Plasma Osmolarity
Estimate plasma osmolarity: 2·Na + glucose/18 + BUN/2.8.
Osmolaridade ≈ — mOsm/L
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.
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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.