Serum Osmolality Calculator — Free Online Calculator
Input
Calculated Osmolality
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About This Calculator
The serum osmolality calculator estimates the osmolality based on serum sodium, BUN, glucose, and optionally ethanol. When a measured osmolality is provided, it calculates the osmolar gap — the difference between measured and calculated osmolality. An elevated osmolar gap (>10 mOsm/kg) suggests the presence of unmeasured osmotically active substances such as toxic alcohols (methanol, ethylene glycol), or less commonly, mannitol or isopropanol.
Formula
Interpretation
| Parameter | Interpretation |
|---|---|
| Osm < 275 | Hypo-osmolar |
| Osm 275–295 | Normal |
| Osm > 295 | Hyperosmolar |
| Osmolar Gap ≤ 10 | Normal |
| Osmolar Gap > 10 | Elevated — consider toxic alcohols |
References
- Purssell RA, et al. Derivation and validation of a formula to calculate the contribution of ethanol to the osmolal gap. Ann Emerg Med. 2001;38(6):653-659.
- Kraut JA, Xing SX. Approach to the evaluation of a patient with an increased serum osmolal gap and high-anion-gap metabolic acidosis. Am J Kidney Dis. 2011;58(3):480-484.
Frequently Asked Questions
When should I suspect toxic alcohol ingestion?
Suspect toxic alcohols when there is an elevated osmolar gap (> 10) combined with high anion gap metabolic acidosis. Methanol causes visual symptoms, ethylene glycol causes renal failure, and isopropanol causes ketosis without acidosis. An elevated osmolar gap with normal anion gap may suggest early ingestion.
Why does the osmolar gap change over time in toxic alcohol ingestion?
The parent alcohol (methanol, ethylene glycol) initially raises the osmolar gap. As alcohol dehydrogenase metabolizes it into organic acids, the osmolar gap decreases while the anion gap increases. A normal osmolar gap does NOT rule out toxic alcohol ingestion if presentation is delayed.
What causes a low osmolar gap?
A low or negative osmolar gap can occur with hypoalbuminemia, hyperlipidemia (pseudo-hyponatremia), or hyperproteinemia (multiple myeloma). These conditions affect measured osmolality or sodium differently than calculated values.
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⚠ Medical Disclaimer
This tool is for educational and informational purposes only. It is not intended to replace professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional for clinical decisions.