Water Balance, Sodium, and Potassium

Body water makes up 40 to 75% of body weight, lower with age, obesity, and in women because adipose tissue holds little water. Intracellular fluid holds about two-thirds of total body water. Extracellular fluid, the remaining third, divides into intravascular plasma, interstitial fluid, and transcellular fluid such as cerebrospinal fluid. Plasma is about 93% water. The sodium-potassium ATPase pumps three sodium ions out for every two potassium ions in, sustaining steep transmembrane gradients. Total exchangeable sodium is a major determinant of extracellular fluid volume; plasma sodium concentration chiefly reflects water balance and effective tonicity, which governs cell hydration.¹

Osmolality and its measurement

Osmolality is total dissolved particle concentration per kilogram of solvent, expressed in mOsm/kg. It is a colligative property that depends on particle number, never identity. Osmolality may be measured by freezing-point or vapor-pressure osmometry; the physics of each instrument belongs to Laboratory Operations, and the measurement practice for urine belongs to Urinalysis. Suspected volatile-alcohol exposure requires freezing-point depression osmometry, because volatile alcohols enter the vapor headspace and produce a falsely low result with vapor-pressure methods. Osmolarity, expressed per liter of solution, can misestimate true tonicity in hyperlipidemia, hyperproteinemia, and urine, and is not the preferred reporting unit.¹

Plasma osmolality is defended at 275 to 295 mOsm/kg, and sodium with its anions supplies about 90% of that value.

Thirst, vasopressin, and volume regulation

Thirst and arginine vasopressin (AVP) respond to a 1 to 2% osmolality change. Thirst drives water intake to dilute rising osmolality. AVP, released from the posterior pituitary, increases collecting-duct water reabsorption; a 1 to 2% osmolality rise quadruples AVP, and a 15 to 20 minute half-life allows minute-to-minute titration. With AVP suppressed, the kidney can excrete 3 to 20 L of dilute urine daily. Persistent hypo-osmolality therefore nearly always reflects impaired renal water excretion rather than simple overdrinking.¹

Volume is regulated separately from tonicity by the renin-angiotensin-aldosterone system (RAAS). Falling perfusion triggers juxtaglomerular renin release, which converts angiotensinogen to angiotensin I; angiotensin-converting enzyme generates angiotensin II, a vasoconstrictor that also drives aldosterone release; aldosterone promotes distal sodium and water retention. Atrial natriuretic peptide (ANP), released on volume expansion, opposes this system by promoting natriuresis.

Calculated osmolality and the osmolal gap

Because the standard formula counts only sodium salts, glucose, and urea, the difference between measured and calculated osmolality exposes unmeasured osmotically active solutes:¹

Calculated osmolality (mOsm/kg) = 2 × [Na⁺] + [glucose]/18 + [BUN]/2.8, with glucose and BUN in mg/dL

Osmolal gap = measured osmolality − calculated osmolality

The osmolal gap is interpreted against the laboratory’s method- and formula-specific interval, often approximately −10 to +10 mOsm/kg. An increased gap indicates an unmeasured osmole: ethanol, methanol, ethylene glycol, isopropanol, acetone, or beta-hydroxybutyrate. The gap is a nonspecific adjunct, and toxic-alcohol poisoning can occur with a normal gap after the parent alcohol has been metabolized; the toxicology module develops that application.

Worked example. Measured serum osmolality is 326 mOsm/kg, with Na⁺ 137 mmol/L, glucose 126 mg/dL, and BUN 14 mg/dL. Calculated osmolality = 2(137) + 126/18 + 14/2.8 = 274 + 7 + 5 = 286 mOsm/kg. The osmolal gap is therefore 40 mOsm/kg. In an ill patient with compatible acid-base findings, a gap of this magnitude should trigger evaluation for an unmeasured osmole such as a toxic alcohol.

Intervals vary with laboratory, method, and formula. The clinical reference intervals topic keeps the cross-discipline comparison, and the specific gravity and osmolality topic owns urine measurement practice.

Sodium

Sodium supplies about 90% of extracellular cation charge. Plasma sodium is governed by thirst-driven water intake, AVP-driven water excretion, and sodium excretion mediated by aldosterone, angiotensin II, and ANP. The proximal tubule reabsorbs 60 to 75% of filtered sodium.

Hyponatremia

Hyponatremia, generally below 135 mmol/L, becomes urgent below 120 mmol/L. Classification combines mechanism with concurrent serum osmolality:¹

Corrected sodium for hyperglycemia. Na⁺(corrected) = measured Na⁺ + 1.6 × [(glucose mg/dL − 100)/100]. This traditional factor is an estimate; studies support a correction near 2.4 mmol/L per 100 mg/dL glucose above 100 in marked hyperglycemia, and laboratories should state which factor their clinical protocol uses.²

Worked example. Measured Na⁺ is 130 mmol/L while glucose is 430 mg/dL. Using 1.6 mmol/L per 100 mg/dL glucose above 100: corrected Na⁺ = 130 + 1.6 × (330/100) = 130 + 5.28 = 135.3 mmol/L. The correction shows that much of the apparent hyponatremia is attributable to hyperglycemia, while the patient’s current tonicity must still be assessed separately.

Pseudohyponatremia is purely analytic. Sodium dissolves only in the aqueous portion of plasma, roughly 93% of its volume. Gross hyperproteinemia or hyperlipidemia expands the nonaqueous fraction, so an indirect, pre-diluted ISE under-reports sodium relative to the true aqueous-phase concentration. A direct, undiluted ISE, as used on blood gas analyzers, is immune. In-vitro hemolysis causes a separate, true dilutional decrease by releasing low-sodium erythrocyte contents into the aqueous phase.¹

Symptoms depend on both severity and rate of decline. Acute hypotonic hyponatremia can cause severe neurologic symptoms at concentrations tolerated in chronic disease. The rate of sodium correction is determined by the treating service and institutional protocol; the laboratory reports serial sodium and osmolality and flags critical values.

