🧪 Serum Osmolality (Calculated)
Calculate serum osmolality and provide the basis for the osmolal gap.
Serum Osmolality (Calculated)
Sodium (mmol/L)
Glucose (mg/dL)
BUN (mg/dL)
Ethanol (optional) (mg/dL)
When to use
Work up hyponatraemia and suspected toxic alcohol ingestion.
How it works
Calculated osmolality = 2·Na + glucose/18 + BUN/2.8 (+ ethanol/3.7), mg/dL units. Reference ~275–295 mOsm/kg.
Key points
- Osmolal gap = measured − calculated; > 10 suggests unmeasured osmoles.
- A raised gap may indicate methanol/ethylene glycol — urgent.
- Add the ethanol term when measured.
References
Decision support for licensed clinicians only; not a substitute for clinical judgement, diagnosis or local protocols.