AskCalc
Home › Nephrology & Electrolytes › Osmolal gap

Serum Osmolal Gap

Osmolal gap = measured osmolality − (2 × Na + glucose/18 + BUN/2.8 [+ ethanol/3.7]). A gap above about 10 mOsm/kg suggests unmeasured osmoles such as methanol, ethylene glycol or isopropanol; a normal gap does not exclude toxic alcohol ingestion.

  • Measured serum osmolality
  • Serum sodium (mmol/L or mEq/L)
  • Serum glucose (mg/dL or mmol/L)
  • BUN / serum urea (mg/dL or mmol/L)
  • Serum ethanol (mg/dL or mmol/L)
—

Interpretation

ResultMeaning
≤ 10Normal
10–20Elevated
> 20Markedly elevated: toxic alcohol likely

Formula

OG = measured Osm − [2 × Na⁺ + glucose (mg/dL)/18 + BUN (mg/dL)/2.8 + ethanol (mg/dL)/3.7]. SI: 2Na + glucose + urea + 1.25 × ethanol (mmol/L).

Clinical pearls

  • Late after ingestion the parent alcohol is metabolised: the osmolal gap falls while the anion gap rises.
  • Measured osmolality must be by freezing-point depression; vapour-pressure osmometers miss volatile alcohols.
  • Baseline osmolal gap varies roughly −10 to +10 between people, so a "normal" gap does not rule out a dangerous level.

Examples

osmolal gap measured osm 320 na 140 glucose 90 bun 14osm gap 330 na 138 glucose 6 mmol/L urea 5 mmol/L ethanol 20 mmol/Lsuspected ethylene glycol: measured osmolality 340, sodium 140, glucose 100, BUN 10

FAQ

What osmolal gap suggests toxic alcohol ingestion?

A gap above about 10 mOsm/kg is abnormal; gaps above 20–25 strongly suggest methanol or ethylene glycol, but a normal gap does not exclude them.

How do you account for ethanol in the osmolal gap?

Add ethanol (mg/dL) ÷ 3.7, or 1.25 × ethanol in mmol/L, to the calculated osmolality.

Evidence

  1. Purssell RA et al. Derivation and validation of a formula to calculate the contribution of ethanol to the osmolal gap. Ann Emerg Med 2001;38:653–9.
  2. Dorwart WV, Chalmers L. Comparison of methods for calculating serum osmolality from chemical concentrations. Clin Chem 1975;21:190–4.

Nephrology & Electrolytes · Reviewed 2026-10-14 · Methodology