Clegg DJ. Electrolyte disturbances in patients with chronic alcohol-use disorder. N Engl J Med 2017; 377: 1368–77.

42. Berl T. Impact of solute intake on urine flow and water excretion. J Am Soc Nephrol 2008; 19: 1076–8.

43. Karin A, Brinc D, Leung F, Jung BP. Inaccuracy of sodium measurement in patients with severe hypernatremia. JALM 2021; 6 (2): 463–7.

44. Ellison DH, Welling P. Insights into salt handling and blood pressure. N Engl J Med 2021; 385: 1981–93.

45. Lefevre CR, Gilbert C, Maucorps L, Vase J, Michel M, Chupin M, et al. Pseudohyponatremia: interference of hyperglycemia on indirect potentiometry. doi: 10.1515/cclm-2022-0766.

8.3 Chloride

Lothar Thomas

Chloride is the essential anion (Cl^–) in the extracellular fluid compartment (ECF); it makes up 2/3 of all of the ECF anions, the concentration of which is 154 mmol/L. Chloride, as the counter ion to Na⁺ plays an essential role in the maintenance of the distribution of water between the ECF and the intracellular fluid compartment (ICF). See Section 8.1.5 – Volume homeostasis.

8.3.1 Indication

• Disturbances of acid-base balance
• Disturbances of Na⁺ and water balance
• Acute situations in intensive internal, surgical and pediatric medicine
• Calculation of the anion gap
• Determination of the strong ion difference.

8.3.2 Method of determination

Ion-selective electrode

Principle: the ion-selective electrode contains a silver chloride crystal which is separated from the specimen by a membrane. The Cl^– in the specimen shifts across the membrane and become embedded in the crystal grid structure. During this process, a potential is created, which is measured against the constant voltage of a reference electrode. The difference of the potential is proportional to the Cl^– concentration in the specimen.

Coulometric titration at the chloride meter

Principle: silver ions are released from metallic silver in an acid buffer by means of electrolysis. Together with the Cl^– in the specimen, these silver ions form a silver chloride precipitate. The turnover is quantitatively determined by using the measurement set-up described here in detail. The set-up consists of two generator silver electrodes and two measuring electrodes. All four electrodes are submerged in an acid buffer. In order to measure the Cl^– concentration a defined sample volume is added to the buffer, a stabilized voltage is attached to the silver electrodes, and a chronometer is started. A constant amount of silver anions is released per unit of time, leading to the precipitation of Cl^– in the form of silver chloride. In order to detect the titration endpoint, the conductivity of the buffer is continuously monitored by means of two measuring electrodes. The titration endpoint is reached when free silver ions occur. The required time period is proportional to the Cl^– concentration in the specimen. In modern instruments, the impulse rate of the time counter has been adjusted so that the results are shown in mmol/L.

Mercurimetric titration

Principle: the Cl^– concentration of the specimen is titrated in an acidic solution with mercury (II) nitrate in the presence of the indicator diphenylcarbazone. Excessive mercury and the indicator react to yield a purple color, thus indicating the endpoint titration.

Spectrophotometric determination

Reaction with mercury chloranilate

Principle: Cl^– of the sample release equivalent amounts of the purple-colored chloranilinic acid from mercury chloranilate. The color intensity is proportional to the Cl^– concentration in the specimen and is measured spectrophotometrically at 500 nm.

Reaction with mercury thiocyanate

Principle: Cl^– of the sample release equivalent amounts of thiocyanate from mercury thiocyanate. Together with iron ions, thyocyanate forms a red-colored complex. The color intensity of the complex is proportional to the Cl^– concentration in the specimen and is measured spectrophotometrically at 480 nm.

Reference method

Inductively coupled plasma-isotope dilution mass spectrometry (ICP-IDMS).

8.3.3 Specimen

Serum, heparin anticoagulated blood: 1 mL

8.3.4 Reference range

Refer to references /1, 2/ and Tab. 8.3-1 – Reference intervals for chloride.

8.3.5 Clinical significance

Chloride (Cl^–) is the most important anion from Na⁺ and is co-responsible for the extracellular fluid volume (ECFV) and the osmolality of the plasma /3/. The Cl^– follow the Na⁺ passively whenever the Na⁺ concentration changes in the body compartments. Cl^– is thereby also subject to the influence through mechanisms that are responsible for volume homeostasis. The Cl^– concentration in serum often behaves in a manner parallel to that of Na⁺. Refer to Tab. 8.1-1 – Neurohormonal mechanisms with regulatory effects on volume homeostasis.

Cl^– and HCO₃^– in plasma often have a reciprocal relationship. This holds true for both acidosis and alkalosis, and has led to the terms hyperchloremic acidosis and hypochloremic alkalosis. Plasma HCO₃^– concentrations are regulated by the strong ion difference, where Cl^– and lactate are the strong ions.

8.3.5.1 Strong ion difference (SID)

Cl^– belongs, with Na⁺, K⁺, Ca²⁺ and Mg²⁺ to the group of strong ions, in contrast to albumin, phosphate and HCO₃^–, which are weak ions /4/. Some authors count, apart from Cl^–, lactate as well among the strong anions. Strong ions, but not weak ions, are completely dissociated in plasma. In plasma, the strong cations outnumber the strong anions. The difference between strong cations (mainly Na⁺) and strong anions (predominantly Cl^–) is 40–42 mmol/L. For the maintenance of the principle of electroneutrality, the remaining negative charges come from CO₂ and from weak acids.

Calculation of the SID without lactate:

SID (mmol/L) = Na⁺ (mmol/L) + K⁺ (mmol/L) + Ca⁺⁺ (mmol/L) + Mg⁺⁺ (mmol/L) – Cl^– (mmol/L)

The SID has a strong electrochemical effect on water dissociation and thereby on the plasma H⁺concentration:

• With an increase in SID the concentration of H⁺ decreases, the HCO₃^– in the plasma and the pH increase
• When the SID declines, acidosis increases and HCO₃^– in the plasma decreases.

Strong anions and cations are ingested with the diet in similar quantities. The regulation of their concentrations in the body, their excretion, and thereby the SID value is determined by the kidneys. The kidneys regulate the accumulation of acids via the excretion of Cl^–. In order that Na⁺ and K⁺ are spared, renal ammonia formation is utilized and Cl^– in the form of NH₄Cl is excreted.

In vomiting, the loss of Cl^– and not the loss of H⁺ is the determining factor with regard to plasma pH. If Cl^–, as a strong anion, is lost without the simultaneous loss of a strong cation, the SID is raised, the H⁺ concentration in the plasma is reduced, and the pH is increased. If H⁺ is lost as water, and not as HCl, neither the SID nor the pH change.

8.3.5.2 Chloride level in metabolic acidosis

Metabolic acidosis is the consequence of a reduction of the SID due to the increase in Cl^– (Cl^– loss is less marked than Na⁺ loss). Due to electrochemical forces, increased H⁺ is released from water and its concentration is raised; the pH decreases. A situation such as this occurs in:

• Conditions which are associated with the formation of free anions (ketone bodies, lactate)
• Cation loss (diarrhea)
• Renal-tubular disorders (renal-tubular acidosis)
• Iatrogenic acidosis (intake of acids and poisons).

The differentiation of renal and non-renal causes of metabolic acidosis are listed in Tab. 8.3-2 – Differential diagnosis of the metabolic acidosis by determining the SID in the urine. The differentiation of diseases and disorders that go hand in hand with elevated serum Cl^– and a decrease in the SID is provided in Tab. 8.3-3 – Diseases and disorders associated with an increase in serum chloride.

8.3.5.3 Chloride level in metabolic alkalosis

Metabolic alkalosis results from inappropriately high SID and is based upon a marked loss of Cl^– relative to Na⁺. Causes are:

• Vomiting, diuretics
• High exogenous intake of strong cations relative to strong anions (large volumes of banked blood).

Refer to Tab. 8.3-4 – Diseases associated with a decrease in serum chloride.

In all forms of metabolic alkalosis with an increase in HCO₃^– a corresponding decrease of other anions occurs, particularly of Cl^–.

Two forms of hypochloremic metabolic alkalosis are distinguished from one another:

• Cl^–-sensitive form, which can be corrected with salt intake and which occurs in vomiting or medication with diuretics, due to the loss of H⁺ and Cl^– and also due to the loss of Cl^– in the stools because of villous adenoma or congenital factors. Significant characteristics are a decrease of the ECFV and urine Cl^– concentrations greater than 20 mmol/L.
• Cl^–-resistant form, which cannot be corrected with salt administration and is observed in hyperaldosteronism and in the Bartter syndrome. The Cl^– excretion in the urine corresponds to its intake.

8.3.6 Comments and problems

Possibilities for methodological errors

Bromide and iodide interfere with the Cl^– determination. Results with chloride meter determinations are elevated simply by the extent of the halogen concentration (additive effect). Major errors occur with measurements using ion selective electrodes and the photometric method (multiplicative effect) /5/.

ISE measurements /12/

Measurement of Cl^– by direct and indirect methods lacks harmonization and exceeds the desirable bias based on biological variation. Accuracy of indirect Cl^– electrodes of ISE platforms show a HCO₃^– concentration-dependent systemic bias compared to the ICP-IDMS method. All BGA analyzers show a HCO₃^– dependent bias for Cl^– from the reference method ranging from –3.8% for the low HCO₃^– concentrations to +4.85 for samples with high HCO₃^– concentrations.

BGA (blood gas analysis) overestimates Cl^– compared to the reference method in high HCO₃^– concentration samples. The Cl^– in BGA shows variation in bias between different platforms.

Stability

In serum up to 1 week in a closed tube; prompt separation of the serum (plasma) following blood sample collection is necessary, otherwise erroneously low Cl^– concentration is determined.

References

1. Beeler MF. SI-units and the AJCP. AJCP 1987; 87: 140–51.

2. Soldin SJ, Brugnara C, Wong EC. Pediatric reference ranges. Washington; AACC Press, 2003: 51.

3. Weinstein AM. Sodium and chloride transport. In: Seldin DW, Giebisch G (eds). The kidney: physiology and pathophysiology, 2nd ed. New York; Raven 1992: 1925–2039.

4. Kellum JA. Determinants of plasma acid-base balance. Crit Care Clin 2005; 21: 329–46.

5. Story DA, Morimatsu H, Bellomo R. Hyperchloremic acidosis in the critically ill III: one of the strong acidoses? Anesth Analg 2006; 103: 144–8.

6. Ring T, Frische S, Nielsen S. Clinical review: renal tubular acidosis – a physiochemical approach. Crit Care 2005; 9: 573–80.

7. Stokes JB. Potassium intoxication: pathogenesis and treatment. In: Seldin DW, Giebisch G (eds). The regulation of potassium balance. New York: Raven, 1989: 269–301.

8. Emancipator K, Kroll MH. Bromide interferences: is less really better? Clin Chem 1990; 36: 1470–3.

9. Bhandari S, Turney JH. The molecular basis of hypothalaemic alkalosis: Bartter’s and Gitelman’s syndromes. Nephron 1998; 80: 373–9.

10. Clive DM. Bartter’s syndrome: the unresolved puzzle. Am J Kidney Dis 1995; 6: 813–23.

11. Zimmermann J, Reincke M, Schramm L, Harlos J, Allolio B. Das Gitelman-Syndrom – eine Differentialdiagnose zum Bartter-Syndrom. Med Klin 1994; 89: 40–4.

12. Kootstra-Ros JE, van der Hagen EAE, van Schrojenstein Lantman M, Thelen M, van Berkel M. (In)direct chloride ISE measurements, room for improvement. Clin Chem Lab Med 2022; 60 (7): e168–e171.

8.4 Anion gap

Lothar Thomas

The anion gap in plasma or serum equals the difference between the serum concentrations of the major cation (Na⁺) and the major measured anions (Cl^– and HCO₃^–). The anion gap is a useful tool for identifying the cause of a metabolic acidosis and is of value when evaluating a variety of unmeasured anions in conditions such as monoclonal gammopathy or bromism. The normal anion gap results from anions such as phosphate, sulphate, organic acids and anionic proteins, of which albumin is the most important.

8.4.1 Indication

Diagnostic workup of patients with metabolic acidosis e.g.,

• Inborne errors of metabolism
• Lactic acidosis
• Uremia
• Toxicity from methanol, ethylene glycol, isopropanol, diethylene glycol, paraldehyde, salicylates
• Dysproteinemias such as hypoalbuminemia, multiple myeloma, polyclonal hypergammaglobulinemia.
• Bromism.

8.4.2 Method of determination

The anion gap is mostly calculated by subtracting the concentrations of anions Cl^– and bicarbonate HCO₃^– from the concentration of the cation sodium Na⁺.

Anion gap (mmol/L) = Na⁺ (mmol/L) – Cl^– (mmol/L) – HCO₃^– (mmol/L)

8.4.3 Specimen

Serum: 1 mL

8.4.4 Reference interval

Anion gap: 3–11 mmol/L /1/

Clinical laboratorians need to establish (or at least verify) the anion gap reference interval for the instrumentation used in their laboratory /2/.

8.4.5 Clinical significance

8.4.5.1 Metabolic acidosis

When organic acids enter the extracellular fluid, the dissociated H⁺ reacts with HCO₃^- to generate CO₂ and water. As a consequence , the HCO₃^- concentration decreases and the salt level of the organic acid concentration increases; this accounts for the increase in the anion gap. The excretion of the organic acid salt into the urine with sodium or potassium (rather than hydrogen and ammonium) produces contraction of the extracellular fluid volume and stimulates renal retention of dietary sodium chloride /3/.

Acidemia should not be confused with acidosis. Acidemia refers to a blood pH less than 7.40 /4/.

If the baseline anion gap is low, it might not rise above the upper reference limit despite considerable accumulation of organic acid anions. Also, the anion gap rises as metabolism progresses.

The anion gap differentiates two types of metabolic acidosis:

• normal anion gap acidosis; decreased HCO₃^– concentration is compensated by increased Cl^– concentration
• elevated anion gap acidosis; the concentration of other anions than Cl^– or HCO₃^– is increased.

8.4.5.2 Normal anion gap metabolic acidosis

The metabolic acidosis results from:

• An increase of both Cl^– and H⁺
• A decrease of HCO₃^– with the retention of Cl^–.

In both cases electroneutrality is preserved.

Patients with normal anion gap metabolic acidosis may have /5/:

• Primary respiratory alkalosis (with secondary metabolic acidosis)
• Primary metabolic acidosis with the etiologies described in Tab. 8.4-1 – Normal anion gap metabolic acidosis.
• Other causes with a reduction in HCO₃^–. Diarrhea is the prototype of this group. Due to intestinal loss of HCO₃^– increased tubular reabsorption of Cl^– occurs, with resulting hyperchloremia, thus the HCO₃^– loss is compensated for. The anion gap remains normal.

A normal anion gap acidosis may occur with several toxins that produce an anion gap; therefore, a normal anion gap should not be used to exclude a possible cause of metabolic acidosis /4/.

