Question 1

What does an elevated osmolar gap mean?

Accepted Answer

An osmolar gap >10 mOsm/kg suggests **unmeasured osmoles**. The most concerning causes are toxic alcohols (methanol, ethylene glycol, isopropanol). Other causes: propylene glycol (vehicle for IV lorazepam, phenobarbital), mannitol, severe alcoholic ketoacidosis, contrast dye, and severe renal failure.

Question 2

Does a normal osmolar gap rule out toxic alcohol ingestion?

Accepted Answer

**No.** The osmolar gap is most sensitive **early** in the course, before the parent alcohol is metabolized. Once methanol or ethylene glycol is metabolized to its toxic acid metabolites, the osmolar gap falls and the anion gap rises (the "osmolar-to-anion gap transition"). A normal osmolar gap with the right clinical context still warrants a serum methanol/ethylene glycol level.

Question 3

How does ethanol affect the osmolar gap?

Accepted Answer

Ethanol contributes to measured osmolality and will widen the osmolar gap. If the patient has a known ethanol level, include it in the calculated osmolality (ethanol/4.6) to avoid a falsely elevated gap and missing or overestimating other unmeasured osmoles.

Question 4

What is the formula for calculated serum osmolality?

Accepted Answer

`Calculated Osm = 2 × Na + Glucose/18 + BUN/2.8 (+ Ethanol/4.6 if known)`. Subtract from measured osmolality to get the osmolar gap. Normal range: −10 to +10 mOsm/kg.

Osmolar Gap Calculator

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