Calculated Serum Osmolality Calculator
Calculated Serum Osmolality Calculator
Calculate serum osmolality from sodium, glucose, BUN and ethanol to compare against the measured value and detect an osmolal gap.
Calculated Serum Osmolality
4 inputs → osmolalitySodium 140 mmol/L, glucose 90 mg/dL, BUN 14 mg/dL, ethanol 0 mg/dL
Formula
- 18
- converts glucose from mg/dL to mmol/L
- 2.8
- converts BUN from mg/dL to mmol/L
- 3.7
- converts ethanol from mg/dL to mmol/L
- 2 × Na
- accounts for sodium and its accompanying anions, which together dominate normal plasma osmolality
Worked example
Sodium 140 mmol/L, glucose 90 mg/dL, BUN 14 mg/dL, ethanol 0 mg/dL
2 × 140 = 280
90 ÷ 18 = 5.0, 14 ÷ 2.8 = 5.0, 0 ÷ 3.7 = 0
280 + 5.0 + 5.0 + 0 = 290.0 mOsm/kg
Causes of a raised osmolal gap
| Cause | Notes |
|---|---|
| Methanol | Metabolic acidosis, visual symptoms |
| Ethylene glycol | Metabolic acidosis, renal failure, oxalate crystalluria |
| Isopropanol | Ketosis without acidosis |
| Propylene glycol | A diluent in intravenous lorazepam and other infusions |
| Mannitol | Iatrogenic, from osmotic diuretic administration |
Calculated osmolality, the osmolal gap, and effective osmolality
Each divisor converts a mass concentration to molar terms — 18 for glucose, 2.8 for urea nitrogen, 3.7 for ethanol — so that all four inputs are combined on the same osmolar scale. The result approximates what a laboratory osmometer measures directly, and the two values should normally agree closely.
Comparing the calculated value against a directly measured osmolality gives the osmolal gap. A gap above about 10 mOsm/kg suggests an osmotically active substance is present that this formula does not account for — classically methanol, ethylene glycol, isopropanol, propylene glycol (a diluent found in intravenous lorazepam and some other infusions), or mannitol given as an osmotic diuretic. An elevated gap in an unexplained metabolic acidosis is a recognised trigger for urgent toxic alcohol workup.
A distinct and important point is that effective osmolality, also called tonicity, excludes urea. Urea crosses cell membranes freely and equilibrates across the intracellular and extracellular compartments, so it contributes to total osmolality without drawing water between them — it does not affect tonicity. Sodium and glucose, which do not cross membranes freely, are what actually drive water movement and cellular swelling or shrinkage. This distinction is why effective osmolality, not total osmolality, is the value that matters in hyperosmolar hyperglycaemic state, where a very high glucose genuinely threatens cerebral oedema through osmotic water shifts.
Frequently asked questions
What is a normal calculated serum osmolality?
Approximately 275 to 295 mOsm/kg in a healthy adult, driven mainly by sodium and its accompanying anions.
What is the osmolal gap and when does it matter?
The difference between measured and calculated osmolality. A gap above about 10 mOsm/kg suggests an unmeasured osmotically active substance, such as a toxic alcohol, and should prompt urgent evaluation in the right clinical context.
Why is urea excluded from effective osmolality?
Urea crosses cell membranes freely and equilibrates between compartments, so it does not drive water movement. Effective osmolality (tonicity) excludes it and uses only sodium and glucose, which is why it is the relevant value in hyperosmolar hyperglycaemic state.
Does ethanol affect the osmolal gap even at levels that don't cause obvious intoxication?
Yes. Ethanol is osmotically active and, once included correctly in the calculation, should account for its own contribution — an ethanol level should be included whenever intoxication is suspected, so the remaining gap is not falsely attributed to a toxic alcohol.
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References
- Smithline N, Gardner KD. Gaps — anionic and osmolal. JAMA. 1976;236(14):1594–1597.
- Hoffman RS, Smilkstein MJ, Howland MA, Goldfrank LR. Osmol gaps revisited: normal values and limitations. J Toxicol Clin Toxicol. 1993;31(1):81–93.