Osmolal Gap
Compute the calculated serum osmolality from sodium, glucose, BUN, and (optionally) ethanol, then subtract from a measured serum osmolality to produce the osmolal gap.
Open the calculator → Runs in your browser. Data leaves only if you deliberately send a problem report from the interactive tool.
Example
- Measured serum osmolality
- 300 mOsm/kg
- Sodium
- 140 mEq/L
- Glucose
- 90 mg/dL
- BUN
- 14 mg/dL
- EtOH
- 0 mg/dL
Result: Calculated osm ~290; gap ~10 (within normal range).
The tool opens with these values already filled in. Replace them with your own.
What you enter
- Measured serum osmolality
- Sodium
- Glucose
- BUN
- EtOH (optional; omit unless an ethanol level is known)
What this is
The formula is 2*Na + glucose/18 + BUN/2.8 (+ EtOH/4.6 when present); a gap greater than 10 mOsm/kg is the classic threshold that raises suspicion for an unmeasured osmole - most importantly methanol or ethylene glycol, but also isopropanol, propylene glycol, and a short list of others.
When to use it
Use this when an unexplained high-anion-gap metabolic acidosis raises the question of a toxic alcohol ingestion, or when a poisoned-patient evaluation needs an immediately available screen before formal gas-chromatography levels come back from the reference lab. The osmolal gap is sensitive but imperfect - a normal gap does not rule out late-presenting methanol or ethylene glycol once the parent alcohol has been metabolized to its toxic anion - so use the number to escalate suspicion, not to dismiss it.
How this is calculated
Calculated osm = 2*Na + glucose/18 + BUN/2.8 (+ EtOH/4.6). Gap >10 raises suspicion of toxic alcohols.
A reference and educational tool. Not medical, legal, or financial advice, and not a substitute for clinician judgment.
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