Osmolar Gap

What This Calculator Measures

Calculates the difference between measured and calculated serum osmolality to detect the presence of unmeasured osmoles, which may indicate toxic alcohol ingestion.

How to Use

1. Enter the measured serum osmolality (mOsm/kg) from the laboratory.
2. Enter serum sodium (mmol/L), glucose (mg/dL), and BUN (mg/dL).
3. Review the osmolar gap and its interpretation.

Interpretation Guide

An osmolar gap >10 mOsm/kg may indicate the presence of unmeasured osmoles such as toxic alcohols (methanol, ethylene glycol), ethanol, or other ingestions. A normal osmolar gap does not completely exclude toxic ingestion in delayed presentations.

Clinical Significance

An elevated osmolar gap in the appropriate clinical setting suggests the presence of an unmeasured osmole, which may be a toxic alcohol. Early identification is critical because specific antidotes (fomepizole, ethanol) and dialysis can be life-saving.

When to Use

• Suspected toxic alcohol ingestion (methanol, ethylene glycol, isopropanol)
• Emergency department evaluation of unknown ingestion
• Altered mental status with metabolic acidosis
• Evaluation of elevated anion gap with suspected ingestion

When Not to Use

• When measured osmolality is not available
• As a standalone diagnostic test for toxic ingestion
• When the clinical picture clearly points to a non-toxicological cause

Limitations

• A normal osmolar gap does not exclude toxic ingestion, particularly in delayed presentations where metabolism has occurred.
• Requires a measured serum osmolality from the laboratory.
• Ethanol ingestion elevates the osmolar gap but is not necessarily toxic.
• Uremia, ketoacidosis, and alcohol can also elevate the osmolar gap.

Worked Example

A 30-year-old man presents with altered mental status and metabolic acidosis. Measured serum osmolality is 330 mOsm/kg, sodium is 140 mmol/L, glucose is 100 mg/dL, and BUN is 15 mg/dL.

Calculated osmolality ≈ 2 × 140 + 100/18 + 15/2.8 = 280 + 5.6 + 5.4 = 291 mOsm/kg. Osmolar gap ≈ 330 − 291 = 39 mOsm/kg. This elevated osmolar gap, in the context of metabolic acidosis, raises concern for unmeasured osmoles such as toxic alcohol ingestion.

Evidence

Source

Emergency Medicine / Toxicology

Reference

Brent J, et al. Fomepizole for the treatment of methanol poisoning. N Engl J Med. 2001;344:424-429.

Reviewed By

MedCalcHub Clinical Team

Version

1.0

Updated

2026-08

1. Brent J, et al. N Engl J Med. 2001;344:424-429.

References

1. Brent J, et al. Fomepizole for the treatment of methanol poisoning. N Engl J Med. 2001;344:424-429. (Level I)

Frequently Asked Questions