Osmol gaps revisited: normal values and limitations

Area of Science:

• Clinical Chemistry
• Toxicology
• Internal Medicine

Background:

• The osmol gap is a calculated difference between measured and estimated serum osmolarity.
• Accurate osmol gap calculation is crucial for diagnosing conditions like toxic alcohol ingestion.
• Existing equations for osmolarity calculation may not be universally applicable.

Purpose of the Study:

• To define the osmol gap in patients with known serum ethanol levels.
• To reevaluate and compare different equations for calculating osmolarity.
• To assess the clinical utility of the osmol gap in suspected toxic alcohol ingestions.

Main Methods:

• Prospective enrollment of adult patients requiring serum ethanol and electrolyte determination.
• Exclusion of 16 patients from the final analysis of 321 enrolled.
• Stepwise multiple linear regression to determine optimal coefficients for sodium, blood urea nitrogen, and ethanol.

Main Results:

• The calculated osmol gap was -2 +/- 6 mOsm using a specific regression-derived formula.
• Different osmolarity equations yielded varying osmol gap values, ranging from -5 to +15 mOsm.
• Standard deviations and correlation coefficients were comparable across different calculation methods.

Conclusions:

• Significant variability exists in osmol gap values depending on the calculation formula employed.
• Absolute osmol gap values are highly dependent on the specific osmolarity equation used.
• Small osmol gaps should not be relied upon to exclude the possibility of toxic alcohol ingestion due to equation-dependent variations.