Medical AICalculated serum osmolality and osmolar gap calculator
Calculated serum osmolality is estimated from sodium, glucose, and urea (BUN). When a measured osmolality is available, the osmolar gap (measured minus calculated) screens for unmeasured osmoles. A large gap with metabolic acidosis raises concern for toxic alcohols (methanol, ethylene glycol).
How to measure each input
- Sodium, glucose, BUN
- Use values from the same blood draw. The formula uses Na in mmol/L, and glucose and BUN in mg/dL (divided by 18 and 2.8 respectively to convert to mOsm/kg).
- Ethanol term
- If a serum ethanol level is available, entering it adds ethanol/3.7 so a known alcohol load does not inflate the apparent gap. The optimal ethanol divisor is debated (3.7 versus higher values), so interpret the corrected gap cautiously.
- Measured osmolality
- The osmolar gap requires a lab-measured osmolality by freezing-point depression (not vapour pressure, which can miss volatile alcohols). Draw it close in time to the chemistries used in the calculation.
Interpretation
| Band | Meaning |
|---|---|
| Below the usual reference range | Calculated osmolality below roughly 275 mOsm/kg. Consider hyponatraemia or low solute states, and interpret in clinical context. If a measured value was entered, focus on the osmolar gap rather than the absolute number. |
| Within the usual reference range | Calculated osmolality of about 275 to 295 mOsm/kg is within the usual reference range. The diagnostic value here is the osmolar gap: a measured value much higher than calculated suggests unmeasured osmoles even when the absolute number looks normal. |
| Above the usual reference range | Calculated osmolality above roughly 295 mOsm/kg. Look for hypernatraemia, hyperglycaemia, uraemia, or measured ethanol. Pair with the osmolar gap and clinical picture before acting. |
Pitfalls, exclusions and caveats
- The osmolar gap has a wide normal range (about -10 to +10 mOsm/kg), so individual baseline variation can mask a meaningful rise. A modest gap is not reliably normal or abnormal in isolation.
- A normal osmolar gap does not exclude toxic alcohol poisoning, especially late in the course: as methanol or ethylene glycol is metabolised to acids, the gap falls while the anion-gap acidosis rises.
- Measure osmolality by freezing-point depression. Vapour-pressure osmometers do not detect volatile alcohols and will underestimate the gap.
- Hyperlipidaemia, hyperproteinaemia, and chronic kidney disease can raise the osmolar gap without toxic alcohols. Interpret alongside the anion gap, history, and clinical state.
- The ethanol divisor (3.7) is an empirical correction; recent data suggest a higher coefficient may fit better, so the ethanol-corrected gap is an estimate.
Calculated osmolality (mOsm/kg) = 2 x Na(mmol/L) + glucose(mg/dL)/18 + BUN(mg/dL)/2.8, plus ethanol(mg/dL)/3.7 when ethanol is entered. Osmolar gap = measured osmolality - calculated osmolality.Frequently asked
What is a normal osmolar gap?
A calculated-versus-measured osmolar gap below about 10 mOsm/kg is generally considered normal, though the range is wide (roughly -10 to +10). The gap is most useful when clearly elevated alongside a suggestive history and an anion-gap acidosis.
Does a normal osmolar gap rule out methanol or ethylene glycol poisoning?
No. Early on the parent alcohol raises the gap, but as it is metabolised to toxic acids the gap falls and the anion gap rises. A normal or near-normal gap late in the course does not exclude poisoning, so use the clinical picture and anion gap too.
Why is ethanol divided by 3.7 rather than its molecular weight?
Ethanol does not behave as an ideal osmole in serum, so an empirical divisor of 3.7 (rather than its molecular weight of about 46/10 in mg/dL terms) is commonly used. Recent data suggest a higher coefficient may fit better, so treat the ethanol-corrected gap as an estimate.
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