8.4.5.3 Elevated anion gap metabolic acidosis

This acidosis occurs when an organic acid is associated with an unmeasured anion (e.g. lactate, toxic alcohol) /4/. An increased anion gap occurs only if electrically neutral substances e.g., salts of organic acids, are formed. Increased anion gaps are associated with renal failure, ketoacidosis, lactic acidosis and with poisoning. Conditions of metabolic acidosis with increased anion gap are described in Tab. 8.4-2 – Metabolic acidosis with increased anion gap.

The metabolic pathways of toxic alcohols are described in Tab. 8.4-3 – Metabolic pathways of toxic alcohols.

References

1. Winter SD, Pearson JR, Gabow PA, et al. The fall of the serum anion gap. Arch Intern Med 1990; 150: 311–3.

2. Roberts WL, Johnson RD. The serum anion gap. Has the reference interval really fallen? Arch Pathol Lab Med 1997; 121: 568–72.

3. Palmer BF,Clegg DJ. Electrolyte disturbances in patients with chronic alcohol-use disorder. N Engl J Med 2017; 377: 1368–77.

4. Hoshitsuki K, Molinelli A, Inaba H, Rubnitz J, Barker PJ. Metabolic acidosis in a peditric patient with leukemia and fungal infection. Clin Chem 2020; 66, 4: 518–24.

5. Walmsley RN, White HG. Normal anion gap (hyperchloremic) acidosis. Clin Chem 1985; 31: 309–13.

6. Hoshitsuki K, Molinelli AR, Inaba H, Rubnitz JE, Barker P. Metabolic acidosis in a pediatric patient with leukemia and fungal infection. Clin Chem 2020; 66 (4): 518–24.

7. Felton D, Ganetsky M, Berg AH. Osmolal gap without anion gap in a 43-year-old man. Clin Chem 2014; 60: 446–50.

8. Adams BD, Bonzani TA, Hunter CJ. The anion gap does not accurately screen for lactic acidosis in emergency department patients. Emerg Med J 2006; 23: 179–82.

9. Kraut JA, Mullins ME. Toxic alcohols. N Engl J Med 2018; 378: 270–80.

8.5 Osmolality in serum and urine

Lothar Thomas

If two aqueous solutions with different concentrations of solutes are separated by a semi-permeable membrane, water will flow across the membrane from the compartment with the low solute concentration into that with the higher concentration. This shift of water is called osmosis and the pressure that is required to stop the flow of water is called osmotic pressure. Osmotic pressure is determined by the number of particles and is independent of their molecular structure /1/. The number of particles depends upon their dissociation in water. A NaCl solution exerts an osmotic pressure that is double that of a glucose solution of the same molarity. The unit of osmolality is the osmol and, in accordance with the SI system, is expressed as mmol/kg. The measurements are made predominantly in serum, plasma and urine.

Calculation of osmolality

The formula that is acceptable for clinical purposes is as follows /2/:

mmol/kg = 2 × Na⁺ (mmol/L) + urea-N (mg/dL) + glucose (mg/dL)/18

or

mmol/kg = 2 × Na⁺ (mmol/L) + urea-N (mg/dL) + glucose (mmol/L)

Tonicity

In clinical usage the expressions osmolality and tonicity are often considered to be synonyms. Attention should be paid to the fact that osmolality is a physical property, relating to all of the particles in a solution, while tonicity is determined by the selectivity of the biological membrane. In this regard urea, alcohol and acetone permeate freely across the cell membrane, they therefore have no effect on tonicity, but they do increase osmolality. Tonicity describes the distribution of water between two compartments.

Colloid osmotic pressure

The term colloid is used to describe particles in solution of molecular weight greater than 30 kDa. Colloid osmotic pressure, also known as oncotic pressure, describes the pressure required to maintain two solutions which are separated by a semi-permeable membrane, and one of which is a colloid, in equilibrium. The measurement of colloid osmotic pressure in pulmonary edema fluid is of prognostic value in critically ill patients /3/.

8.5.1 Indication

Serum

• Assessment of the distribution of water between the intra- and extracellular fluid compartments (tonicity) with serum Na⁺ concentrations outside the reference interval
• Disturbances of water metabolism (e.g., in suspected diabetes insipidus, primary polydipsia, water intoxication or hypodipsia)
• Suspicion of non-ionic low molecular weight substances in the blood, particularly in suspected intoxications e.g., toxic alcohols
• Recognition of pseudo-hyponatremia
• Investigation of the osmotic gap and of free water clearance.

Urine

• Evaluation of poly uric states
• Assessment of renal concentrating capacity
• As part of a water loading test or a standard fluid deprivation test
• Investigation of free water clearance.

8.5.2 Method of determination

Freezing point osmometer, vapor pressure osmometer, and colloid osmotic pressure osmometer are employed /4/. Most frequently used is the freezing point osmometer.

Freezing point osmometer

Principle: the osmometer consists of a cooling element and an electric thermometer whose resistance is proportional to the temperature. Initially the specimen is cooled. Then, using a vibrator, the process of crystallization is initiated. During this process, warmth is generated, the temperature rises and reaches a plateau below the freezing point which is compared to the plateau of known standard solutions. Finally, the measuring scale of the instrument directly displays osmolality.

8.5.3 Specimen

Serum, heparin anticoagulated blood, urine: 1 mL

8.5.4 Reference interval

Refer to references /5, 6/ and Tab. 8.5-1 – Reference intervals for osmolality.

8.5.5 Clinical significance

Poisonings by toxic alcohols (methanol, ethylene glycol, isopropanol, diethylene glycol, and propylene glycol refer to Ref. /15/ and Tab. 8.4-2 – Metabolic acidosis with increased anion gap.

8.5.5.1 Osmolality in serum and plasma

The osmolality of the plasma and its essential determinant, the Na⁺ concentration, are kept constant within a narrow range. In spite of greatly varying intake of water and salt, as well as of other solutes, only changes of ± 2% from the mean plasma osmolality of 287 mmol/kg occur physiologically. This constancy is ensured through the concerted action of two feedback systems which regulate total body water and thereby counteract a change in the concentration of Na⁺, its anions and the osmolality /6, 7, 8/.

The objective is the maintenance of normal water distribution between the intracellular fluid compartment (ICF) and the extracellular fluid compartment (ECF).

The two feedback systems that regulate water balance are:

• The secretion of arginine vasopressin (AVP). At a plasma osmolality of below 280 mmol/kg, AVP is not secreted. The result is water diuresis, in consequence plasma osmolality rises, accompanied by a linear increase in plasma AVP level, thus renal water excretion is restrained. Volume status is less relevant with regard to AVP secretion. The most important role of AVP secretion is to protect the organism from water intoxication.
• The thirst mechanism. An increase in plasma osmolality to above 290 mmol/kg activates the thirst mechanism. Under such conditions, the ingestion of water results in the normalization of plasma osmolality, a complete inhibition of thirst, and a reduction in AVP secretion. The most important function of the thirst mechanism is the prevention of dehydration. The thirst mechanism by itself is capable of maintaining the plasma osmolality, if adequate quantities of water are available for oral intake.

Changes in plasma osmolality due to water loss or water intake result in a readjustment of the water distribution between the ECF and the ICF. This implies that either cellular edema or cellular dehydration occurs. Volume changes affecting nerve cells result in fatal neuropsychiatric symptoms.

The clinical symptoms accompanying abnormal plasma osmolality depend upon the etiology, the abruptness of the change and the nature of the solute /9/. Thus, as an example, slow decreases in plasma osmolality of 60–80 mmol/kg can occur without fatal consequences.

Plasma osmolality increases due to water loss, or increases if solutes are infused that do not permeate the cell membrane, such as Na⁺ and glucose. Hyper osmolality can lead to coma and subsequent death if the plasma osmolality is elevated by 40–60 mmol/kg. Freely diffusible substances like urea, acetone and ethanol, on the other hand, are harmless since they do not create large osmolality gradients between ECF and ICF.

In summary, the following remains to be noted:

• The plasma osmolality is the most important measured variable for the assessment of internal water balance (between ICF and ECF), while the monitoring of body weight is the most useful parameter for the evaluation of external fluid balance (intake and excretion)
• In euglycemic patients and with normal renal function, changes in plasma osmolality usually follow any changes in Na⁺ concentration in a parallel fashion
• Knowledge of the simultaneously determined Na⁺­ value is therefore an important differential diagnostic criterion for the clinical assessment of the measured osmolality. Urea and glucose are clinically relevant only if present at abnormally high concentrations.

8.5.5.2 Serum/plasma osmolal gap

Plasma osmolality is determined by the physiological solutes Na⁺, Cl^–, HCO₃^–, glucose and urea. Since every Na⁺­ is accompanied by an anion, only Na⁺­, urea and glucose have to be measured for the calculation of osmolality /7, 8/.

An osmolal gap between measured and calculated osmolality is encountered, if a hyper osmolar state occurs in the presence of other solutes, in addition to Na⁺, Cl^–, HCO₃^–, glucose and urea.

The osmolal gap is the difference between the serum osmolarity measured by the freezing-point depression and the serum osmolarity estimated from the equation given in the section "calculation of osmolality".

Osmolal gap (mmol/kg) = Measured osmolality – calculated osmolality

An osmolal gap is present in conditions where the measured osmolality exceeds the calculated one by > 10 mmol/kg (sensitivity 100%, specificity 86%)

Tab. 8.5-2 – Plasma osmolality and osmotic gap for diseases and conditions with hypo- and hypernatremia reviews diseases and clinical settings which are associated with changes in plasma osmolality and a normal or increased osmolal gap.

The calculation of the osmolal gap is important (see also Section 8.4 – Anion gap):

• In intoxications for the recognition and monitoring of exogenous low molecular weight substances which lead to a rise in plasma osmolality such as ethanol, methanol, ethylene glycol, isopropanol, dichloromethane
• In metabolic diseases as an indication of enhanced formation of endogenous substances like ketone bodies and organic acids.

8.5.5.3 Osmolality in the urine

The measurement of urine osmolality is an important investigation for the assessment of free water formation by the kidney and is indicated for the evaluation of increased urine volume /7, 12/. See Section 8.1.6 – Renal regulation of water and sodium excretion). It is necessary to differentiate between renal concentrating defects with, generally, a urine volume of less than 2 liters/24 h, and polyuria due to osmotic diuresis or water diuresis with a urine volume of ≥ 2.5–3 liters/24 h.

8.5.5.3.1 Urine osmolality in healthy persons

Renal concentrating capacity

Healthy individuals excrete 450–600 mmol of solutes in 24 hours (e.g., 150 mmol of Na⁺, 75 mmol of K⁺, 400 mmol of urea and 50 mmol of other non-electrolytes). The 24-hour urine volume is 1–1.5 L. Since, the urine osmolality, in complete fluid deprivation, rises to 1,000–1,200 mmol/kg within 8–18 hours, a urine volume of about 500 mL/24 hours is required for the excretion of solutes. Oliguria is present if, with maximally concentrated urine, such a volume is not reached.

Renal excretion of free water

In order to prevent hyponatremia the kidneys are capable of excreting large quantities of water (up to 0.1 liters/minute). The excretion begins 30 minutes following the excess water intake. The urine osmolality can fall to below 50 mmol/kg. In the presence of euvolemia (normal total body sodium), urine osmolality of below 180 mmol/kg confirms the excess of free water due to exogenous intake. The Na⁺ concentration in the urine is below 20 mmol/L.

The amount of actual excess of free water (i.e. water free from solutes) can be calculated according to the following equation by means of free water clearance:

K, K⁺ (mmol/L); Na, Na⁺ (mmol/L); V, urine volume (mL); U, urine; P, plasma

Free water clearance describes the difference, in mL, between the actual urine volume per unit time and the volume that is required in order to excrete isotonic (to plasma) urine. It is positive in cardiac insufficiency and negative in the presence of the SIADH.

8.5.5.3.2 Urine osmolality in renal disease

A prerequisite for the preservation of the concentrating capacity of the kidneys is the maintenance of hypertonicity in the medullary interstitium so that concentration of the urine can take place in the loop of Henle. Prerequisites for this are /13/:

• A sufficient number of nephrones, in order to maintain a sufficient GFR. If this is not the case, the urine cannot be concentrated and its osmolality will be equal to that of plasma (isosthenuria). Due to a lower GFR, the maximal concentrating capacity is reduced in the elderly in comparison to young adults.
• Adequate release and efficacy of arginine vasopressin. The efficacy can be disturbed in tubulointerstitial nephropathy, hypokalemia, hypercalcemia, or is drug-induced (e.g., due to lithium).

In the presence of renal concentrating defects, urine osmolality generally does not rise to more than 400–500 mmol/kg.

8.5.5.3.3 Urine osmolality in poly uric states

In adults, polyuria is associated with a urine volume of above 2.5–3.0 L/24 h, or a excretion rate of greater than 2 mL/min., and is due to osmotic or water diuresis. In the differentiation of the causes of the diuresis, appropriate and inappropriate forms are differentiated /13/.

Osmotic diuresis

Osmotic diuresis is based upon an accumulation of solutes such as glucose, mannitol, salt, and urea in the ECF. It results in a rise in serum osmolality, osmotic diuresis, in the loss of Na⁺ and water, an ECF decline, and a shrinking of the cells. Loop diuretics like furosemide inhibit the Na⁺ reabsorption in the loop of Henle, thus leading to osmotic diuresis via impaired urine concentrating capacity.

Water diuresis

Increased excretion of free water is present. The water diuresis may be:

• The normal response to an excess of water
• The result of primary polydipsia
• or it may be due to diabetes insipidus.

Appropriate and inappropriate diuresis

Appropriate diuresis is the result of water overload, while inappropriate diuresis is caused by diabetes insipidus. Appropriate diuresis is due to, for example, the accumulation of glucose and urea or administration of mannitol, while inappropriate diuresis is the result of volume expansion. The concentration of Na⁺ in urine is 50–80 mmol/L in inappropriate diuresis. The diagnosis can be made based on /13/:

• Glucosuria, where excretion is usually greater than 45 mg/dL and the urine osmolality is above 250 mmol/kg.
• Urea nitrogen excretion above 0.7 g/dL (117 mmol/L) due to, for example, high protein intake or hyper-alimentation. Urine osmolality is 700–900 mmol/kg. A decrease of the ECFV can develop due to hypernatremia.

Differentiation between water diuresis and osmotic diuresis

Osmolality of over 400 mmol/kg in a random urine sample is an indication of osmotic diuresis. A value of around 300 mmol/kg also speaks for the likelihood of osmotic diuresis, but in this case further clarification is necessary /13/.

Osmolality of less than 150 mmol/kg is indicative of water diuresis. For the etiological clarification and for the differentiation of water diuresis from mixed water-electrolyte diuresis with an osmolality of 150–300 mmol/kg, the fluid deprivation test is performed (Tab. 8.5-3 – Investigation of water diuresis by the fluid deprivation test and evaluation of DDAVP sensitivity).

Further findings that permit a differentiation are:

• An abrupt onset of the diuresis, suggesting central diabetes insipidus (DI) since nephrogenic DI develops slowly
• Mild polyuria of 4–5 L/24 h; this is indicative of acquired DI
• The desire for ice water suggests the presence central DI.

Typical changes in Na⁺, osmolality, and effective osmolality in serum in various clinical conditions are shown in Tab. 8.5-4 – Sodium, osmolality and tonicity in serum for various conditions with hyponatremia or hypernatremia.

8.5.6 Comments and problems

Specimen

The osmolalities in serum and plasma are almost identical, since fibrinogen which precipitates during the clotting process is not osmotically active. Plasma and urine protein content has only a minor effect on osmolality. Heparin that is present in heparinized plasma does not lead to a relevant change in osmolality.

Dilution of the sample

The relationship between osmolality and concentration is linear only in solutions of monovalent ions such as Na⁺, Cl^–, and specifically up to 1–2 mmol/kg. In serum and urine as in solutions containing calcium chloride, sucrose, dextrose, mannitol, or sorbitol, the osmolality is higher than predicted if the sample is diluted. Therefore, if an adequate amount of sample is available for the measurement, it must not be diluted /4/.

Stability

Serum and urine can be stored for several days at 4 °C in a tightly sealed container. The sample should be brought to room temperature prior to the measurement in order to reverse any sedimentation.

Calculation of osmolality

Serum osmolality (mmol/L): Serum Na⁺ × 2 (Blood glucose/18) + (Blood urea nitrogen/2.8)

Urine osmolality (mmol/L): Urine K⁺ × 2 (Blood glucose/18) + (Blood urea nitrogen/2.8)

Blood glucose and blood urea nitrogen are expressed in mg/dL

References

1. Gennari FE. Serum osmolality: uses and limitations. N Engl J Med 1984; 310: 102–5.

2. Krahn J, Khajuria A. Osmolality gaps: diagnostic accuracy and long-term variability. Clin Chem 2006; 52: 737–9.

3. Sprung CL, Isikoff SK, Hauser M, Eisler BR. Comparison of measured and calculated colloid osmotic pressure of serum and pulmonary edema fluid in patients with pulmonary edema. Crit Care Med 1980; 8: 613–5.

4. Sweeney TE, Beuchat CA. Limitations of methods of osmometry: measuring the osmolality of biological fluids. Am J Physiol 1993; 33: R 469–80.

5. Davies DP. Plasma osmolality and protein intake in preterm infants. Arch Dis Child 1973; 48: 575–9.

6. Robertson GL. Regulation of vasopressin secretion. In: Seldin DW, Giebisch G (eds). Clinical disturbances of water metabolism. New York: Raven, 1993: 99–118.

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, 1993: 211–24.

8. Knepper MA, Kwon TH, Nielsen S. Molecular physiology of water balance. N Engl J Med 2015; 372: 1349–58.

9. Macknight ADC, Grantham J, Leaf A. Physiologic responses to changes in extracellular osmolality. In: Seldin DW, Giebisch G (eds). Clinical disturbances of water metabolism. New York: Raven, 1993: 31–49.

10. Davidson DF. Excess osmolal gap in diabetic acidosis explained. Clin Chem 1992; 38: 755–7.

11. Demedts P, Theunis L, Wauters A, Franck F, Daelemans R, Neels H. Excess serum osmolality gap after ingestion of methanol: a methodology-associated phenomon? Clin Chem 1994; 40: 1587–90.

12. Howard RL, Bichet DG, Schrier RW. Pathogenesis of hypernatremic and polyuric states. In: Seldin DW, Giebisch G (eds). Clinical disturbances of water metabolism. New York: Raven, 1993: 189–209.

13. Oster JR, Singer I, Thatte L, Grant-Taylor I, Diego JM. The polyuria of solute diuresis. Arch Intern Med 1997; 157: 721–9.

14. Oster JR, Singer I. Hyponatremia, hypoosmolality, and hypotonicity. Arch Intern Med 1999; 159: 333–6.

15. Kraut JA, Mullins ME. Toxic alcohols. N Engl J Med 2018; 378: 270–80.

8.6 Arginine vasopressin (AVP), copeptin (CT-proAVP)

Lothar Thomas

In spite of marked differences in water intake, plasma osmolality is kept within narrow limits (280–295 mmol/kg). The concerted action of the thirst mechanism, renal water management and neurohypophyseal antidiuretic hormone is responsible for the organism’s water balance /1/. The antidiuretic hormone arginine vasopressin (AVP) is formed from pre pro vasopressin, a molecule of 164 amino acids (AA). AAs 1–19 are the signal peptide, AAs 20–28 are the AVP, AAs 32–124 are the neurophysin, and AAs 126–164 are the copeptin, also known as CT-proAVP. Following proteolytic cleavage, the peptides are stored in neurosecretory vesicles. The function of CT-proAVP is unknown but it’s secretion is equimolar to that of AVP /2/.

8.6.1 Indication

Differentiation of the polyuric-polydipsia syndrome:

• Nephrogenic diabetes insipidus
• Central diabetes insipidus
• Primary polydipsia
• Unclear hyponatremia
• The SIADH from the cerebral salt wasting syndrome.

8.6.2 Method of determination

Arginine vasopressin

Radioimmunoassay following extraction of the sample (e.g., with column chromatography). The assays employ antibodies directed against AVP which is bound to a protein such as thyroglobulin /3/. Commercial kits are usually calibrated against the 1^st International Standard for arginine vasopressin 77/501.

cT-proAVP (copeptin)

Commercial immunoluminometric sandwich assay /4/.

8.6.3 Specimen

AVP: EDTA plasma 1 mL; blood collection with chilled tubes, centrifugation within 30 min. at 4 °C, plasma removed and deep frozen at –20 °C.

cT-proAVP: serum, plasma (EDTA, heparin)

8.6.4 Reference interval

See:

• Tab. 8.6-1 – Reference intervals for AVP and CT-proAVP in dependence of serum osmolality
• Fig. 8.6-1 – Relationship between osmolality and AVP in plasma and AVP in plasma and osmolality in urine.

8.6.5 Clinical significance

The concentration of AVP is regulated primarily via osmotic stimuli. Increases in osmolality lead to enhanced AVP secretion. Secondary non-osmotic stimuli are low blood pressure, reduced blood volume, stress, nausea, vomiting, pain, hypoxia, hypoglycemia, fever and medicines.

Nonetheless, there exists inter individual variability in osmolality thresholds for the release of AVP. This is likely due to different osmoreceptor sensitivity. Below the threshold (280–284 mmol/kg) AVP is not released, while above the threshold a steep increase in AVP secretion occurs. Plasma hypo-osmolality is associated with AVP values below the limit of detection, and with maximally diluted urine /5/.

The measurement of AVP or CT-proAVP is not recommended in suspected AVP deficiency with urine osmolality of > 100 mmol/kg, because as of this value AVP and CT-proAVP are always detectable /6/.

Physiological AVP release occurs in the plasma osmolality range of 280–295 mmol/kg; there is a linear relationship between plasma osmolality and AVP concentration (Fig. 8.6-1 – Relationship between osmolality and AVP in plasma (top) and AVP in plasma and osmolality in urine). Thus, a 1% change in plasma osmolality leads to an increase or decrease in AVP levels of 1 ng/L (0.93 pmol/L). With plasma osmolality of less than 280–284 mmol/kg, AVP is undetectable. With > 295 mmol/kg the AVP level is greater than 3–4 ng/L (2.8–3.7 pmol/L). Maximal urine anti diuresis is reached at around 5 ng/L (4.7 pmol/L) (Fig. 8.6-1) /7/.

The osmotic regulation of plasma osmolality is genetically determined and varies in an inter individual manner. Sensitive persons manifest a change in AVP concentrations with osmolality fluctuations of around 0.5 mmol/kg, others only at of 5 mmol/kg /8/.

The relationship between plasma osmolality and AVP concentration, illustrated in Fig. 8.6-1, is only valid if hypovolemia, hypoglycemia, hyperglycemia, hypercalcemia, urea elevation, treatment with lithium and angiotensin-dependent vasoconstriction have been ruled out. If this is not the case, the slope of the curve is steeper.

If an increase in urea or glucose is present, the AVP concentration can be converted to a corrected plasma osmolality (cPos) according to the following equation:

cPos = mPos - (U + G – 7.5)

cPos, corrected plasma osmolality; mPos, measured plasma osmolality; U, urea (mmol/L); G, glucose (mmol/L)

8.6.5.1 Elevation of the effective arterial blood volume

The effective arterial volume is registered by the low pressure baroreceptor in the right atrium of the heart and in the lungs, and by the high pressure baroreceptor in the aortic arch. The release of AVP occurs, however, only if the effective arterial volume decreases by approximately 10%. A volume reduction of over 10% leads to a marked increase in AVP and in CT-proAVP in the plasma. Thus, decreases in the effective arterial volume of 20–30% cause a 20 to 30-fold elevation of the AVP level /8/. The relationship of blood volume, osmolality and AVP concentrations is shown in Fig. 8.6-2 – Relation of blood volume, osmolality and AVP concentration.

8.6.5.2 Assessment of plasma AVP concentration

AVP and CT-proAVP must be assessed relative to plasma osmolality. Within the range of 280–295 mmol/kg there exists a linear relationship between osmolality and AVP or CT-proAVP. Deviations from this relationship have been found in systemic disorders (Fig. 8.6-3 – Diseases and syndromes with disturbed relationship between plasma osmolality and AVP secretion):

• With excess AVP secretion, also named SIADH
• With insufficient AVP secretion, also called diabetes insipidus (DI). Central DI is differentiated from renal DI.
• With the cerebral salt wasting syndrome.

8.6.5.3 Syndrome of inappropriate antidiuretic hormone (ADH) secretion (SIADH)

The SIADH is based on elevated AVP secretion. The result is decreased excretion of free water and hyponatremia, which is secondary in nature and due to the increase in total body water. SIADH patients release AVP in spite of the fact that their serum osmolality lies below the threshold for the stimulation of AVP secretion.

SIADH is a diagnosis of exclusion /1/; the patients must fulfill the criteria provided in Tab. 8.6-2 – Laboratory results in SIADH .

The SIADH is manifested clinically by neuropsychiatric symptoms, since hyponatremia and water intoxication lead to cerebral edema and metabolic encephalopathy. The symptoms are weakness, apathy, headache, nausea, seizures and disturbances of concentration ability. Disturbances of consciousness and coma only occur with Na⁺ values below 125 mmol/L. Focal neurological disturbances may also be consequences of an SIADH.

The causes of SIADH are listed in Tab. 8.6-3 – Diseases and conditions associated with SIADH.

The concentration of AVP in relation to the plasma osmolality is shown in Fig. 8.6-3 – Diseases and syndromes with disturbed relationship between plasma osmolality and AVP secretion.

The determination of AVP is hardly helpful in SIADH, because hyponatremia is usually associated with elevated AVP or CT-proAVP. Elevated AVP is, in fact, a symptom of SIADH but it is not the diagnostic criterion. Approximately 10–20% of the patients manifest all the criteria of an SIADH, but do not have measurably elevated AVP or CT-proAVP /6/.

More than 50% of SIADH cases are associated with a malignant tumor. In the majority of the cases small cell lung cancer, with the formation of ectopic AVP, is present. The prevalence of clinically manifest SIADH with neuropsychiatric symptoms is 1.3–9.5% in small cell lung cancer, while that of SIADH identified through laboratory investigations is 5–40% /1/.

Exclusion of volume depletion in suspected SIADH

It is important to exclude volume depletion before making a diagnosis of SIADH, because this is associated with increased AVP secretion /6/. If there is uncertainty, volume depletion can be excluded by the infusion of sodium chloride. The normalization of the hyponatremia following sodium chloride infusion indicates volume depletion. Two liters of physiological NaCl are infused over a period of 24–48 hours.

Differentiation of the SIADH from the cerebral salt wasting syndrome (CSWS)

It can be difficult to separate the SIADH from the CSWS (Tab. 8.6-4 – Differentiation of CSWS from SIADH). In subarachnoid bleeding, the hyponatremia is based upon SIADH in 71% of the cases /12/.

8.6.5.4 Diabetes insipidus (DI)

DI is characterized by elevated excretion of diluted urine. Excretion is > 40 mL/kg body weight in adults, and > 100 mL/kg body weight in children, over 24 hours. Excretion in adults is > 3 liters in 24 hours /7/. The organism is incapable of conserving free water. Independent of an organism’s water balance, a large volume of water passes through the kidneys, and this leads to a marked sensation of thirst, exaggerated water intake, exsiccosis and constipation. DI is characterized by the excretion of a large volume of urine and polydipsia. One of the following mechanisms is responsible:

• The partial or absolute reduction in osmotically regulated AVP secretion (central, hypothalamic DI)
• The partial or total resistance of the kidneys to respond antidiuretically to AVP (nephrogenic DI)
• Primary excessive drinking of water in the presence of normal AVP secretion and action.

The relationship between osmolality and AVP in DI is shown in Fig. 8.6-4 – Relationship between osmolality and AVP in cases of diabetes insipidus (DI).

DI must be differentiated from osmotic diuresis in poorly controlled diabetes mellitus and from renal insufficiency. In both cases, plasma and urine osmolality are high (see also Section 8.5 – Osmolality).

8.6.5.5 Laboratory diagnostics

Important laboratory diagnostic criteria of DI are:

• Polyuria (> 2.5 to 3 liters/24 h)
• Urine osmolality. A value below 200 mmol/kg is indicative, an osmolality higher than 300 mmol/kg and a high glucose concentration of higher than 1 g/L indicate diabetes mellitus, while elevated serum creatinine points to renal disease.
• The determination of CT-proAVP. In nephrogenic DI, the serum concentration is > 20 pmol/L, while in complete central DI it is < 2.6 pmol/L /13/
• Functional tests, or the CT-proAVP increase in relation to serum Na⁺ level in the standard fluid deprivation test. The test enables the differentiation between partial central DI and primary polydipsia.

The copeptin, also known as CT-proAVP, correlates strongly over a wide range of osmolalities in healthy individuals. Therefore, the measurement of copeptin is an alternative to vasopressin measurements /14/.

Functional tests

The following functional tests are employed for the diagnosis of DI:

• Fluid deprivation test in combination with the determination of DDAVP sensitivity (Tab. 8.6-5 – Interpretation of the standard fluid deprivation test in combination with the DDAVP sensitivity for the diagnosis of diabetes insipidus (DI). See also Tab. 8.5-3 – Investigation of water diuresis by the fluid deprivation test and evaluation of DDAVP sensitivity.
• Osmotic stimulation test (Tab. 8.6-6 – Osmotic stimulation test).
• Standard fluid deprivation test and measurement of the CT-proAVP rise (Tab. 8.6-7 – cT-proAVP in the standard fluid deprivation test).

The clinical and laboratory findings in diseases with a disturbance of water balance and altered AVP and CT-proAVP are described in Tab. 8.6-8 – Disorders of the water balance with increase of AVP and CT-proAVP and Tab. 8.6-9 – Disorders of water balance with reduced or normal AVP and CT-proAVP.

