Serum Osmolality calculator
Result
290mOsm/kg
Normal (275–295)
How to use it
- 1
Calc osmolality = 2×Na + glucose + urea (mmol/L). UK units. US version: 2×Na + glucose/18 + BUN/2.8.
- 2
Compare to measured osmolality. Osmolal gap = measured − calculated. Gap > 10: suspect toxic alcohols.
- 3
HHS threshold. Hyperosmolar hyperglycaemic state: effective osmolality > 320 mOsm/kg.
When to use it
- HHS (Hyperosmolar Hyperglycaemic State)
- Glucose-driven hyperosmolality — effective osmolality uses 2×Na + glucose.
- SIADH vs. osmotic hyponatraemia
- Low serum osmolality (< 275) = hypotonic hyponatraemia — confirms SIADH territory.
- Toxic alcohol screen
- Osmolal gap > 10: check ethanol, methanol, ethylene glycol.
A reference for clinicians and students, not medical advice. A score supports a decision; it does not make one. Check every result against the patient in front of you and your local protocol.
At a glance
- Specialty
- Fluids and electrolytes
- Inputs
- 3
- Type
- Formula
More in fluids and electrolytes
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- Cockcroft-Gault CrClNephrology · 4 inputs
- MDRD eGFRNephrology · 3 inputs
- Fractional Excretion of Sodium (FENa)Nephrology · 4 inputs
- Fractional Excretion of Urea (FEUrea)Nephrology · 4 inputs
- Urine Anion GapNephrology · 3 inputs
- AKI Staging (KDIGO)Nephrology · 2 inputs
- Osmolal GapNephrology · 4 inputs
