Acid-Base
Osmolar Gap Calculator
Measured minus calculated osmolality; a gap over 10 points to an unmeasured osmole such as methanol or ethylene glycol.
Formula
Gap = Measured Osmolality - Calculated Osmolarity
Calculated Osm = (2 × Na) + (Glucose/18) + (BUN/2.8) + (EtOH/4.6)
How it is used
- Normal Osmolar Gap is < 10.
- An elevated gap indicates unmeasured osmotically active substances, most notably toxic alcohols (Methanol, Ethylene Glycol, Propylene Glycol).
Related one-pagers
- ABG Interpretation in 6 StepsThe osmolar gap finds uncharged toxins. Calculated osmolality = 2 × Na⁺ + glucose ÷ 18 + BUN ÷ 2.8 (+ ethanol ÷ 3.7); the gap, measured minus calculated, is normally under 10 mOsm/kg (Berend 2014). Na⁺ 140, glucose 90 and BUN 14 give 290; a measured 320 is a gap of 30. Early after methanol or ethylene glycol the osmolar gap is high and the anion gap normal; as they are metabolized to formate, glycolate and oxalate the osmolar gap falls and the anion gap rises, so a normal osmolar gap late does not exclude poisoning. Mannitol, propylene glycol (IV lorazepam vehicle) and severe ketoacidosis also raise it.
- Sedation, Pain and Delirium ScalesLorazepam · 0.01–0.1 mg/kg/h (≤10 mg/h) · Propylene glycol: osmolar gap, acidosis
- Neuro: ICP and StrokeMannitol 20% · 0.5–1 g/kg over 5–15 min; in-line filter · Osmolar gap, BP, urine output, creatinine
- Toxidromes and AntidotesToxic alcohols: osmolar gap early, anion gap late; methanol blinds, ethylene glycol makes oxalate crystals and AKI.
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For education and exam preparation only. Follow your institution’s protocols. For education and exam preparation only. Verify results independently and follow your institution’s protocols. Not a substitute for clinical judgment.