Hypernatremia

Hypernatremia reflects a water deficit relative to sodium and is less common than hyponatremia because intact thirst normally prevents it. Its presence indicates that thirst or access to water has failed, as in infants, altered mental status, and older adults with blunted osmotic thirst. Urine osmolality localizes the defect: low or inappropriately normal urine osmolality with hypernatremia points to renal water wasting from central or nephrogenic diabetes insipidus or tubular disease, while concentrated urine points to extrarenal loss or sodium loading with an appropriately conserving kidney. Symptoms are neurologic, including altered mental status, seizures, and hyperreflexia; sodium above 160 mmol/L carries substantial mortality.¹

Determination. Specimens include serum, plasma (lithium heparin, ammonium heparin, or lithium oxalate), whole blood, and 24-hour urine. Ordinary hemolysis has little effect because erythrocyte sodium is low relative to plasma. The method is ion-selective electrode (ISE) potentiometry: a semipermeable membrane develops a potential proportional to the transmembrane activity gradient, translated to concentration through the Nernst relationship. Sodium electrodes use a glass ion-exchange membrane, and scheduled cleaning prevents protein-fouling drift.

Each laboratory verifies its own intervals for its population and instrument under CLIA, 42 CFR 493.1253, and reports patient results against its verified interval. Examination answers follow the ASCP BOC composite set.

Potassium

Potassium is the dominant intracellular cation, roughly 20-fold more concentrated inside cells than outside. Only about 2% of body potassium is extracellular, and this is the pool the laboratory samples. Potassium determines the resting membrane potential: hyperkalemia depolarizes toward threshold, producing hyperexcitability, peaked T waves, then paralysis and fatal arrhythmia, while hypokalemia hyperpolarizes, producing weakness, ileus, and arrhythmia. Potassium also trades against hydrogen ion: acidemia can shift potassium extracellularly, especially in mineral acidosis, but no universal potassium change per 0.1-unit pH is reliable. Organic acidosis, insulin status, renal function, and total-body potassium modify the response, and alkalosis often shifts potassium into cells.³

Regulation. The proximal tubule reabsorbs about 65% of filtered potassium and the thick ascending limb reabsorbs most of the remainder; distal nephron secretion determines final urinary excretion. Aldosterone directs distal secretion, exchanged for sodium and competing with hydrogen ion. Acute loads are buffered by cellular uptake through insulin, beta-adrenergic stimulation, and the sodium-potassium ATPase. Pump failure from hypoxia, hypomagnesemia, or digoxin toxicity, or beta-blockade, releases potassium.

Transcellular shifts change plasma potassium without changing total body potassium. They include exercise, from +0.3 up to +3 mmol/L, including the venipuncture fist-clenching artifact; hyperosmolality, which drags potassium out with water; acidosis; and cellular breakdown from trauma, tumor lysis, rhabdomyolysis, or massive transfusion.

Hypokalemia and hyperkalemia

Hypokalemia is usually silent above 3.4 mmol/L. Below 3.0 mmol/L, weakness, fatigue, and constipation progress toward paralysis and respiratory compromise, with the greatest arrhythmic risk in digitalized cardiac patients. Hyperkalemia produces weakness, tingling, and confusion, with ECG change starting around 6 to 7 mmol/L and cardiac arrest possible above 10 mmol/L. Potassium of 6.0 to 6.5 mmol/L or any ECG change is a critical value requiring immediate physician notification; the exact threshold follows each laboratory’s critical-value list.³

Preanalytic artifact in hyperkalemia

Preanalytic artifact is the leading cause of spurious hyperkalemia. Clotting releases platelet potassium, so serum runs 0.1 to 0.7 mmol/L above plasma, worse with thrombocytosis, which favors plasma specimens in that setting. Prolonged tourniquet time and fist-clenching leak potassium from exercised forearm muscle. For combined blood-gas and electrolyte specimens, ice only when the validated protocol allows it, because cooling can alter potassium. Hemolysis is the single most common cause; even mild hemolysis, roughly 50 mg/dL free hemoglobin, raises potassium by roughly 3%, and the exact bias is analyzer- and method-specific.⁴

Determination. Specimens include serum, heparinized plasma, whole blood, and 24-hour urine, which smooths diurnal variation. Hemolysis must be avoided given the erythrocyte-plasma gradient. The method is ISE potentiometry using valinomycin as the potassium-selective ionophore.

References

1. Rifai N, Chiu RWK, Young I, Burnham CAD, Wittwer CT, eds. Tietz Textbook of Laboratory Medicine. 7th ed. Elsevier; 2023.
2. Hillier TA, Abbott RD, Barrett EJ. Hyponatremia: evaluating the correction factor for hyperglycemia. Am J Med. 1999;106(4):399-403. doi:10.1016/S0002-9343(99)00046-6
3. Palmer BF, Clegg DJ. Physiology and pathophysiology of potassium homeostasis. Adv Physiol Educ. 2019;43(4):531-538. doi:10.1152/advan.00121.2019
4. Clinical and Laboratory Standards Institute. Standardization of Sodium and Potassium Ion-Selective Electrode Systems to the Flame Photometric Reference Method. CLSI document C29-A2. Wayne, PA: CLSI; 2000. Specimen-choice guidance for platelet-related serum-plasma differences; hemolysis effect sizes are analyzer- and method-specific.