Serum Osmolality (Calculated) & Osmolar Gap
Calculated serum osmolality and osmolar gap for toxic alcohol evaluation
Calculator
Why use it
Screens for toxic alcohol ingestion. Evaluates hyponatremia etiology (true vs. pseudohyponatremia). Osmolar gap > 10 suggests unmeasured osmoles requiring urgent evaluation.
About
Calculates estimated serum osmolality from sodium, glucose, and BUN. When compared to measured osmolality, the osmolar gap (OG) can identify the presence of unmeasured osmotically active substances such as toxic alcohols (methanol, ethylene glycol), ethanol, or other osmoles.
When to use
- Suspected toxic alcohol ingestion (methanol, ethylene glycol)
- Anion gap metabolic acidosis of unclear etiology
- Hyponatremia workup
- Altered mental status with suspected poisoning
Formula
Pearls & pitfalls
- Normal osmolar gap is -10 to +10 mOsm/kg.
- A normal osmolar gap does NOT exclude toxic alcohol — late in poisoning, the parent compound is metabolized.
- Ethanol contributes to osmolar gap: each 100 mg/dL adds ~22 mOsm/kg.
- If ethanol level is known, include it in the calculation for a more accurate gap.
- Diabetic ketoacidosis can cause an elevated osmolar gap (acetone, β-hydroxybutyrate).
- Measured osmolality must be obtained by freezing point depression (not vapor pressure) for accuracy.
Next steps
Osmolar gap > 10: Consider toxic alcohols (methanol, ethylene glycol, isopropanol). Order specific levels. Consider fomepizole if suspicion is high. Osmolar gap > 25 is highly suggestive.
Evidence
Osmolar gap described by Smithline and Gardner (1976). Widely used in toxicology. Sensitivity for toxic alcohol detection varies; should always be combined with clinical assessment and specific levels when available.