Serum osmolality (calculated)
Calculated serum osmolality from sodium, glucose and urea
Sources last checked: 2026-10-27
Serum osmolality (calculated)
Calculated serum osmolality from sodium, glucose and urea
A calculated value, not a measurement: it does not detect unmeasured osmoles (ethanol, methanol, ethylene glycol, mannitol), which is why the osmolal gap against a measured value matters. Urea crosses cell membranes freely, so the calculated value is total, not effective (tonicity) osmolality.
What is the Serum osmolality (calculated)?
Calculated serum osmolality estimates the concentration of osmotically active particles from the main measured solutes: sodium (counted twice for its accompanying anions), glucose and urea. The formula was popularised by Smithline and Gardner (1976) together with the concept of the osmolal gap — the difference between measured and calculated osmolality that points to unmeasured osmoles.
When to use it
- Work-up of hyponatraemia or hypernatraemia alongside a measured osmolality
- Suspected toxic alcohol ingestion (methanol, ethylene glycol) — to compute the osmolal gap
- Hyperglycaemic crises, where glucose contributes substantially to osmolality
When not to use it
- As a replacement for measured osmolality when the distinction matters (e.g. pseudohyponatraemia, toxic alcohols)
- Severe hyperlipidaemia or hyperproteinaemia with indirect sodium measurement, which underestimates sodium
How it is calculated
Calculated osmolality = 2 × Na + glucose + urea (all in mmol/L) Conventional units: 2 × Na + glucose (mg/dL)/18 + BUN (mg/dL)/2.8 Osmolal gap = measured − calculated osmolality
Clinical pearls and limitations
- A calculated value, not a measurement: it does not detect unmeasured osmoles (ethanol, methanol, ethylene glycol, mannitol), which is why the osmolal gap against a measured value matters. Urea crosses cell membranes freely, so the calculated value is total, not effective (tonicity) osmolality.
- Osmolal gap = measured − calculated osmolality; Smithline and Gardner considered a gap above about 10 mOsm/kg abnormal, but the normal range is wide and a normal gap does not exclude toxic alcohol ingestion.
- Ethanol is the most common cause of a raised gap; some formulas add ethanol (mmol/L) to the calculation.
- Urea is an ineffective osmole: effective osmolality (tonicity) = 2 × Na + glucose.
- In mg/dL, glucose/18 and BUN/2.8 convert to mmol/L; German and Turkish labs often report urea rather than BUN, which is why a separate urea option is offered.
Frequently asked questions
What is the formula for calculated serum osmolality?
2 × Na + glucose + urea, all in mmol/L. In US units: 2 × Na + glucose/18 + BUN/2.8.
How do I calculate the osmolal gap?
Subtract the calculated value from the measured osmolality (freezing-point osmometer). A gap well above 10 mOsm/kg suggests unmeasured osmoles such as ethanol, methanol, ethylene glycol or mannitol.
Osmolality or osmolarity?
Osmolality is per kg of water and is what the laboratory measures; the calculated formula gives an osmolarity-like estimate that is compared with it directly in practice.
Evidence and guidelines
Have a follow-up question?
Medelix answers clinical questions from current guidelines and studies — every statement with its source.
Related calculators
- Anion gapClassification of metabolic acidosis
- Corrected sodium in hyperglycaemiaSodium corrected for the dilutional effect of high glucose (Hillier factor 2.4)
- Glucose mg/dL → mmol/LConvert blood or plasma glucose from mg/dL to mmol/L
- Urea mmol/L → BUN mg/dLConvert SI urea (mmol/L) to US blood urea nitrogen (BUN, mg/dL)
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