Serum Osmolality Calculator with Osmolar Gap, Ethanol Correction and Effective Osmolality
Calculated serum osmolality from sodium, glucose and urea or BUN (2 × Na + glucose + urea, or the Dorwart 1.86 × Na + 9 variant) in mmol/L or mg/dL, with ethanol correction, the osmolar gap against measured osmolality, effective osmolality (tonicity) for HHS, and the toxic alcohol level a gap could represent.
How it works
Calculated osmolality = 2 × Na + glucose + urea (mmol/L) = 2 × Na + glucose (mg/dL) ÷ 18 + BUN (mg/dL) ÷ 2.8 · + 1.25 × ethanol (mmol/L) = ethanol (mg/dL) ÷ 3.7 · Dorwart variant: 1.86 × Na + glucose + urea + 9 · osmolar gap = measured − calculated (normal −10 to +10) · effective osmolality = 2 × Na + glucose
Sodium with its accompanying anions, glucose and urea make up almost all the osmotically active particles in plasma, so a simple formula predicts the measured (freezing-point) osmolality to within about 10 mOsm/kg. When the measured value is well above the calculated one, something unaccounted for is dissolved in the blood: classically methanol or ethylene glycol in a poisoned patient, where the gap appears early, before their acid metabolites produce a high anion gap acidosis. Ethanol is the most common cause of a gap, so a measured ethanol level is added to the calculation (each mmol/L adds about 1.25 mOsm/kg, which is ethanol in mg/dL ÷ 3.7). Effective osmolality, or tonicity, leaves out urea because urea crosses cell membranes freely; it is what moves water in and out of brain cells and is used to diagnose hyperosmolar hyperglycaemic state (above 320 mOsm/kg). In mg/dL, glucose is divided by 18 and BUN by 2.8 to give mmol/L; where urea rather than BUN is reported in mg/dL, divide by 6. The Dorwart and Chalmers variant (1.86 × Na + glucose + urea + 9) is offered under More options. If the gap is raised, the calculator shows the concentration of each toxic alcohol that would explain it on its own, which is gap × molecular weight ÷ 10 in mg/dL.
Worked example
Sodium 140 mmol/L, glucose 6 mmol/L, urea 5 mmol/L and no ethanol: calculated osmolality = 2 × 140 + 6 + 5 = 291 mOsm/kg, within the normal 275–295. A measured osmolality of 318 mOsm/kg gives an osmolar gap of 318 − 291 = 27 mOsm/kg, which is elevated. If methanol alone explained it, that would be about 27 × 32 ÷ 10 = 87 mg/dL; for ethylene glycol about 168 mg/dL. Effective osmolality is 2 × 140 + 6 = 286 mOsm/kg.
Sources: Osmol gap: calculated osmolality with ethanol ÷ 3.7, causes of a raised gap (Wikipedia) · Plasma osmolality: 2 Na + glucose + urea, normal 275–299 mOsm/kg (Wikipedia) · Formulas: calculated serum osmolality (WikEM)
Frequently asked questions
What is the formula for serum osmolality?
The standard formula is 2 × sodium + glucose + urea, all in mmol/L. In US units it is 2 × Na + glucose (mg/dL) ÷ 18 + BUN (mg/dL) ÷ 2.8. Add ethanol (mg/dL) ÷ 3.7 when an ethanol level is available.
What is a normal osmolar gap?
Usually below 10 mOsm/kg; individual baselines range from about −10 to +10. A gap above 10 suggests unmeasured osmoles, and larger gaps (above about 20–25) make toxic alcohol poisoning more likely, but a normal gap never rules it out.
Why multiply ethanol by 1.25?
Ethanol does not behave ideally in plasma; each mmol/L raises measured osmolality by about 1.25 mOsm/kg, so dividing ethanol in mg/dL by 3.7 (instead of 4.6) fits measured values better. Some references still use 1.0.
What is the difference between osmolality and effective osmolality?
Total osmolality counts every particle, including urea. Effective osmolality (tonicity) leaves out urea, which crosses cell membranes and does not move water between cells and plasma. Effective osmolality above 320 mOsm/kg is a criterion for hyperosmolar hyperglycaemic state.
Is osmolality the same as osmolarity?
Not exactly: osmolality is per kilogram of water (what the lab measures by freezing point), osmolarity is per litre of solution. In plasma the difference is small, and the calculated value is usually reported as osmolality.
How do I convert urea and BUN?
Urea in mmol/L equals BUN in mg/dL divided by 2.8. If your laboratory reports urea (not BUN) in mg/dL, divide by 6 to get mmol/L, or divide by 2.14 to get BUN.
Related calculators
Clinical decision tool for education and for use by trained clinicians alongside judgement, not a diagnosis. A normal osmolar gap does not exclude toxic alcohol poisoning; suspected poisoning needs urgent specific testing and discussion with a poison centre.