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Serum Osmolality Calculator
Input
mEq/L
mg/dL
mg/dL
mg/dL
mOsm/kg
Calculated Osmolality
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About
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
Calculated Osm = 2 × Na + BUN/2.8 + Glucose/18 + Ethanol/4.6
Osmolar Gap = Measured Osm − Calculated Osm
Normal serum osmolality: 275–295 mOsm/kg
Normal osmolar gap: < 10 mOsm/kg
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.
FAQ
Disclaimer
For educational and informational purposes only. Not intended to replace professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional.