Osmolal Gap
Osmolal Gap - a deterministic tool in Sophie Well's Clinical Math & Conversions group.
Open the Osmolal Gap → Runs in your browser. No signup, no tracking.
What this is
Compute the calculated serum osmolality from sodium, glucose, BUN, and (optionally) ethanol, then subtract from a measured serum osmolality to produce the osmolal gap. 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.
References
Calculated osm = 2*Na + glucose/18 + BUN/2.8 (+ EtOH/4.6). Gap >10 raises suspicion of toxic alcohols.
Worked example: Calculated osm ~290; gap ~10 (within normal range).
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