Plasma Osmolarity

Serum osmolality: calculation and osmolar gap

Calculated serum osmolality estimates the concentration of osmotically active solutes in plasma. Using Na⁺ (mEq/L), glucose (mg/dL), and urea/BUN (mg/dL): Osm_calc = 2·Na⁺ + Glucose/18 + Urea/2.8. The result is in mOsm/L. Normal range: 280–295 mOsm/L. The osmolar gap (measured − calculated) >10 mOsm/L suggests unmeasured osmoles: methanol, ethylene glycol, isopropanol, mannitol, iodinated contrast, or acute kidney injury. Hypernatremia (Na >145) causes cellular dehydration; hyponatremia (Na <135) drives cerebral edema. Example: Na 140, glucose 90, BUN 14 → Osm ≈ 2·140 + 90/18 + 14/2.8 = 290 mOsm/L (normal).

Clinical context

Used in emergency medicine (acute alcohol intoxication, toxic alcohol poisoning), ICU (dysnatremias, sepsis), nephrology and hemodialysis (osmolar shifts during ultrafiltration), and neurosurgery (mannitol dosing for intracranial hypertension). A widened osmolar gap with high anion gap metabolic acidosis is the classic signature of methanol or ethylene glycol poisoning, prompting fomepizole and dialysis.

FAQ

Osmolality or osmolarity? Osmolality (mOsm/kg of water) is measured by freezing-point depression; osmolarity (mOsm/L of solution) is what the formula returns. In plasma the values are nearly identical.

Why divide glucose by 18 and urea by 2.8? Those are conversion factors from mg/dL to mmol/L based on molecular weight (glucose 180 g/mol; urea nitrogen 28 g/mol).

When is the osmolar gap most useful? In altered mental status with metabolic acidosis of unclear etiology — a gap >25 strongly suggests toxic alcohol ingestion until proven otherwise.