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Calculates the urine osmolal gap — the difference between measured and estimated urine osmolality — to detect unmeasured osmoles in the urine (e.g., ethylene glycol, mannitol, or other toxins).
Estimated UOsm = 2 × (Urine Na + Urine K) + (Urine Urea ÷ 2.8) + (Urine Glucose ÷ 18); UOG = Measured UOsm − Estimated UOsm
Near zero (typically < 10 mOsm/kg) in health
The urine osmolal gap detects osmotically active solutes that are not routinely measured. It is a helpful adjunct when toxic alcohol poisoning is suspected and the serum osmolar gap is unreliable or negative.
Specialty
Internal MedicineCategory
NephrologyDifficulty
Basic
Estimated Time
30 seconds
Version
1.0
Last Updated
2026-08-16
Formula
Keywords
Normal Range
Near zero (typically < 10 mOsm/kg) in health
| Classification | Range |
|---|---|
Normal no significant unmeasured osmoles | ≤10 mOsm/kg |
Elevated (unmeasured osmoles) e.g., ethylene glycol, mannitol, low-molecular-weight alcohols | >10 mOsm/kg |
Calculates the urine osmolal gap — the difference between measured and estimated urine osmolality — to detect unmeasured osmoles in the urine (e.g., ethylene glycol, methanol, mannitol).
The estimated urine osmolality is 2 × (Na + K) + urea/2.8 + glucose/18 (urea and glucose in mg/dL). A gap of 10 mOsm/kg or less is normal; a gap above 10 suggests unmeasured osmoles such as ethylene glycol or mannitol. A normal gap does not exclude toxic alcohol ingestion.
The urine osmolal gap detects osmotically active solutes not captured by routine urine chemistries and complements the serum osmolar gap in the evaluation of toxic alcohol and mannitol exposure.
A 35-year-old man with suspected ethylene glycol ingestion has a measured urine osmolality of 600 mOsm/kg, urine sodium 80 mmol/L, urine potassium 40 mmol/L, urine urea 560 mg/dL, and urine glucose 0 mg/dL.
Inputs:
Estimated urine osmolality = 2 × (80 + 40) + 560/2.8 + 0 = 440 mOsm/kg. The gap of 160 mOsm/kg indicates substantial unmeasured osmoles.
It compares measured urine osmolality to the osmolality predicted from urine sodium, potassium, urea, and glucose. The difference represents unmeasured osmotically active solutes.
Mainly in suspected toxic alcohol or mannitol exposure when the serum osmolar gap is normal or negative, and as part of the metabolic evaluation of unmeasured osmoles.
Disclaimer: This calculator is intended for educational and clinical decision support purposes only. Suspected toxic alcohol ingestion requires specific assays and urgent clinical management regardless of the urine osmolal gap.