ROUNDS·
Toxicology

Osmolar Gap Calculator

Screens for unmeasured osmotically active substances, including toxic alcohols, in a patient with suspected ingestion.

What is the osmolar gap?

The osmolar gap is the difference between a lab-measured serum osmolality and a calculated osmolality from routine labs. A significant gap suggests the presence of an osmotically active substance not accounted for in the standard calculation — classically, toxic alcohols.

How to calculate it

Calculated osmolality = 2 × Sodium + Glucose ÷ 18 + BUN ÷ 2.8 (add Ethanol ÷ 3.7 if an ethanol level is included).

Osmolar gap = Measured osmolality − Calculated osmolality.

Interpretation

A gap above roughly 10 mOsm/kg is considered elevated and should raise suspicion for toxic alcohol ingestion (methanol, ethylene glycol, isopropyl alcohol), mannitol administration, or severe ketoacidosis. For the broader picture, see the approach to the poisoned patient guide and the anion gap calculator.

Limitations

A normal osmolar gap doesn't fully exclude toxic alcohol ingestion — once the parent alcohol is metabolized into its toxic acid metabolites, the osmolar gap narrows even as toxicity (and the anion gap) worsens. Clinical suspicion and timing both matter.

Frequently Asked Questions

What causes an elevated osmolar gap?

Toxic alcohols (methanol, ethylene glycol, isopropyl alcohol), mannitol, and severe ketoacidosis are common causes of an elevated, unexplained osmolar gap.

Does a normal osmolar gap rule out toxic alcohol ingestion?

No — a normal gap doesn't fully exclude it, especially later after ingestion once the alcohol has been metabolized. Clinical suspicion still matters.

Why include ethanol in the formula?

Ethanol is itself osmotically active. If a level is available, including it gives a more accurate calculated osmolality and avoids falsely attributing the gap to other toxic alcohols.