Osmolal Gap Calculator
Osmolal Gap Calculator
Calculate the osmolal gap from measured osmolality, sodium, glucose, urea nitrogen and ethanol, and see why a normal gap does not exclude a toxic alcohol.
Osmolal gap
5 inputs → mOsm/kgMeasured osmolality 310 mOsm/kg, sodium 140 mmol/L, glucose 100 mg/dL, BUN 14 mg/dL, no ethanol
Formula
Osmolal gap = measured osmolality − calculated osmolality
- 2 × Na
- sodium with its accompanying anions, the dominant contributor to plasma osmolality
- ÷ 18
- glucose in mg/dL divided by one tenth of its molecular weight, 180.16
- ÷ 2.8
- urea nitrogen in mg/dL divided by one tenth of the nitrogen content of urea
- ÷ 4.6
- ethanol in mg/dL divided by one tenth of its molecular weight, 46.07
Worked example
Measured osmolality 310 mOsm/kg, sodium 140 mmol/L, glucose 100 mg/dL, BUN 14 mg/dL, no ethanol
2 × 140 = 280
100 ÷ 18 = 5.6 · 14 ÷ 2.8 = 5.0 · 0 ÷ 4.6 = 0
Calculated osmolality = 280 + 5.6 + 5.0 = 290.6 mOsm/kg
310 − 290.6 = 19.4 mOsm/kg → raised
Where each divisor comes from
| Term | Molecular weight | Divisor | Converts |
|---|---|---|---|
| Glucose | 180.16 | 18 | mg/dL → mOsm/kg |
| Urea nitrogen (BUN) | urea 60.06, nitrogen fraction 28.01 | 2.8 | mg/dL → mOsm/kg |
| Ethanol | 46.07 | 4.6 | mg/dL → mOsm/kg |
Causes of a raised osmolal gap other than a toxic alcohol
| Cause | Comment |
|---|---|
| Ketoacidosis | Acetone and other ketones are osmotically active |
| Lactic acidosis | Contributes a modest gap |
| Renal failure | Retained small solutes |
| Hypertriglyceridaemia or paraproteinaemia | Pseudohyponatraemia lowers the calculated value and widens the apparent gap |
| Mannitol, glycine, propylene glycol diluents | Iatrogenic osmotically active substances |
Reading the gap, and its two failure modes
The osmolal gap is the difference between the osmolality a laboratory measures and the osmolality predicted from the solutes that were measured directly. Sodium and its anions dominate the calculation, and glucose, urea nitrogen and ethanol each add their molar contribution. The divisors are not arbitrary: 18 for glucose, 2.8 for urea nitrogen and 4.6 for ethanol are each one tenth of the relevant molecular weight, which is exactly what converts a concentration in mg/dL into mOsm/kg. Anything left over is an unmeasured osmotically active solute.
The most dangerous misreading is treating a normal gap as reassurance. The gap reflects the parent alcohol, and as methanol or ethylene glycol is metabolised to formic or glycolic and oxalic acid the osmolal gap falls while an anion gap acidosis takes its place. A patient presenting late can therefore have a normal osmolal gap and a lethal poisoning at the same time. The osmolal gap and the anion gap should be read together and interpreted against the time since ingestion.
The second problem is the reference range itself. The gap in healthy people varies widely between individuals, with a spread of roughly minus 10 to plus 10 mOsm/kg, so a single value sitting a few units above the conventional cut-off is weak evidence either way. Serial measurement in the same patient is far more informative than one number, because the direction of travel carries the signal. Ketoacidosis, lactic acidosis, renal failure and hypertriglyceridaemia all widen the gap without any toxic alcohol being present.
The measurement method matters. Osmolality must be determined by freezing-point depression, because vapour-pressure osmometry drives off volatile alcohols and does not detect them at all — the very substances the test is being used to find. Toxic alcohol poisoning is time-critical, and management including the decision to give fomepizole or ethanol and to dialyse should be guided by a poisons centre or clinical toxicology service. This calculator supports that discussion and never replaces it.
Frequently asked questions
What is a normal osmolal gap?
Conventionally below 10 mOsm/kg, but healthy individuals span roughly minus 10 to plus 10, so a single borderline value is weak evidence. Serial measurements in the same patient are more informative than one reading.
Does a normal osmolal gap exclude methanol or ethylene glycol poisoning?
No. The gap reflects the parent alcohol and falls as it is metabolised to its acid, with an anion gap acidosis taking its place. A late presenter can have a normal osmolal gap and a lethal poisoning.
Why do the formulas divide glucose by 18 and ethanol by 4.6?
Each divisor is one tenth of the molecular weight — glucose 180.16, ethanol 46.07 — which converts a concentration in mg/dL into mOsm/kg. The 2.8 for BUN comes from the nitrogen content of urea.
Does it matter how the laboratory measures osmolality?
Yes. It must be by freezing-point depression. Vapour-pressure osmometry does not detect volatile alcohols, so it will miss precisely the substances the gap is being used to screen for.
Should a raised osmolal gap be discussed with a poisons centre?
Yes. Toxic alcohol poisoning is time-critical, and antidote and dialysis decisions should be guided by a poisons centre or clinical toxicology service. This calculator supports that advice rather than replacing it.
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References
- Kraut JA, Kurtz I. Toxic alcohol ingestions: clinical features, diagnosis, and management. Clin J Am Soc Nephrol. 2008;3(1):208–225.
- Purssell RA, Pudek M, Brubacher J, Abu-Laban RB. Derivation and validation of a formula to calculate the contribution of ethanol to the osmolal gap. Ann Emerg Med. 2001;38(6):653–659.
- Kruse JA, Cadnapaphornchai P. The serum osmole gap. J Crit Care. 1994;9(3):185–197.