8.6.6 Comments and problems

Reference range

AVP shows a circadian rhythm with high values during the night and lower values during the daytime.

Stability

AVP is hydrolyzed by peptidases and it is therefore necessary, following blood sampling, to perform the additional pre-analytical steps at 4 °C. CT-proAVP is stable for 3 days at room temperature.

For the determination of AVP in pregnant women, a peptidase inhibitor should be present in the blood collection tubes; this is because lysine amino peptidase, which hydrolyse AVP, is sometimes found in the plasma of pregnant women.

8.6.7 Pathophysiology

AVP is synthesized as a pre pro hormone (Fig. 8.6-5 – Primary structure of vasopressin andits analogues). The pre pro hormone is packaged in secretory vesicles and is converted into the final secretory form when the vesicles, located in magno cellular neurons, migrate to the nerve endings in the arteries of the adenohypophysis. There, AVP is stored along with neurophysin II, and is secreted in response to increased firing of the vasopressinergic neurons. The primary stimulus is the rise in plasma osmolality, while secondary stimuli are a decline in blood pressure and volume depletion. The circulating half-life time of AVP is 10–20 minutes.

8.6.7.1 Effects of AVP

The effect of AVP is mediated via the three receptors V1–V3, which are located in the plasma membrane. These receptors belong to the family of G-protein-coupled receptors, which form the intracellular messenger cyclic AMP via adenylate cyclase. See Fig. 6.2-4 – Gs-protein-mediated signal transmission).

The distribution of the receptors is as follows:

• V1 on smooth muscle cells of the blood vessels, in the liver, in thrombocytes, and in the central nervous system
• V2 on the basolateral membrane of the tubular cells of the distal nephron
• V3 in the corticotrophe part of the adenohypophysis.

8.6.7.2 Renal water management

Renal management of water is regulated osmotically and is under the direct control of AVP. A rise in osmolality of 1–2% causes the release of AVP, which binds to the V2 receptors in the basolateral membrane of the distal nephron. The result is the secretion of proteins for the formation of aquaporin water channels (AQP) from intracellular vesicles of the apical membrane of the collecting duct cells (Fig. 8.6-6 – Effect of AVP for increasing the water permeability in the renal collecting duct cells). These are integrated into the cell membrane and lead to water reabsorption along an osmotic gradient. Of the 11 known AQPs of the organism, 7 are found in the kidney. AQP1 is localized in the apical and basolateral membrane of the proximal tubular cell. AQP2 is found in the collecting ducts, where it is responsible for AVP-dependent water transport. Activation of the V2 receptor leads to the expression of the genes coding the AQP2 water channels. AQP2 proteins are synthesized, and these are then arranged to form water channels. In consequence, increased quantities of water are transported from the collecting ducts into the interstitium.

References

1. Wong LL, Verbalis JG. Systemic diseases associated with disorders of water hemostasis. Endocrinol Metab Clin N Am 2002; 31: 121–40.

2. Fenske W, Quinkler M, Lorenz D, Zopf K, Haagen U, Papassotiriou J, et al. Copeptin in the differential diagnosis of the polydipsia-polyuria syndrome – revisiting the direct and indirect water deprivation tests. J Clin Endocrin Metab 2011; 96: 1506–15.

3. Robertson GL, Mahr EA, Athar S, et al. The development of clinical application of a new method for the radioimmunoassay of arginine vasopressin in human plasma. J Clin Invest 1973; 52: 2340–52.

4. Morgenthaler NG, Struck J, Alonso C, Bergmann A. Assay for the measurement of copeptin, a stable peptide derived from the precursor of vasopressin. Clin Chem 2006; 52. 112–9.

5. Robertson GL. The use of vasopressin assays in physiology and pathophysiology. Sem Nephrol 1994; 4: 368–83.

6. Ellison DH, Berl T. The syndrome of inappropriate antidiuresis. N Engl J Med 2007; 356: 2064–72.

7. Ball SG. Vasopressin and disorders of water balance: the physiology and pathophysiology of vasopressin. Ann Clin Biochem 2007; 44: 417–31.

8. Robertson GL. Regulation of vasopressin secretion. In: Seldin DE, Giebisch G (eds). Clinical disturbances of water metabolism. New York: Raven, 1993: 99–118.

9. Robertson GL, Shelton RL, Athar S. The osmoregulation of vasopressin. Kidney Int 1976; 10: 25–37.

10. Howard RL, Bichet DG, Schrier RW. Pathogenesis of hypernatremic and polyuric states. In: Seldin DW, Giebisch G, eds. Clinical disturbances of water metabolism. New York: Raven, 1993: 189–209.

11. Beyersdorf S, Albrecht C, Wallaschofski H. Differentialdiagnostik des Syndrom der inadäquaten ADH-Sekretion gegenüber dem zerebralen Salzverlust-Syndrom. J Lab Med 2008; 32: 19–25.

12. Hannon MJ, Behan LA, O’Brien MM, Tormey W, Ball SG, Javadpur M, et al. Hyponatremia following mild/moderate subarachnoid hemorrhage is due to SIAD and glucocorticoid deficiency and not cerebral salt wasting. J Clin Endocrinol Metab 2014; 99: 291–8.

13. Fenske W, Christ-Crain M. Stellenwert von CT-proAVP (Copeptin) in der Abklärung des Polyurie-Polydipsie Syndroms. Med Welt 2011; 62: 39–44.

14. Balanescu S, Kopp P, Gaskill MB, Morgenthaler NG, Schindler C, Rutishauser J. Correlation of plasma copeptin and vasopressin concentrations in hypo-, iso-, and hyperosmolar states. J Clin Endocrinol Metab 2011; 96: 1046–52.

15. Bichet DG. Nephrogenic diabetes insipidus. Adv Chron Kidney Dis 2006; 13: 96–104.

16. Sorensen JB, Andersen MK, Hansen HH. Syndrome of inappropriate secretion of antidiuretic hormone (SIADH) in malignant disease. J Intern Med 1995; 238: 97–110.

17. Jochberger S, Mayr VD, Luckner G, Wenzel V, Ulmer H, Schmid S, et al. Serum Vasopressin concentrations in critically ill patients. Crit Care Med 2006; 34: 293–9.

18. Vincent JL. Vasopressin in hypotensive and shock states. Crit Care Clin 2006; 22: 187–97.

19. Landry DW, Levin Hr, Gallant EM, et al. Vasopressin deficiency contributes to the vasodilation in septic shock. Circulation 1997; 95: 122–5.

20. Liu BA, Mittmann N, Knowles SR, Shear NH. Hyponatremia and the syndrome of inappropriate secretion of antidiuretic hormone associated with the use of selective serotonin reuptake inhibitors: a review of spontaneous reports. Can Med Ass 1996; 155: 519–27.

21. Buonocore CM, Robinson AG. The diagnosis and management of diabetes insipidus during medical emergencies. Endocrinol Metab Clin N A 1993; 22: 411–23.

22. Baylis PH, Phillips EMG. The endocrine investigation of disorders of sodium and water homeostasis. JIFCC 1994; 6: 158–63.

23. Star RA. Pathogenesis of diabetes insipidus and other polyuric states. In: Seldin DW, Giebisch G (eds). Clinical disturbances of water metabolism. New York: Raven, 1993: 211–24.

8.7 Potassium

Lothar Thomas

Quantitatively, potassium is the most important intracellular cation (K⁺). It plays an important role in the control of cellular volume, in the maintenance of the electrochemical potential across the cell membrane of excitable (nerve, muscle) and non-excitable tissues, and in acid-base balance. Potassium also plays a meaningful role in numerous cell functions such as growth, DNA and protein synthesis, and the activity of various enzymes /1/.

The regulation of K⁺ homeostasis between the intracellular fluid compartment (ICF) and the extracellular fluid compartment (ECF) occurs rapidly through the Na⁺-K⁺ pumps of the cell membrane, and time-delayed by gastrointestinal K⁺ uptake and renal K⁺ excretion. The plasma K⁺ is only a moderate indicator of total body potassium, but it is physiologically important for the assessment of the transmembrane electrochemical gradient. See also Section 8.1 – Water balance and fluid compartments).

8.7.1 Indication

• Hypertension
• Cardiac arrhythmias
• Chronic ingestion of potassium depleting medications (diuretics, laxatives)
• Long-term therapy with corticosteroids
• Acute and chronic renal insufficiency
• Diarrhea, vomiting
• Disorders of water and electrolyte balance
• Disorders of acid-base balance
• Monitoring of intensive care patients
• Hypomagnesemia
• Suspicion of renal tubular acidosis
• Decrease in renal function.

8.7.2 Method of determination

Flame photometry

Principle: see Section 8.2.2 – Method of determination. Flame photometry, although used only to a minor extent in routine diagnostics for the determination of electrolytes, is the reference method according to the recommendation of the National Committee for Clinical Laboratory Standards (NCCLS) /2/.

Potentiometer with ion selective electrode (ISE)

Principle: see Section 8.2.2 – Method of determination. The determination using ISE is performed in routine diagnostics on both undiluted and diluted samples. The ion-selective membrane of the measurement electrode contains valinomycin as the ionophore. It has high selectivity for K⁺ (e.g., K⁺ relative to Na⁺ 5,000 : 1).

Enzymatic spectrometric determination

Principle: in the presence of K⁺ the enzyme pyruvate kinase (EC 2.7.1.40) is activated. The concentration of K⁺ is the rate-limiting step for the enzymatically catalyzed conversion of phosphoenolpyruvate into pyruvate. Pyruvate is then reduced to lactate. During this latter process NADH₂ is consumed, and its decline is measured kinetically at 340 nm. In order to obtain a good measurement signal within the clinically relevant range, a cryptand is added to the reaction mixture prior to the start of the enzymatic reaction. This cryptand captures a constant proportion of K⁺ /3/.

8.7.3 Specimen

Serum, plasma (lithium, ammonium heparinate): 1 mL

8.7.4 Reference range

Refer to Ref. /4, 5, 6/ and Tab. 8.7-1 – Reference intervals for potassium.

8.7.5 Clinical significance

Regulation of body K⁺ balance requires that the kidney excrete most of the K⁺ gained each day from diet. Filtered K⁺ is largely reabsorbed by proximal nephron segments, and that excreted K⁺ is secreted by distal segments. High K⁺ intake not only lowers blood pressure but also reduces salt sensitivity.

8.7.5.1 Basic principles for assessing the plasma potassium concentration

The total-body potassium content is 50–55 mmol/kg body weight. The plasma K⁺ concentration is /7/:

• On the one hand, regulated by total body K⁺ and reflects this. It must be noted, however, that with normal plasma serum K⁺ level and total body potassium of 3,500 mmol, of which 10% is extracellular, the acute loss of 1% of total body K⁺ (35 mmol) leads to a significant disturbance of the K⁺ balance between the intracellular fluid (ICF) and the extracellular fluid (ECF) and to a decrease in serum K⁺ concentration. Thus, reduced serum levels are not automatically the indicator of a marked reduction in total body K⁺.
• On the other hand, changes in circulating K⁺ are controlled by mechanisms like the renin-angiotensin-aldosterone system, which attempts to maintain total body K⁺ constant by altered renal K⁺ excretion.

Increases or decreases in serum K⁺ levels are always the result of a disturbance in K⁺ distribution between the ICF and the ECF.

8.7.5.2 Disorders of the external potassium balance

The external K⁺ balance is regulated by K⁺ secretion of the kidneys. In high potassium intake serum K⁺ level rises within 2 hours after the meal and returns to the reference interval before the next meal. Dietary intake of 400 mmol/day causes progressively increased renal excretion so that the external balance is restored within 3 days. Modulators of renal K⁺ excretion and, accordingly, of the external K⁺ balance, are /8/:

• The amount of K⁺ that is provided with food
• Sodium content and flow rate in the distal tubule
• The current acid-base status
• The activity of mineralocorticoids and mineralocorticoid-like substances
• The responsiveness of the distal tubule to mineralocorticoids
• The type and availability of anions.

Amount of K⁺ ingested

Maintenance of a healthy steady state requires a continuing balance between intake and excretion of K⁺. The rates are 50–100 mmol/day (e.g., around 1 mmol/kg body weight in 24 hours).

Sodium content and flow rate in the distal tubule

A rise in the ECF volume leads, via an increase in the glomerular filtration rate (GFR), to the delivery of greater quantities of water and Na⁺ to the distal tubule; the consequence is increased K⁺ excretion. A reduction of the ECF has the opposite effect, via the same mechanisms.

8.7.5.3 Acid-base status

Acid-base derangements affect the excretion of K⁺. Extracellular acidosis leads to hyperkalemia through the loss of K⁺ from the intracellular to the extracellular fluid compartment, while alkalosis causes hypokalemia via the displacement of K⁺ from the extracellular to the intracellular fluid compartment. In metabolic acidosis, K⁺ exits from the cells in exchange for H⁺.

Acute metabolic acidosis

Gavage of NH₄Cl or HCl cause acute hyperkalemia and renal secretion of K⁺ due to the direct efflux of K⁺ from the cell. Acidosis caused by the accumulation of organic acids like lactate or ketone bodies does not directly lead to hyperkalemia. Instead, hyperkalemia only develops secondarily due to volume depletion and decreased urinary flow rate which is present under such circumstances. In addition the renin-angiotensin aldosterone system is activated.

Acute and chronic respiratory alkalosis

Both of these disturbances have a minimal effect on K⁺ balance and manifest a mild tendency toward hypokalemia.

Chronic metabolic acidosis

With high PCO₂ and increased renal HCO₃^– reabsorption, a high K⁺ excretion with hypokalemia develops.

8.7.5.4 Mineralocorticoids and glucocorticoids

Mineralocorticoids and glucocorticoids stimulate tubular K⁺ excretion. Mineralocorticoids have a direct stimulatory effect on K⁺ secretion at the distal tubule. The K⁺ secretion is dependent upon the delivery of Na⁺­ to the distal tubule. If this is low, hypokalemia does not develop. Glucocorticoids act indirectly. They elevate the GFR and hence the urinary flow and, via increased Na⁺ delivery to the distal tubule, lead to increased K⁺ excretion.

Responsiveness of the distal tubules to mineralocorticoids

Diseases such as interstitial nephritis can lead to damage to the distal tubules and the collecting ducts and, in consequence, to decreased responsiveness to aldosterone. A K⁺ excretion disorder is the result, with the development of hyperkalemia and severe acidosis. Derangements of K⁺ excretion are also described in obstructive nephropathy, systemic lupus erythematosus and sickle cell anemia, and following kidney transplantation.

Potassium-sparing diuretics such as triamterene and amiloride suppress renal K⁺ secretion by blocking the Na⁺ channels of the luminal cell membrane. Spironolactone blocks the aldosterone receptor and thereby inhibits renal K⁺ secretion.

8.7.5.5 Type and availability of anions

Normally, Na⁺ is reabsorbed in the distal tubule together with an anion such as Cl^– in order to preserve electroneutrality.

