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Osmolar Gap Calculator

About this Calculator

The Osmolar Gap calculator helps clinicians detect unmeasured osmotically active substances in plasma — most importantly toxic alcohols such as methanol and ethylene glycol. It compares the laboratory-measured osmolality against an osmolality calculated from routine solutes (sodium, glucose, and urea). A meaningful difference between the two suggests that something not captured by the calculation is contributing to plasma osmolality. The calculator reports two complementary approaches: the conventional additive formula (with a corrected variant accounting for incomplete sodium dissociation and the plasma water fraction) and a regression-derived equation by Lepeytre and colleagues.

Osmolar Gap = Measured Osmolality − Calculated Osmolality
Conventional: 2 × Na⁺ + Glucose + Urea
Corrected: (1.86 × Na⁺) + Glucose + Urea 0.93
Lepeytre regression: (2.0 × Na⁺) + (1.2 × Urea) + (1.4 × Glucose) + (1.2 × Ethanol)
All solute concentrations in mmol/L.
mOsm/kg
Optional: required to compute the gap
mmol/L
mmol/L
mmol/L
mmol/L
Optional: defaults to 0. Used by the Lepeytre equation.
Method 1 — Conventional formula
Calculated osmolarity
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Osmolar gap
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Calculated osmolarity (corrected)
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Osmolar gap (corrected)
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Method 2 — Lepeytre regression
Calculated osmolarity
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Osmolar gap
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Interpretation
Enter sodium, glucose, and urea, then press Calculate

Interpretation of Results

  • Normal gap (< 10 mOsm/kg): No significant unmeasured osmoles. A normal osmolar gap does not fully exclude early toxic alcohol ingestion, since the parent alcohol may not yet have accumulated or may already have been metabolized to its toxic acids.

  • Borderline gap (10–20 mOsm/kg): Indeterminate. Consider the clinical context, the timing of any suspected ingestion, and unmeasured osmoles. The wide reference range seen in hospitalized patients means values in this band should be interpreted cautiously.

  • Elevated gap (≥ 20 mOsm/kg): Suggests a substantial burden of unmeasured osmoles, classically toxic alcohols (methanol, ethylene glycol), but also acetone, propylene glycol, mannitol, or unmeasured ethanol. An elevated gap warrants prompt clinical correlation and consideration of specific testing or treatment.

  • Clinical Considerations: Ethanol is a common cause of an elevated gap; the conventional formula excludes it, so unmeasured ethanol inflates the conventional gap, whereas the Lepeytre equation incorporates it directly. The conventional and corrected formulas carry opposing analytic errors that approximately offset under ideal conditions, which is why a corrected calculation and method-specific thresholds improve accuracy. The osmolar gap is a screening adjunct, not a stand-alone diagnostic test — decisions should integrate history, acid–base status, the anion gap, and direct toxin levels where available.

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