🧪 Serum Osmolality (Calculated)
Calculate serum osmolality and provide the basis for the osmolal gap.
Clinical takeaway
Osmolal gap = measured − calculated; > 10 suggests unmeasured osmoles.
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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.
Worked calculation
The values below come from this tool's own example placeholders and are computed server-side with the formula shown on this page, so the arithmetic can be checked quickly. It demonstrates how to substitute values only — it is not clinical advice and not a real case.
| Sodium | 140 mmol/L |
|---|---|
| Glucose | 90 mg/dL |
| BUN | 14 mg/dL |
| Ethanol (optional) | 0 mg/dL |
→Calculated osmolality290mOsm/kg
- Formula：2·Na + glucose/18 + BUN/2.8
- Reference：~275–295 mOsm/kg
Frequently asked questions
- What is Serum Osmolality (Calculated)?
- Calculate serum osmolality and provide the basis for the osmolal gap.
- How is Serum Osmolality (Calculated) calculated? What is the core formula?
- Calculated osmolality = 2·Na + glucose/18 + BUN/2.8 (+ ethanol/3.7), mg/dL units. Reference ~275–295 mOsm/kg.
- When is Serum Osmolality (Calculated) used?
- Work up hyponatraemia and suspected toxic alcohol ingestion.
- What are the key clinical points for Serum Osmolality (Calculated)?
- Osmolal gap = measured − calculated; > 10 suggests unmeasured osmoles. A raised gap may indicate methanol/ethylene glycol — urgent. Add the ethanol term when measured.
- What are the limits and cautions when using Serum Osmolality (Calculated)?
- For licensed clinicians and clinical researchers. Interpret results with history, investigations and local protocols; not a diagnosis or prescription, and not a substitute for multidisciplinary decision-making or local guidelines.
- How is Serum Osmolality (Calculated) calculated in practice? Can you show a worked example?
- Inputs: Sodium 140 mmol/L, Glucose 90 mg/dL, BUN 14 mg/dL, Ethanol (optional) 0 mg/dL → Result: Calculated osmolality 290 mOsm/kg（Formula: 2·Na + glucose/18 + BUN/2.8, Reference: ~275–295 mOsm/kg）