If, in metabolic acidosis, the fraction of poorly permeable anions like HCO₃^– in the distal tubule is increased, more K⁺ is secreted, and hypokalemia results.

In secondary hyperaldosteronism the reabsorption of Na⁺ as well as the secretion of K⁺ and H⁺ is increased in the distal tubules; hypokalemia develops. If the availability of exchangeable Na⁺ is low due to strict limitation of salt intake, no Na⁺-K⁺ exchange occurs and hypokalemia does not develop.

8.7.5.6 Disorders in internal potassium balance

Internal balance disorders alter the serum level of K⁺, but not total body K⁺.

Catecholamines

The effect of catecholamines on internal K⁺ distribution is mediated by β₂-receptors. The stimulation of the receptors leads to a decline in K⁺ concentrations in the ECF due to a shift of K⁺ to the intracellular fluid compartment. Medication with β₂-blockers elevates serum K⁺. The α-adrenergic system has the opposite effect. Post-operatively, the administration of catecholamines protects against the development of potential hyperkalemia /7/.

Insulin

Insulin increases the K⁺ uptake by non-renal tissues and can rapidly lower the serum K⁺ level. This is the case in non-diabetic individuals following a meal rich in carbohydrates.

In diabetics, the deficiency of insulin reduces the cellular uptake of K⁺. In consequence, hyperkalemia should, theoretically, develop. However, due to the existing osmotic diuresis and the associated hyperkaluria, this does not occur. If, however, the diabetic patient has renal insufficiency or hyporeninemic hypoaldosteronism, hyperkalemia develops /7/.

8.7.5.7 Important differential diagnostic examinations

For the assessment of a pathological plasma K⁺ level, blood and urine evaluations can provide, in addition to anamnestic and clinical data, important differential diagnostic information.

Blood

• Sodium, chloride, creatinine
• Magnesium, calcium, phosphate
• Acid-base status
• Cortisol, (ACTH)
• Renin, aldosterone
• Digoxin, digitoxin

Urine

• Excretion of potassium, chloride, sodium
• Diuretics.

8.7.5.8 Hypokalemia

More than 20% of hospitalized patients have hypokalemia with a lower reference value of 3.6 mmol/L /7, 9/. The prevalence of hypokalemia in patients taking diuretics is as high as 50%.

Hypokalemia causes concentration-dependent muscular symptoms /10/:

• Levels of 3.0–3.5 mmol/L may be associated with mild muscular weakness, myalgia and easy fatigability. In persons with normal heart function, hypokalemia of 3.0–3.5 mmol/L generally does not lead to cardiac problems; in individual cases, however, ventricular arrhythmia may occur.
• Concentrations < 3.0 mmol/L are considered to be severe hypokalemia, since at these levels the risk of cardiac arrhythmia is elevated. Therefore, potassium substitution should immediately be provided. In the range of 2.5–3.0 mmol/L, muscular weakness (particularly of the proximal limbs and the head musculature) occurs. In one study /11/ 2.6% of the 37,458 hospital admissions had severe hypokalemia. Of these, 0.7% had a level below 2.0 mmol/L, 8.5% a value of 2.0–2.4 mmol/L, and 91% a value of 2.5–2.9 mmol/L. The hypokalemia was associated with the ingestion of a medication in 75% of the cases, primarily with furosemide, with other diuretics, and with corticosteroids and amphotericin B.
• Concentrations below 2.5 mmol/L can lead to rhabdomyolysis with segmental muscle necrosis, vacuolar degeneration of the muscle fibers and myoglobinuria. Tetany occurs in association with alkalosis. Cerebral symptoms are rare.

Usually, clinical symptoms only accompany rapid, and not slow, decreases in K⁺ levels. The redistribution of K⁺ from the ECF to the ICF has only minimal effects on symptoms as seen:

• After strenuous physical activity.
• During infusion of glucose and/or insulin.

The causes of hypokalemia are classified as follows:

• Renal loss
• Extrarenal loss
• Shift from the extracellular to the intracellular fluid compartment.

Diseases and conditions of hypokalemia are listed in Tab. 8.7-2 – Diseases and conditions associated with hypokalemia.

8.7.5.8.1 Hypokalemia and urinary potassium excretion

If hypokalemia is present, the determination of urinary K⁺ excretion is the most important test for establishing the etiology /7, 11/. The findings of a random sample can only be assessed if, anamnestically, it can be assumed that the urine volume is normal (1–1.5 L). Otherwise the determination must be performed using a timed urine collection. This is particularly important in patients with hypokalemia, since reduced total body K⁺ is associated with polydipsia and a renal concentrating defect.

Based upon urinary K⁺ excretion, hypokalemia is differentiated etiologically into:

• Internal balance disorders. In these cases an increase in catecholamines or in insulin causes a trans cellular shift in K⁺ from the extracellular to the intracellular fluid compartment. The result is hypokalemia, but no change in renal K⁺ excretion.
• External balance disorders with reduced total body K⁺. The cause can be reduced nutritional intake of K⁺, or renal or extrarenal loss of K⁺.

Based on the premise that in hypokalemia compensatory renal K⁺ excretion is limited, the urine findings can be interpreted as follows:

• The combination of hypokalemia and low K⁺ excretion (below 10 mmol/L) suggests non-renal K⁺ loss
• The combination of hypokalemia and K⁺ excretion of greater than 10 mmol/L indicates renal loss
• A dissociation between serum K⁺ level and urinary K⁺ excretion suggests stress-related hypokalemia caused by catecholamines or diuretic-induced hypokalemia.

Apart from the urinary K⁺ excretion, the blood pH and Cl^– excretion in the urine provide further important differential diagnostic information for hypokalemia (see Section 8.8 – Renal electrolyte excretion).

Hypokalemia in association with reduced renal potassium excretion

With additional blood pH information, the following types of hypokalemia can be differentiated:

• With metabolic acidosis (Cl^– in the serum elevated) in diarrhea, with villous adenomas of the intestine and in laxative abuse
• With metabolic alkalosis (Cl^– reduced in the serum) with Cl^– losing diarrhea
• With normal pH (Cl^– in the serum is normal) with decreased K⁺ intake or K⁺ loss through the skin and the gastrointestinal tract.

Hypokalemia in association with increased renal potassium excretion

Based upon blood pH, the following types of hypokalemia can be differentiated:

• With metabolic hyperchloremic acidosis (like renal tubular acidosis) or acidosis with increased anion gap (as in diabetic or alcoholic ketoacidosis)
• With normal blood pH due to trans cellular K⁺ displacement due to catecholamine increases (e.g., as seen in stressful situations and in the alcohol withdrawal syndrome)
• With metabolic alkalosis. In these cases the Cl^– excretion allows further differentiation.

Hypokalemia in association with increased renal chloride excretion

Cl^– excretion of over 20 mmol/L and metabolic alkalosis are found in:

• The Bartter syndrome, as well as in patients who take non-potassium-sparing diuretics such as furosemide, bumetanide and thiazides. If these diuretics are discontinued shortly before the visit to the doctor, however, Cl^– excretion is below 10 mmol/L.
• Hypertensive patients with hyperaldosteronism or hypercortisolism. Hyperaldosteronism can be primary or secondary in nature; it is differentiated by the determination of plasma renin. Hypercortisolism can be medication-dependent or endogenous (e.g., hypophyseal adenoma with increased ACTH secretion, para neoplastic ectopic ACTH synthesis or adrenocortical adenoma). In hyperaldosteronism or hypercortisolism, renal excretion of both K⁺ and Cl^– is higher than 20 mmol/L.
• Hypertensive patients with low aldosterone (e.g. in adrenal hyperplasia with 11β-hydroxylase deficiency, 17α-hydroxylase deficiency, or in the Liddle syndrome). In the Liddle syndrome, plasma cortisol values are normal, while in adrenal hyperplasia they are elevated.

Hypokalemia in association with reduced renal chloride excretion

Cl^– excretion of below 10 mmol/L in hypokalemia occurs with vomiting, after the discontinuation of non-potassium-sparing diuretics, and in chronic hypercapnia due to respiratory insufficiency of a wide variety of etiologies.

8.7.5.9 Hyperkalemia

Hyperkalemia is a potentially life-threatening disease. In the United Kingdom by the Royal College of Pathologists’ Critical Communication Document, which requires that all K⁺ results ≥ 6.5 mmol/L be communicated within 2 h. Clinically relevant are serum K⁺ concentrations of ≥ 5.5 mmol/L. Hyperkalemia of 5.6–6.0 mmol/L is considered to be mild, levels of 6.1–6.9 moderate to severe, and values ≥ 7.0 mmol/L as very severe  /9/.

Hyperkalemia is most commonly caused in renal disease due to a decline of glomerular filtration rate (GFR). As approximately 98% of the body’s K⁺ is in the intracellular space any shift of K⁺ from the intracellular to extracellular space will lead to hyperkalemia. Such hyperkalemias are true hyperkalemias as the K⁺ concentration in serum reflects the in-vitro picture /44/.

Refer to Tab. 8.7-3 – Diseases and conditions associated with hyperkalemia.

Recently publicated data /44/ showed that more than half of serum K⁺ results of ≥ 6.0 mmol/L in specimen from primary care were due to pseudohyperkalemia. An estimated GFR (eGFR) of ≥ 90 [mL × min^–1 × (1.73 m²)^–1] gave a negative predictive value (NPV) of 100%, that is, true hyperkalemia was not associated with normal renal function. A second study /45/ found that for initial K⁺ results of ≥ 6.0 mmol/L, eGFR of ≥ 90 [mL × min^–1 × (1.73 m²)^–1] had an NPV of 72%; this increased to 83% when only initial K⁺ results of ≥ 6.5 mmol/L were included and to 100% when only those samples with initial K⁺ results of ≥ 6.5 mmol/L and concomitant normal blood count were included. The authors of both publications suggest that there is no need to telephone K⁺ results ≥ 6.5 mmol/L to out-of hours primary care providers if GFR is ≥ 90 [mL × min^–1 × (1.73 m²)^–1].

Hyperkalemia in association with reduced renal potassium excretion

Causes of decreased K⁺ excretions are:

• A marked reduction of the GFR in acute or chronic renal insufficiency. The kidneys are the key organ with regard to K⁺ excretion; 90% of the K⁺ that is ingested with food is eliminated by the renal route and 10% by the gastrointestinal route. Nonetheless, the capability of excreting K⁺ is maintained for long periods of time in spite of severe renal insufficiency. Therefore, hyperkalemia is manifested only in acute renal failure or in stages IV and V of chronic renal insufficiency.
• Selective hypo aldosteronism, which can be idiopathic in nature or occurs in a secondary manner in diabetics with chronic tubular interstitial nephritis. In some patients cardiac arrhythmias are a consequence of the hyperkalemia.
• Adrenocortical insufficiency. Due to the deficiency in mineralocorticoids the patients manifest, apart from hyperkalemia, hyponatremia, hypovolemia, and a marked decline in blood pressure when moving from supine to upright position.
• ACE inhibitors and angiotensin receptor blockers are used in hypertensive patients, in order to reduce cardiovascular risks. Hyperkalemia can develop as a side effect. This is particularly the case in patients who likely have an existing deficiency in K⁺ excretion, like those with chronic renal insufficiency and with diabetes mellitus. If, additionally, a K⁺ sparing diuretic is administered, as in patients with congestive heart failure, hyperkalemia is even more likely to occur and strict serum K⁺ monitoring is necessary /19/.

Hyperkalemia in association with increased renal potassium load

The kidneys are normally capable of eliminating all of the excess ingested K⁺. This is, however, not the case in patients:

• With chronic renal insufficiency or in dialysis patients who do not observe their dietary instructions
• With the tumor lysis syndrome, rhabdomyolysis, intravascular hemolysis, catabolic state because of high fever with K⁺ efflux from the intracellular fluid. Marked hyperkalemia does not, however, develop until renal function becomes limited.

Redistribution of potassium

For the assessment of K⁺ redistribution from the intracellular fluid (ICF) to the extracellular fluid (ECF) following is to be noted:

• The K⁺ release from the ICF into the ECF takes place in the following situations: in metabolic acidosis, renal insufficiency, ketoacidosis, lactic acidosis, heparin therapy, digitalis overdose, infusion of hypertonic solutions such as 50% dextrose or mannitol, infusion of arginine chloride or lysine chloride for the treatment of metabolic alkalosis, and under therapy with β-receptor blockers
• In metabolic acidosis, a decrease in pH of 0.1 leads to an increase in serum K⁺ of 0.5–1.2 mmol/L, in respiratory acidosis of 0.1–0.9 mmol/L
• The extent to which serum K⁺ is influenced is dependent upon the duration of the acidosis, the acid equivalents, and the HCO₃^– concentrations. Thus, inorganic acids (HCl and NH₄Cl) are more efficacious than organic acids (lactate, ketoacids).

Deficient tubular response to aldosterone

Deficient tubular secretion of K⁺ occurs due to possibly faulty binding of aldosterone to its receptors in the distal tubule. This can be the case in systemic lupus erythematosus, sickle cell anemia, amyloidosis, and following kidney transplantation.

8.7.5.9.1 Clinical symptoms in hyperkalemia

Clinical manifestations of hyperkalemia include muscle weakness or paralysis, cardiac arrhythmias and death.

Clinical symptoms occur less frequently in hyperkalemia than in hypokalemia. The consequences of hyperkalemia are, however, more severe, since a reduced ratio of intracellular to extracellular K⁺ causes depolarization of the cell membrane with a decrease of the resting membrane potential. On electrically excitable cells, this leads to a delay in the rate of rise of the action potential and to a delay in the spread of the excitation. This has consequences, in particular, with regard to cardiac and skeletal muscle cells. At admission, many of the patients with hyperkalemia have, anamnestically, chronic kidney disease, diabetes mellitus, or high blood pressure.

Cardiac disorders /13/

At serum K⁺ levels of 6.0 mmol/L some 30% of patients have hyperkalemia-dependent electrocardiogram (ECG) changes; with values over 7.5 mmol/L almost all patients. With the increase in the K⁺ concentration the following ECG changes are caused chronologically: elevation of the T-wave amplitude, prolongation of the PQ interval, broadening of the QRS complex. The cardiac events are amplified by simultaneous hypocalcemia, hyponatremia and acidosis.

Skeletal muscles /13/

General muscle weakness, especially of the lower limbs. Paresthesia that begins distally, weakened tendon reflexes, and a feeling of heaviness in the limbs occur at serum K⁺ levels of ≥ 8 mmol/L.

8.7.5.9.2 Pseudo­hyper­kalemia

Pseudohyperkalemia is an in vitro increase in serum K⁺ and does not reflect pathological in vivo changes. To distinguish between true hyperkalemia and pseudohyperkalemia renal function (eGFR) should be assessed. It is rare to develop true and severe hyperkalemia in the absence of decreased renal function. Normal ECG finding is a hint for pseudohyperkalemia.

