Plasma Osmolality Calculator

Osmolality measures all dissolved solutes regardless of whether they cross cell membranes. Tonicity (effective osmolality) measures only solutes that are effectively excluded from cells — primarily sodium and glucose. Urea diffuses freely across cell membranes and therefore does not cause osmotic water shifts between intracellular and extracellular compartments. This is why uraemia (high urea) causes a high measured and calculated osmolality but does not cause cell dehydration or shrinkage — urea equilibrates across membranes and is an ineffective osmole.

Sodium is the dominant extracellular cation. To maintain electroneutrality, each sodium ion is accompanied by an anion — predominantly chloride and bicarbonate. Although these anions contribute separately to osmolality, the factor of 2 applied to sodium provides a practical approximation that accounts for sodium plus its associated anions in one step. This is an approximation: the true value depends on the specific anion composition of the plasma.

A low calculated osmolality (<280 mOsm/kg) reflects hyponatraemia as the primary driver, since sodium (×2) is the dominant term. The clinical causes of hypo-osmolality include SIADH, hypothyroidism, adrenal insufficiency, cardiac failure with free water excess, and excess hypotonic fluid administration. Hypo-osmolality causes cellular swelling — neurons in the brain are particularly vulnerable, causing symptoms ranging from headache and nausea to seizures and coma.

Syndrome of Inappropriate ADH secretion (SIADH) causes free water retention with dilutional hyponatraemia and hypo-osmolality. The diagnostic criteria require: plasma osmolality <280 mOsm/kg AND urine osmolality >100 mOsm/kg (inappropriately concentrated urine despite plasma hypo-osmolality) AND urine sodium >30 mEq/L in a euvolaemic patient without thyroid or adrenal disease. Plasma osmolality calculation is the first step in confirming that hyponatraemia is truly hypotonic rather than pseudohyponatraemia.

HHS is defined partly by effective osmolality >320 mOsm/kg, calculated by excluding urea: Eosm = 2×Na + Glucose/18. Consciousness level in HHS correlates with effective osmolality, not total osmolality, because urea equilibrates across the blood-brain barrier. An effective osmolality above 320–330 mOsm/kg causes severe neurological impairment. Treatment targets gradual reduction of effective osmolality at <3–4 mOsm/kg/hour to avoid cerebral oedema.

Pseudohyponatraemia is an artefactual lowering of measured serum sodium caused by severe hyperlipidaemia or hyperproteinaemia (e.g., multiple myeloma). Excess lipid or protein displaces water in the plasma sample, reducing sodium concentration per unit plasma volume while plasma osmolality remains normal. The measured (laboratory) osmolality is normal while calculated osmolality (based on the falsely low sodium) appears low, creating an apparent large negative osmolal gap. Modern direct ISE (ion-selective electrode) methods are less prone to this artefact.

This rapid approximation assumes glucose and BUN are approximately 5 mOsm/kg each (corresponding to glucose ~90 mg/dL and BUN ~14 mg/dL), contributing a combined 10 mOsm/kg. It is useful for quick mental arithmetic but becomes inaccurate when glucose or BUN are significantly abnormal (e.g., diabetic hyperglycaemia, renal failure with high urea, or hypoglycaemia). Always use the full formula when chemistry values are available.

Urine osmolality provides crucial context alongside plasma osmolality: urine osmolality <100 mOsm/kg in the setting of hyponatraemia suggests psychogenic polydipsia or beer potomania (dilute urine appropriately). Urine >100 mOsm/kg in hyponatraemia suggests SIADH or other causes of inappropriate ADH. In hypernatraemia, urine >800 mOsm/kg indicates appropriate renal water conservation (extrarenal losses); urine <300 mOsm/kg suggests central or nephrogenic diabetes insipidus.