Serum osmolality, effective osmolality and osmolal gap

Calculates serum osmolality from sodium, glucose and BUN (and ethanol), the effective (tonicity) osmolality used to diagnose HHS, and the osmolal gap when a measured value is available.

What goes into the Serum osmolality and osmolal gap

Sodium (mEq/L)
Glucose (mg/dL)
BUN (mg/dL)
Ethanol (optional) (mg/dL)
Measured osmolality (optional) (mOsm/kg)

How to interpret the result

Effective osmolality > 300 mOsm/kg supports HHS (2024 consensus); altered mental status is more likely above 320
Osmolal gap > 10 mOsm/kg: consider methanol, ethylene glycol, isopropanol, ketoacidosis or lactic acidosis

Clinical pearls

BUN crosses cell membranes and does not count toward effective osmolality.
Late after toxic alcohol ingestion the gap may be normal while the anion gap is high.
In HHS, aim to lower osmolality by no more than about 3–8 mOsm/kg per hour.

Evidence

Umpierrez GE, et al. Hyperglycemic crises in adults with diabetes: a consensus report. Diabetes Care. 2024;47:1257-1275.

Related workups

Hyperosmolar hyperglycemic state (HHS)Diabetic ketoacidosis Hyponatremia Hypernatremia

Related calculators

Anion gap and delta ratio Sodium corrected for glucose Sodium change per liter (Adrogué-Madias)All calculators

For education only. Check the inputs and the population the score was validated in before acting on it. Not a substitute for clinical judgment.