Osmolal Gap
Compares measured and calculated osmolality to detect unmeasured osmoles.
When to use
Suspected toxic alcohol ingestion or acidosis with high anion gap when osmolality was measured.
Pearls and pitfalls
- Calculated osmolality uses 2 × sodium + glucose/18 + urea/2.8.
- The gap can be normal early in intoxication and rise later as the alcohol is metabolized.
- Glucose and urea must be in the correct units: mg/dL.
- The gap does not replace toxic substance measurement or clinical assessment.
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Formula
Gap = measured osmolality − (2 × sodium + glucose/18 + blood urea nitrogen/2.8).
Evidence
Osmolal gap used to detect unmeasured osmoles such as toxic alcohols in patients with acidosis or neurologic change.
References
- Kraut JA, Kurtz I. Toxic alcohol ingestions: clinical features, diagnosis, and management. Clin J Am Soc Nephrol. 2008;3(1):208-225. doi:10.2215/CJN.03220807
Frequently asked questions
What is Osmolal Gap?
Compares measured and calculated osmolality to detect unmeasured osmoles. Suspected toxic alcohol ingestion or acidosis with high anion gap when osmolality was measured.
How is the result calculated?
Gap = measured osmolality − (2 × sodium + glucose/18 + blood urea nitrogen/2.8).
How should the result be interpreted?
Osmolal gap used to detect unmeasured osmoles such as toxic alcohols in patients with acidosis or neurologic change.
What are the sources?
Kraut JA, Kurtz I. Toxic alcohol ingestions: clinical features, diagnosis, and management. Clin J Am Soc Nephrol. 2008;3(1):208-225.