There are multiple causes of pseudo­hyper­kalemia /44/:

• erythrocytosis, thrombocytosis, leukocytosis; cheque plausibility of blood count
• medications (K⁺ sparing diuretics)
• excessive intake or infusion of K⁺ containing salts
• repeated first clenching during phlebotomy
• sample hemolysis
• potassium-EDTA or potassium-oxalate in sample collection tube
• prolonged tourniquet application
• vigorous tube inversions
• pneumatic tube transport
• delay in sample centrifugation
• seasonal cold temperatures
• salt substitutes such as KCl or NH₄Cl and the ingestion of K⁺ sparing diuretics, beta blockers, ACE inhibitors, and prostaglandin synthetase inhibitors should be inquired anamnestically.

8.7.5.10 Hyperkalemia and urinary potassium excretion

If hyperkalemia is present, urinary K⁺ excretion is the additional test for the differentiation of renal and extrarenal hyperkalemia. K⁺ excretion of:

• Greater than 40 mmol/L (i.e., normal K⁺ excretion, indicates extrarenal hyperkalemia)
• Below 40 mmol/L indicates renal hyperkalemia.

Hyperkalemia in association with normal renal potassium excretion

Different forms are distinguished based on the underlying cause (i.e., due to):

• Reduced trans cellular K⁺ shift from the extracellular fluid compartment (ECF) to the intracellular fluid compartment (ICF). The shift is regulated by the catecholamines, acting via the β₂-receptors, as well as insulin. An inadequate shift in K⁺ into the ICF occurs with the blockade of β₂-receptors by β-blockers, or strenuous physical activity, or in the case of insulin deficiency and digitalis intoxication.
• Increased exogenous K⁺ intake. This may occur with the ingestion with food, salt intake, or for therapeutic reasons (e.g., KCl infusion or the administration of potassium gluconate, potassium phosphate or potassium citrate).
• Increased K⁺ release from the ICF due to cellular damage (intravasal hemolysis, tumor cell lysis), muscle relaxants, hypertonic solutions or metabolic acidosis.

Hyperkalemia in association with reduced renal potassium excretion

From the clinical perspective, this hyperkalemia is differentiated into:

• Conditions with hypo aldosteronism and low renin (similar to acute and chronic renal failure), and those with normal or elevated renin activity (Addison’s disease) or isolated aldosterone deficiency
• Conditions with normal aldosterone in which, however, end organ resistance (lack of responsiveness of the kidneys to aldosterone) is present.

8.7.6 Comments and problems

Blood sampling

K⁺ values in serum are dependent upon the environmental temperature at the time of blood sampling. During the winter time the veins are less well filled than in summer and therefore, in winter, patients are more frequently requested to open and close their hand. This activity leads to the release of K⁺ from blood cells so that the frequency of hyperkalemia above 5.2 mmol/L increases from 0.6% in summer to 0.9% in winter /31/.

Anticoagulant

In vitro sample contamination with potassium EDTA is common and is caused by /32/:

• Decanting of blood from potassium EDTA containing tubes to other tubes
• Direct transfer of blood from potassium EDTA containing tubes to other tubes by back flow. Back flow is the regurgitation of blood from the evacuated blood collection tube back into the needle or vein. If blood is first collected from EDTA containing tubes, the regurgitated blood may be contaminated with EDTA which is then transferred to the following sample tubes.
• If blood is collected with a syringe and distributed into tubes, and in the process the approach cone of the syringe is contaminated with potassium EDTA.

Centrifugation

K⁺ concentrations in erythrocytes are around 25 times higher than in plasma. The blood must be collected without causing hemolysis and the erythrocytes have to be separated within 1 hour in order to prevent hyperkalemia.

Unduly delayed centrifugation of the blood sample is at 12.5% the most frequent cause of spurious hyperkalemia /33/.

Re centrifugation of the sample after 4 hours or more is, likewise, a frequent cause of spurious hyperkalemia. In Japan, for example, samples are re centrifuged in 70% of the laboratories /34/.

Difference between serum and plasma

K⁺ levels in serum are, on the average, higher by 0.3 mmol/L than in heparin plasma. In the presence of hypokalemia, the mean difference is greater than 0.5 mmol/L /4/.

Hemolysis

In vitro hemolysis is defined as the release of intracellular constituents to the extracellular fluid compartment. Hemolysis is visible to the naked eye as a reddish-brown discoloration, if the free hemoglobin (Hb) concentration is ≥ 0.3 g/L. The mean elevations of K⁺ are, respectively, 0.28, 0.70 and 1.4 mmol/L, with mild (1 g Hb/L), moderate (2.5 g Hb/L) and severe (5 g Hb/L) hemolysis /35/.

Pseudohyperkalemia

In chronic lymphatic leukemia the cells manifest increased fragility. Low mechanical stress (compression of the upper arm during blood sampling or blood sampling with Vacutainer or serum separator tubes) leads to pseudo hyperkalemia. This is also the case if the transport for the sample tubes is carried out with pneumatic post /36/.

In myeloid leukemia with an elevated cell count pseudo hypokalemia may occur in vitro. The cause is believed to be increased Na⁺ permeability with activation of the Na⁺-K⁺-ATPase, leading to increased K⁺ uptake in the cells /37/.

Thrombocytosis

Thrombocytoses > 500 × 10⁹/L is associated with a serum to plasma K⁺ difference of > 0.5 mmol/L /38/.

Method of determination

Lipemic samples and samples with total protein of greater than 80 g/L cause spurious hypokalemia if assessed with flame photometry and indirect potentiometry.

Ammonium-containing (e.g., ammonium heparinate) anticoagulants and ammonium values of 20 mmol/L in samples lead to a rise in K⁺ levels of 0.3 mmol/L, if K⁺ is determined by ion-selective electrodes /39/. Disturbances due to ammonium are observed in quality control sera which include ammonium carbonate-containing dilution media, or have these or similar substances in the matrix. ­Ion-selectively measured K⁺ is also increased by ethylene glycol, which is used to stabilize quality control sera, as well as procainamide, employed in cardiac arrhythmias, at concentrations of 8 mg/L.

Storage of samples

The concentration of K⁺ in serum/plasma prior to centrifugation is a function of storage conditions e.g., time and temperature. In a study /42/ a decrease then an increase in K⁺ level was observed at 20–25 °C. A decrease then an increase at 4 °C from –1% to 9.6% between 2 and 24 h was measured due to leakage from cells. In summary K⁺ did not remain stable beyond 4 h after sampling.

Stability in plasma and serum

At least 1 week in closed tubes at room temperature or 4 °C. For direct ISE measurements only fresh serum or plasma are to be used, as an increase in HCO₃^– in stored samples leads to an increase in pH and a decline in measurable ion activity /40/.

References

1. Wright FS, Giebisch G. Regulation of potassium excretion. In: Seldin DW, Giebisch G (eds). The kidney, physiology and pathophysiology. New York; Raven, 1992: 2209–47.

2. NCCLS. Standardization of sodium and potassium ion selective electrode systems to the flame photometric reference method; approved standard. NCCLS Document C29-A, Vol 15 No 1. Villanova: NCCLS, 1995.

3. Berry MN, Mazzachi RD, Pejakovic M, Peake MJ. Enzymatic determination of potassium in serum. Clin Chem 1989; 34: 2295–8.

4. Drogies T, Ittermann T, Lüdemann J, Klinke D, Kohlmann T, Lubenow K, et al. Potassium – reference intervals for lithium-heparin plasma and serum from population-based cohort. J Lab Med 2010; 34: 39–44.

5. Rodriguez-Soriano J. Potassium homeostasis and its disturbances in children. Pediatr Nephrol 1995; 9: 364–74.

6. Soldin SJ, Brugnara C, Wong EC. Pediatric reference ranges. Washington: AACC-Press, 2003: 152.

7. Gumz ML, Rabinowitz L, Wingo CS. An integrated view of potassium homeostasis. N Engl J Med 2015; 373: 60–72.

8. Berns JS, Hayslett JP. Renal and extrarenal excretion of potassium. In Seldin DW, Giebisch G (eds). The regulation of potassium balance. New York: Raven, 1989: 157–74.

9. Mandal AK. Hypokalemia and hyperkalemia. Med Clin North Am 1997; 81: 611–39.

10. Riggs JE. Neurologic manifestations of electrolyte disturbances. Neurologic Clinics 2002; 20: 227–39.

11. Paltiel O, Salakhov E, Ronen I, Berg D, Israeli A. Man-agement of severe hypokalemia in hospitalized patients. Arch Intern Med 2001; 161: 1089–95.

12. Palmer BF. Managing hyperkalemia caused by inhibitors of renin-angiotensin-aldosteron system. N Engl J Med 2004; 351: 585–92.

13. Stein G, Ritz E. Klinik und Diagnostik der Hyperkali­ämie. Dtsch Med Wschr 1990; 115: 899–902.

14. Gennari FJ. Hypokalemia. N Engl J Med 1998; 339: 451–8.

15. Reincke M, Seiler L, Rump LC. Normokaliämischer primärer Hyperaldosteronismus. Dt Ärztebl 2003; 100: B169–74.

16. Beal AL, Scheltema KE, Beilman GJ, Deuser WE. Hypokalemia following trauma. Shock 2002; 18: 107–10.

17. Penney MD, Oleesky DA. Renal tubular acidosis. Ann Clin Biochem 1999; 36: 408–22.

18. Amirlak I, Dawson KP. Bartter syndrome: an overview. Q J Med 2000; 93: 207–15.

19. Barakat AJ, Rennert OM. Gitelman’s syndrome (famial hypokalemia-hypomagnesemia). J Nephrol 2001; 14: 43–7.

20. Jones BJ, Twomey PJ. Comparison of reflective and reflex testing for hypomagnesaemia in severe hypokalemia. J Clin Pathol 2009; 62: 816–9.

21. Lapie P, Lory P, Fontaine B. Hypokalemic periodic paralysis: an autosomal dominant muscle disorder caused by mutations in a voltage-gated calcium channel. Neuromuscular Disorders 1997; 7: 234–40.

22. Duke M. Thiazide-induced hypokalemia: association with acute myocardial infarction and ventricular fibrillation. JAMA 1978; 239: 43–5.

23. Burl RD, Sebastian A, Cheitlin MW, Christiansen M, Schambelan M. Pseudohyperkalemia caused by first clenching during phlebotomy. N Engl J Med 1990; 322: 1290–2.

24. Colussi G, Cipriani D. Pseudohyperkalemia in extreme leukocytosis. Am J Nephrol 1995; 15: 450–2.

25. Wulkan RW, Michiels JJ. Pseudohyperkalemia in thrombocythemia. J Clin Chem Clin Biochem 1990; 28: 489–91.

26. Alani FSS, Dyer T, Hindle E, Newsome DA, Ormerod LP, Mahoney MP. Pseudohyperkalemia associated with hereditary spherocytosis in four members of a family. Postgrad Med J 1994; 70: 749–51.

27. Hawkins RC. Serum potassium in renal impairment: At what concentration of estimated GFR does it rise? Clin Chim Acta 2009; 408: 135–6.

28. Perazella MA. Drug-induced hyperkalemia: old culprits and new offenders. Am J Med 2000; 109: 307–14.

29. Cairo MS, Bishop M. Tumour lysis syndrome: new therapeutic strategies and classification. Br J Haematol 2004; 127: 3–11.

30. Oster JR, Singer I, Fishman LM. Heparin-induced aldosterone suppression and hyperkalemia. Am J Med 1995; 98: 575–86.

31. Bailey IR, Thurlow VR. Is suboptimal phlebotomy technique impacting on potassium results for primary care? Ann Clin Biochem 2008; 45: 266–9.

32. Cornes MP, Ford C, Gama R. Spurious hyperkalaemia due to EDTA contamination: common and not always easy to identify. Ann Clin Biochem 2008; 45: 601–3.

33. Kapoor AK, Ravi A, Twomey PJ. Investigation of outpatients referred to a chemical pathologist with potential pseudohyperkalaemia. J Clin Pathol 2009; 62: 920–3.

34. Hira K, Aoki N, Fukui T. Pseudohyperkalaemia at commercial laboratories in Japan: a questionnaire survey. Ann Clin Biochem 2004; 41: 155–6.

35. Hawkins RC. Poor knowledge and faulty thinking regarding hemolysis and potassium elevation. Clin Chem Lab Med 2005; 43: 216–20.

36. Dsatych M, Cermakova Z. Pseudohyperkalaemia in leukaemic patients: the effect of test tube type and form of transport to the laboratory. Ann Clin Biochem 2014; 51: 110–3.

37. Polak R, Huisman R, Sikma MA, Kersting S. Spurious hypokalaemia and hypophosphaemia due to extreme hyperleukocytosis in a patient with haematological malignancy. Ann Clin Biochem 2010; 47: 179–81.

38. Thurlow V, Ozevlat H, Jones SA, Bailey IR. Establishing a practical blood platelet threshold to avoid reporting spurious potassium results due to thrombocytosis. Ann Clin Biochem 2005; 42: 196–9.

39. Marsoner HJ, Harnoncourt K. Potentiometrische Bestimmung der Kaliumkonzentration im Plasma. Ärztl Lab 1977; 23: 327–9.

40. Boink FBT, Bijster B, Vink KL, Maas AH. Direct potentiometric determination of sodium in blood. III. Influence of bicarbonate. Clin Chem 1985; 31: 523–6.

41. Palmer BF, Clegg DJ. Electrolyte disturbances in patients with chronic alcohol-use disorder. N Engl J Med 2017; 377: 1368–77.

42. Dupuy AM, Cristol JP, Vincent B, Bargnoux AN, Mendes M, Philibert P, Kloche K, Badiou S. Stability of routine biochemical analytes in whole blood and plasma/serum: focus on potassium stability from lithium heparin. Clin Chem Lab Med 2018, 56: 413–21.

43. Ellison DH, Welling P. Insights into salt handling and blood pressure. N Engl J Med 2021; 385: 1981–93.

44. Morris TG, Lambda S, Fitzgerald T, Roulston G, Johnstone H, Mirzazadeh M. The potential role of the GFR in differentiating between true and pseudohyperkalaemia. Ann Clin Biochem 2020; 57(6): 444–55.

45. Bealing E, Lundquist D, Stock S. The use of estimated glomerular filtration rate (GFR) in identifying pseudohyperkalemia in primary care. Ann Clin Biochem 2022; 0 (0): 1–2.

46. Vokoun CW, Murphy MC, Reynolds KL, Haines MS. Case1-2023: a 49-year-old man with hypokalemia and paranoia. N Engl J Med 2023; 388 (2): 165–175.

8.8 Renal electrolyte excretion

Lothar Thomas

The kidneys are responsible for electrolyte and water homeostasis, and they carry out this function through glomerular filtration, re uptake and secretion of water and solutes. This is performed by special nephron transporters, cotransporters and ion channels, which have a specific tubular localization. For urine formation, a daily volume of 160 liters of glomerular filtrate is produced, 99% of which is reabsorbed in the tubules, along with most of the solutes. The excretion of the solutes is determined ultimately by the relationship between the amount of solutes filtered and the quantity reabsorbed into the peritubular capillaries, which is thereby returned to the systemic circulation /1/.

In the proximal tubule specific channels and a Na⁺-K⁺-ATPase dependent transport system permit the movement of selected ions, for glucose, phosphate and amino acids through cellular tubular membranes and the re-uptake from the tubulus lumen into the cell. Cotransporters, responsible for the reabsorption of organic solutes (glucose and amino acids) and inorganic substances (Cl^– and phosphate), are also localized in the tubular cells. The concentrating functions of the filtrate are localized in the distal tubules, while the final composition of the urine occurs in the collecting ducts.

Congenital and acquired disorders of channel proteins, termed channelopathies, have critical roles enabling Na⁺ and K⁺ re uptake or excretion along the nephron, in Mg²⁺ homeostasis, in the control of water reabsorption in the collecting duct, and in determining glomerular permeability /2/. Evaluations of electrolyte excretion are helpful in order to localize disorders of electrolyte and water balance and to quantify their scope.

In a study follow-up  /18/ of 8.8 years participants of mean age 51 and median 24-hour Na⁺ excretion of 3270 mg, higher Na⁺ excretion, lower K⁺ excretion and a higher Na⁺/K⁺ ratio were associated with a higher cardiovascular risk. Each daily increment of 1,000 mg in Na⁺ excretion was associated with an 18% increase in cardiovascular risk (hazard ratio 1.18) and each daily increment of 1,000 mg in K⁺ excretion was associated with an 18% decrease in risk (hazard ratio 0.82).

8.8.1 Urinary electrolyte determination

The investigation of urinary electrolyte excretion includes, depending upon the clinical issue, the determination of Na⁺, K⁺, H⁺ (pH), ammonium ions (NH₄⁺), Cl^– and HCO₃^–. For purposes of differentiation of the disorders, the urinary anion gap, osmotic gap and fractional excretion (FE) of Na⁺ (FE_Na) are calculated.

8.8.1.1 Indication

Sodium

• Etiological evaluation of states associated with hypernatremia or hyponatremia
• Suspection of disturbances of water balance.

Potassium

• Differentiation of renal and extrarenal causes in states associated with hyperkalemia or hypokalemia
• Suspected intake of non-potassium-sparing diuretics.

Chloride, pH, bicarbonate

• Diagnosis of metabolic acidosis.

8.8.1.2 Method of determination

Sodium

Na⁺-selective electrode (ISE), flame photometry; for principle see Section 8.2.2 – Method of determination

Potassium

K⁺-selective ISE, flame photometry; for principle see Section 8.2.2 – Method of determination and Section 8.6.2 – Method of determination

Chloride

Cl^–-selective ISE, coulometric titration (chloride meter), mercurimetric titration; for principle see Section 8.3 – Chloride

Ammonium

Spectrophotometric determination using glutamate dehydrogenase (EC 1.4.1.3) and electrochemical determination with potentiometry or conductimetry

H ions

Determination by means of pH meter

Bicarbonate (HCO₃^–)

Spectrophotometric determination using the enzymes phosphoenolpyruvate carboxylase and malate dehydrogenase. Determination of urinary PCO₂ and pH and calculation of HCO₃^– according to the following equation:

HCO₃^– (mmol/L) = 10^{(pH – pK)} × 0.03 × PCO₂

pK = 6.1

8.8.1.3 Specimen

Na⁺, K⁺, Cl^–

For determination of the concentration: fresh spot urine sample. Send the full urine volume to the laboratory or, after having measured the volume, at least 10 mL.

For determination of the excretion: send 24-hour urine sample without supplement to the laboratory.

pH

In the morning fresh random urine is voided into a vessel containing mineral oil; this covers the fluid surface, thus preventing the loss of CO₂.

Ammonium

Fresh early morning voided urine sample, see Na⁺, K⁺, Cl^–.

Bicarbonate

Fresh early morning voided random urine sample, see Na, K, Cl.

8.8.1.4 Reference interval

Refer to references /3, 4, 5/ and Tab. 8.8-1 – Urinary electrolyte reference intervals.

8.8.2 Disturbances of sodium excretion

Excessive salt consumption is a risk factor for the development of hypertension and cardiovascular disease. Na⁺ excretion in 24-hour urine is an indicator of the amount of salt that is ingested with food. The intake of 0.6 g of salt represents 10 mmol of excreted Na⁺. In the industrialized nations of the Western world, salt intake is 10–15 g (160–250 mmol of Na⁺) per day, instead of the adequate amount of 3 g of NaCl (50 mmol Na⁺). Na⁺ excretion in 24-hour urine is a good marker for the assessment of daily salt intake. With Na⁺ intake of 250 mmol/day, 252 ± 65 mmol/24 h are excreted, while with an intake of 50 mmol/day, 46 ± 27 mmol/24 h are excreted /6/.

Some 30% of the Na⁺ is reabsorbed in the distal tubules and the collecting ducts. In the distal tubules this is accomplished in accordance with the principle shown in Fig. 8.8-1 – Pathways of Na⁺, K⁺, Mg⁺⁺ and Ca⁺⁺ reabsorption in the thick ascending limb of the loop of Henle.

The composition of the Na⁺ and K⁺ concentrations of the final urine takes place in the collecting ducts (Fig. 8.8-2 – Reabsorption of solutes in the collecting duct). Na⁺ is reabsorption takes place in exchange for K⁺ via respective special channels.

The lining of the luminal membrane of the collecting duct cells with ion channels is regulated by aldosterone, which acts via the mineralocorticoid receptors. A deficiency of aldosterone, or resistance of the receptors to aldosterone, reduces the synthesis of ion channels /2/.

Renal salt and water excretion adjusts to the daily dietary intake, so that a dynamic equilibrium between intake and excretion is created and, as a result, the Na⁺ concentration in the extracellular fluid (ECF) is held constant. Small changes in renal reabsorption of Na⁺ and water can lead to considerable alterations in the volume of the ECF. Thus, with a sodium-free diet, Na⁺ excretion in healthy individuals declines to below 3 mmol/24 h after 3–5 days.

Since Cl^– is the principal anion of Na⁺, renal excretion of Na⁺ and Cl^– is, in many ways, identical. The determination of renal Na⁺ excretion is diagnostically important for the differentiation of hyponatremia and hypernatremia. See Section 8.2 – Sodium.

8.8.2.1 Sodium excretion in hyponatremia

Renally induced Na⁺ losses are associated with the excretion of more than 20 mmol/L in the random urine sample /7, 8/.

Conditions with reduced extracellular fluid volume decline (ECFV) due to extra-renal fluid losses (except for vomiting) and Na⁺ losses in the third space (edema) are associated with Na⁺ excretion of less than 20 mmol/L.

In conditions with hyponatremia, renal Na⁺ excretion is an indicator of the volume status of the ECFV. Accordingly, in patients with hypo osmolar hyponatremia, renal Na⁺ excretion is:

• In euvolemia above 20 mmol/L
• In hypovolemia (volume depletion) below 20 mmol/L
• In hypervolemia below 20 mmol/L.

The differential diagnostic significance of Na⁺ excretion in hyponatremia is shown in Tab. 8.8-2 – Differential diagnostic significance of urinary Na⁺ excretion in hyponatremia.

8.8.2.2 Sodium excretion in hypernatremia

The major cause of hypernatremia is the more marked loss of water in comparison with that of Na⁺. For this reason, the serum is hyper osmolar /9/.

In conditions with hypernatremia, renal Na⁺ excretion is an indicator of the volume status of the ECFV. Thus, in patients with hyper osmolar hypernatremia, renal Na⁺ excretion is:

• In volume depletion below 20 mmol/L.
• In hypervolemia above 20 mmol/L.
• In states with loss of free water (renal or hypothalamic) variable. See Section 8.5 – Osmolality).

The differential diagnostic significance of Na⁺ excretion in hypernatremia is shown in Tab. 8.8-3 – Differential diagnostic significance of urinary Na⁺ excretion in hypernatremia.

8.8.3 Fractional sodium excretion (FE_Na)

The FE_Na test determines the excreted fraction of glomerular filtered Na⁺ and is a measure of tubular Na⁺ reabsorption /10/. It serves to differentiate between pre renal azotemia and tubular necrosis in patients with acute renal failure in the oliguric phase. Patients with pre renal failure have a Fe_Na less than 1%, patients with acute tubular necrosis have a Fe_Na of more than 2%.

U, urine; S, serum; Na in mmol/L, creatinine in μmol/L or mg/dL

Interpretation of the FE_Na test

The FE_Na provides better information concerning volume status than does urinary Na⁺ concentration. Healthy individuals with a glomerular filtration rate (GFR) of 120 [mL × min^–1 × (1.73 m²)^–1] and a normal Na⁺ excretion of 120 mmol/L have an Fe_Na of 0.19–0.78%.

In pre renal failure, the nephron attempts to conserve Na⁺. The Na⁺ excretion in a random urine sample is below 20 mmol/L, urine osmolality is above 500 mmol/kg and the Fe_Na is less than 1%. This is the case, for example, in patients with severe heart failure in NYHA stages III and IV with an ejection fraction below 35%.

In renal failure the reabsorption of Na⁺ is diminished, Na⁺ excretion is above 20 mmol/L, urine osmolality is below 300 mmol/kg, and the Fe_Na is greater than 2%.

It must be noted that the FE_Na provides no information in patients treated with diuretics. In these patients it is false high with values of up to 20%.

8.8.4 Disturbances of chloride excretion

Cl^– is the most important counter ion to Na⁺ and is reabsorbed electroneutrally with Na⁺ in the renal tubules. In many pathological conditions the excretion of Na⁺ and Cl^– behave similarly. A dissociation of renal Na⁺ and Cl^– excretion occurs in vomiting.

Vomiting

In sustained vomiting the excretion of Cl^– in the random urine sample is below 20 mmol/L in the presence of elevated Na⁺ and K⁺ excretion. The cause of this is that during continuous vomiting NaCl, HCl and KCl are lost via the gastric juices. Due to the loss of H⁺ the concentration of HCO₃^– is elevated and metabolic alkalosis develops. The increased delivery of HCO₃^– to the distal tubules leads to enhanced excretion of Na⁺ and K⁺. The patient’s volume status in chronic vomiting is more accurately provided by the excretion of Cl^– than by that of Na⁺.

Metabolic alkalosis /9/

In the therapy of metabolic alkalosis Cl^– excretion below 10 mmol/L provides an indication of improvement of the disorder through salt intake. In Cl^–-resistant alkalosis excretion is measured as a function of salt intake. See also metabolic alkalosis in Section 8.3.5.3 – Chloride level in metabolic alkalosis.

8.8.5 Fractional chloride excretion (FE_Cl)

The FE_Cl test determines the fraction of Cl^– filtered by the glomeruli that is excreted in the urine; it is a measure of tubular Cl^– reabsorption. The normal FE_Cl value is 1–3%. In alkalosis and extrarenal Cl^– loss (vomiting, congenital chloridorrhea), the FE_Cl is below 1% and in metabolic acidosis above 3%.

U, urine; S, serum; Cl in mmol/L, creatinine in μmol/L or mg/dL

8.8.6 Urinary anion gap (U_AG)

Polyuria, caused by electrolyte diuresis, usually results from increased NaCl excretion and are the consequence of intravenous administration of NaCl, excessive oral salt intake, renal loss of salt, or the medication of loop diuretics /12/.

For the evaluation of whether an anion other than Cl^– is present in meaningful concentrations as a counter ion to Na⁺ or whether cations other than Na⁺ and K⁺, especially NH₄⁺, are excreted, the urinary anion gap (U_AG) is determined (Tab. 8.8-4 – Interpretation of the urinary anion gap).

The calculation of the U_AG is performed in a pH-dependent manner according to the following equations:

1. Urine pH below 6.5:

U_AG = Na⁺ (mmol/L) + K⁺ (mmol/L) – Cl^– (mmol/L)

2. Urine pH above 6,5:

U_AG = Na⁺ (mmol/L) + K⁺ (mmol/L) – Cl^– (mmol/L) – HCO₃^– (mmol/L)

8.8.7 Disorders of H ions, ammonium and bicarbonate excretion

The kidneys are the principal organ in the regulation of the systemic pH value. Alterations in acid-base status result in corresponding changes in H⁺ secretion, HCO₃^– reabsorption and NH₄⁺ formation. These regulatory changes take place preferentially in the proximal tubule and the collecting duct. Their effectiveness is based upon a rapid change in the net trans epithelial flux of H⁺ and HCO₃^–. Approximately 80–90% of the reabsorption of filtered HCO₃^– occurs in the proximal tubules. Basic mechanisms in the proximal tubules are the allosteric pH controlled Na⁺–H⁺ exchange of the luminal membranes (Fig. 8.8-3 – Acid-base homeostasis in the proximal convoluted tubulus) and an electrogenic Na⁺ and 3 HCO₃^– cotransport system in the basolateral cell membrane /13/.

Proximal tubule mechanisms

Fundamental mechanisms in the proximal tubule are allosteric pH-controlled Na⁺-H⁺ exchange at the luminal cell membrane via the Na⁺-H⁺ exchanger (NHE-3) and the HCO₃^– transport at the basolateral membrane mediated by the 1 Na⁺–3 HCO₃^– cotransporter (NBC-1) (Fig. 8.8-3).The high trans cellular HCO₃^– transport rates occur at a rather constant cytosolic pH. K⁺ channel regulation by cytosolic pH adjusts HCO₃^– export to import.^.If luminal H⁺ secretion exceeds basolateral HCO₃^– transport cytosolic pH alkalinizes. This leads to the activation of K⁺ channels which hyperpolarize the membrane potential, giving an enhanced driving force for electrogenic HCO₃^– export /13, 14/.

Collecting duct mechanisms

In the collecting duct α-type intercalated cells secrete H⁺ via a vacuolar type H⁺-ATPase and a H⁺–K⁺ -ATPase. HCO₃^– is exchanged with Cl^– to the blood site /13, 14/. β-type intercalated cells exchange Cl^– and HCO₃^– at the luminal site. H⁺ leave the cell via basolateral Na⁺–H⁺ exchange (not shown in Fig. 8.8-4 – H⁺ Secretion into the cortical collecting tubule.

Systemic acid disorders

In this condition, the luminal secretion of H⁺ by the nephron exceeds the basolateral (blood side) HCO₃^– export, and the cytosolic pH becomes alkaline. This activates the K⁺ channels, whereby the cell membrane is hyperpolarized and enhanced electrogenic exit of HCO₃^– from the cells into the tubular lumen occurs. In addition, renal glutaminase is induced and hepatic glutamine synthetase is activated. In this way, renal glutamine metabolism is able to form large quantities of NH₄⁺. Contingent upon its charge and the alkaline pK value, NH₄⁺ remains in the tubular lumen /13, 14/.

Systemic alkalosis

In this condition, luminal secretion of H⁺ by the nephron is reduced relative to basolateral (blood side) HCO₃^– secretion, and the cytosolic pH is acidic. In order to maintain intracellular homeostasis, trans cellular transport of HCO₃^– from the tubular lumen into the blood is decreased /14/.

8.8.7.1 Renal-tubular acidoses (RTA)

The term RTA comprises a group of transport defects in the reabsorption of HCO₃^–, the excretion of H⁺ or both. In RTA, the glomerular filtration rate (GFR) is usually normal and metabolic acidosis, hyperchloremia and a normal anion gap in the plasma are present. In contrast, in uremic acidosis a low GFR is found and the metabolic acidosis is associated with an increased anion gap and a normochloremia or hyperchloremia.

RTA is differentiated into three types /15/:

• Distal RTA, type 1. This type is the most common form of RTA and it is subdivided into a classical, complete form and an incomplete form. The complete type is caused by a defect in the vacuolic H⁺-ATPase (Fig. 8.8-4 – H⁺ secretion into the cortical collecting tubule). The result is impaired distal acidification and inability to lower urine pH maximally (< 5.5).
• Proximal RTA, type 2. This type is characterized by a decreased renal HCO₃^– threshold in the proximal tubule (Fig. 8.8-3 – Maintaining the acid-base homeostasis in the proximal tubulus). Distal acidification mechanisms are intact.
• Hyperkalemic RTA, type 4. This type of RTA is mainly caused by impaired ammoniogenesis. Type 4 is most frequently observed in states of hypo- or pseudo hypo aldosteronism.

8.8.7.1.1 Laboratory diagnosis of RTA

In general RTA should be suspected (Tab. 8.8-5 – Renal-tubular acidoses (RTAs)):

• In cases where metabolic acidosis is accompanied by hyperchloremia and normal anion gap in plasma [Na⁺ – Cl^– + HCO₃^–] = 8 – 16 mmol/L, in a patient without evidence of gastrointestinal HCO₃^– losses and who is not taking acetazolamide or ingesting exogenous acid /13/.
• If hypokalemia is present, and diuretics are not being taken.

A series of functional tests are available for the typing of RTA. Of these, the HCO₃^– loading test and the ammonium chloride loading test are employed most frequently. Tests for the functional evaluation of proximal HCO₃^– reabsorption (diagnosis of proximal RTA) and distal acidification of the urine (diagnosis of distal RTA) are listed in Tab. 8.8-6 – Laboratory diagnostics of renal-tubular acidoses (RTAs).

Diagnostic work-up

In suspected RTA, one should proceed as follows /13/:

• Determination of the anion gap and the osmotic gap in a random voided morning urine sample. If the patient has hyperchloremic metabolic acidosis, a negative anion gap or an osmotic gap exceeding 100 mmol/L, proximal RTA is likely. Gastrointestinal loss of HCO₃^– and the intake of acidifying salts must be ruled out (Fig. 8.8-5 – Diagnostic work-up for patients with hyperchloremic metabolic acidosis and negative urinary anion gap). Furthermore, repeated use of laxatives which, in low Na⁺ excretion must be taken into consideration, should be asked about in adults. The definitive diagnosis of proximal RTA (RTA type 2) is established if the fractional HCO₃^– and the urine-plasma PCO₂ difference (U-B PCO₂) is likewise elevated and the anion gap is negative (Tab. 8.8-7 – Differentiation of the renal-tubular acidoses (RTA) by biochemical tests).
• When a random voided morning urine sample from a patient with hyperchloremic metabolic acidosis has a positive anion gap or an osmotic gap below 100 mmol/L, a defect in the distal urinary acidification should be suspected. The next step is the determination of K⁺ in the plasma. If the value is normal or decreased, the inability of the kidneys to lower urine pH below 5.5 after ammonium chloride loading establish the diagnosis of distal RTA (RTA type 1) (Fig. 8.8-6 – Diagnostic work-up for patients with hyperchloremic metabolic acidosis and positive urinary anion gap).
• When in hyperchloremic metabolic acidosis, the random voided morning urine sample anion gap is positive and the osmotic gap is below 100 mmol/L, a defect in the distal urinary acidification should be suspected. The next step is the determination of K⁺ in serum. When the K⁺ level is increased, even modestly, attention should be paid to the value of the urine pH. The finding of pH higher than 5.5 after ammonium chloride loading will permit the identification of patients with hyperkalemic distal RTA (RTA type 4) caused by a voltage dependent defect (Fig. 8.8-6).

8.8.8 Disturbances of potassium excretion

Although trans cellular shifting of K⁺ is a rapid, efficient mechanism to reduce serum K⁺ must be excreted by the kidney to maintain external K⁺ balance. Distal nephron K⁺ delivery is approximately 10% of filtered K⁺ regardless of the glomerular filtration rate. However in addition to ingested potassium additional K⁺ excretion is possible. This situation is impossible with regard to sodium balance.

The most important factors influencing K⁺ secretion by the by the distal nephron are prior to potassium exposure /16/:

• Mineralocorticoids such as aldosterone
• Extracellular fluid volume which inhibits the effect of aldosterone but by raising the flow rate increases the availability of Na⁺ in the distal tubules and thus, leading to an increased excretion of K⁺
• Systemic alkalosis, which enhances K⁺ excretion
• The intake of diuretics such as thiazides, furosemide and bumetanide, which cause an increase in K⁺ excretion by raising the flow rate in the first third of the collecting ducts.

Normal K⁺ excretion is greater than 40 mmol/L. With normal K⁺ intake the kidneys are capable to maintain K⁺ excretion with a decrease of GFR to approximately 10 [mL × min^–1 × (1.73 m²)^–1].

The determination of renal K⁺ excretion is an important step toward differentiation of the various forms of hypokalemia and hyperkalemia.

8.8.8.1 Potassium excretion in hypokalemia

For the K⁺ stores of the organism (e.g.,most is in muscle 3000–5000 mmol), serum K⁺ levels are useful only to a limited degree. In K⁺ deficiency, the K⁺ determination in the random urine sample aids in the localization of the losses.

A value below 10 mmol/L in the presence of hypokalemia is indicative of extrarenal loss, while a value greater than 10 mmol/L suggests renal loss (Tab. 8.8-8 – Renal potassium excretion in hypokalemia).

The diseases associated with hyperkaliuric hypokalemia are listed in Tab. 8.8-9 – Hyperkaliuric hypokalemia. Elevated excretion is also measured in:

• Metabolic acidosis as an effect of distribution disorders of K⁺ (efflux of K⁺ from the intracellular fluid).
• Metabolic alkalosis, as replacement cation for Na⁺, K⁺ along with H⁺ and NH₄⁺ are excreted.

8.8.8.2 Potassium excretion in hyperkalemia

In hyperkalemia, K⁺ excretion below 40 mmol/L indicates renal etiology, while a value of ≥ 40 mmol/L is suggestive of extrarenal causes (Tab. 8.8-10 – Renal potassium excretion in hyperkalemia).

The causes of hypocaliuric hyperkalemia are shown in Tab. 8.8-11 – Hypocaliuric hyperkalemia.

8.8.9 Fractional excretion of urinary electrolytes

Reference intervals

FE_Na 0.19–0.78%; FE_K 4.9–10.3%; FE_Cl 0.35–1.07%; FE_Ca 0.6–1.0%; FE_Mg 1.4–1.8% /17/.

References

1. Sayer JA, Pearce SHS. Diagnosis and clinical biochemistry of inherited tubulopathies. Ann Clin Biochem 2001; 38: 459–70.

2. Loudon KW, Fry AC. The renal chanelopathies. Ann Clin Biochem 2014; 51: 441–58.

3. Krieg M, Gunßer KJ. Vergleichende quantitative ANalytik klinisch-chemischer Kenngrößen im 24-Stunden-Urin und Morgenurin. J Clin Chem Clin Biochem 1986; 24: 863–9.

4. Bingham S, Williams R, Cole TJ, Price C, Cummings JH. Reference values for analytes of 24-h urine collections known to be complete. Ann Clin Biochem 1988; 25: 610–9.

5. Illich JZ, Blanusa M, Orlic ZC, Orct T, Kostial K. Comparison of calcium, magnesium sodium, potassium, zinc, and creatinine concentration in 24-h and spot urine samples in women. Clin Chem Lab Med 2009; 4: 216–22.

6. Pimeta E, Gaddam K, Oparil S, Aban I, Husain S, Dell’Ìtalia LJ, et al. Effects of dietary sodium reduction on bood pressure in subjects with resistant hypertension: results from a randomized trial. Hypertension 2009; 54: 475–81.

7. Bichet DG, Kluge R, Howard RL, Schrier RW. Pathogenesis of hypoNatremic states. In: Seldin DW, Giebisch G (eds). Clinical disturbances of water metabolism. New York: Raven, 1993: 169–88.

8. Bichet DG, Kluge R, Howard RL, Schrier RW. Pathogenesis of hyponatremic states. In: Seldin DW, Giebisch G (eds). Clinical disturbances of water metabolism. New York: Raven, 1993: 169–88.

9. Howard RL, Bichet DG, Schrier RW. Pathogenesis of hyperNatremic and polyuric states. In: Seldin DW, Giebisch G (eds). Clinical disturbances of water metabolism. New York: Raven, 1993: 189–209.

10. Espinel CH. The FeNa test. Use in the differential diagnosis of acute reNal failure. JAMA 1976; 236: 579–81.

11. Schmidt LH, Wessing H, Ehrhardt W. Electrolytes in urine: possibilities and limits of interpretation. Klin Lab 1992; 38: 616–8.

12. Oster JR, Singer I, Thatte L, Grant-Taylor I, Diego JM. The polyuria of solute diuresis. Arch Intern Med 1997; 157: 721–9.

13. Bleich M. Tubular function and acid – base disturbances. Nephrol Dial Transplant 1995; 10: 1540–1.

14. Soriano JR. Renal tubular acidosis: the clinical entity. J Am Soc Nephrol 2002; 13: 2160–70.

15. Ring T, Frische S, Nielsen S. Clinical review: renal tubular acidosis – a physicochemical approach. Crit Care 2005; 9: 573–80.

16. Linas SL, Berl T. Clinical diagnosis of abnormal potassium balance. In: Seldin DW, Giebisch G (eds). The regulation of potassium balance. New York: Raven 1989: 177–205.

17. Gross P, Reimann D. Das Gitelman-Syndrom. Nephro-News 2002; 3: 1–5.

18. Ma Y, He FJ, Sun Qui, Changzheng Y, Kieneker LM, Curhan GC, et al. 24-hour urinary sodium and potassium excretion and cardiovascular risk. N Engl J Med 2021. doi: 10.1056/NEJMoa2109794.

8.9 Cystic fibrosis

Lothar Thomas

Cystic fibrosis is an autosomal recessive disease caused by mutations in the gene CFTR. The cFTR protein is involved in the regulation of transepithelial ion and water transport. The CFTR gene encodes for the ion channel protein cFTR that is evolved in the regulation of water- and electrolyte balance on the surface of many organs. Deletion of three base pairs in the CFTR gene lead to the loss of the amino acid phenylalanine at position 508 in the cFTR protein. The number of CFTR gene variants is more than 2000 and about 700 disease-causing mutations are registered by the Cystic fibrosis Foundation /1/. Cystic fibrosis variants have different implications with respect to cFTR production. The absence or dysfunction of cFTR protein leads to decreased Cl^– and HCO₃^– secretion at the apical membrane of the secretory cell.

8.9.1 Indication

Newborn screening

8.9.2 Method of determination

The diagnosis of cystic fibrosis is based on:

• a positive sweat chloride test
• genetic testing for common CFTR variants: in the case of three disease-causing CFTR variants cystic fibrosis is proven
• most screening programs use blood trypsinogen concentrations, which are increased in infants with cystic fibrosis.

8.9.3 Specimen

• Sweat 0.01 mL; sweat chloride test
• Blood 0.5 mL for testing of immunoreactive trypsinogen determination in the first 24 to 72 hours of life: above the 99. percentile
• Blood 0.5 mL for discovery of the CF gene.

8.9.4 Reference ranges

• Sweat chloride test: Cl^– concentration < 60 mmol/L
• Blood: immunoreactive trypsinogen determination in the first 24 to 72 hours of life: above the 99. percentile
• Genetic testing: no gene variant.

8.9.5 Clinical significance

The diagnosis of cystic fibrosis is based on many organ manifestations that is consistent with /2/:

• a positive sweat test (refer to section 47.10 – Sweat), or
• genetic testing (three base base pairs in the CFTR leading to the loss of the amino acid phenylalanin at position 508).

About 85% of the cystic fibrosis variants in patients in the United States of America are F508del and results in defective protein folding /1/.

Cystic fibrosis causing variants have different implications in the production of cFTR protein expression, processing, and function. The implications can result in /2/:

• premature termination codons with reduced/absent cFTR formation (class I variant)
• premature degradation due to protein misfolding (class II variant)
• abnormalities of channel gating (class III variant)
• conductance deficiency (class IV variant)
• splicing abnormalities (class V variant)
• increased cell-surface turnover (class VI variant).

A given variant can cause more than one molecular defect.

Cystic fibrosis is based on one or more organ manifestations.

Clinically important are /2/:

• lung infection with acute pulmonary exacerbations /3/ (S. aureus, Pseudomonas aeruginosa, Burkholderia complex, Stenotrophomonas maltophilia, Burkholderia species)
• pancreatic insufficiency
• effects on nutrition; a positive relationship between body mass index and survival is documented
• effects on the liver (cystic fibrosis related liver disease can cause isolated, often transient increase of bilirubin, ALT, and AST)
• fertility: in females thickened secretions in the fallopian tubes and the cervix and in males thickened secretion of the vas deferens can impede pregnancy.

8.9.6 Pathophysiology

On the surface of many organ systems the protein cFTR is involved in the regulation of transepithelial ion transport. Normal CFTR gene function in the sweat gland results in the absorption of Cl^– from isotonic sweat fluid.

In individuals with cystic fibrosis and CFTR dysfunction that causes decreased absorption of Cl^– in the ducts of the sweat glands and elevated concentrations of Cl^– are measured in the sweat test.

In the airway epithelium dysfunction of CFTR leads to decreased Cl^– and HCO₃^– secretion at the apical cell membrane. The persistent Na⁺ absorption through loss of CFTR-mediated inhibition of the epithelial Na⁺ channel, which causes absorption of airway surface fluid, has consequences in fluid balance. The fluid imbalance causes thickened secretions and reduced mucocillary transport, resulting in mucus retention and plugging of airways. This pathophysiology stimulates an inflammatory response in the airways /4/. Retention of mucus favors bacterial infections and consecutively increased inflammation.

References

1. Cystic fibrosis Foundation patient registry 2021 annual data report. Bethesda, MD: Cystic Fibrosis Foundation 2022. www.cff.org/media/26771/download.

2. Grasemann H, Ratjen F. Cystic fibrosis. N Engl J Med 2023; 389: 1693–1707.

3. Stoltz DA, Meyerholz DK, Welsh MJ. Origins of cystic fibrosis lung disease. N Engl J Med 2015; 35–62.

4. Despotes KA, Donaldson SH. Current state of CFTR modulators for treatment of cystic fibrosis. Curr Opin Pharmacol 2022; 65: 102239. doi: 10.1016/j.coph.2022.102239.