Osmolal Gap Calculator

Osmolal Gap Calculator for Endocrinology. A positive gap means measured osmolality exceeds the chosen estimate; a negative gap is retained rather than clamped. Neither direction identifies a particular solute. Changing the ethanol equation can change the gap even when the same sample is used.

For shared guidance on choosing a tool and documenting results, read the clinical calculator workflow.

How this calculator works

The gap is measured osmolality minus [2 x sodium + glucose + urea + ethanol/divisor]. Sodium, glucose and urea are in mmol/L; ethanol is in mg/dL. The divisor is explicitly selected as ideal 4.6 or empirical 3.7. US glucose is divided by 18.0182 and BUN by 2.8; BUN is not urea mass.

When to use this calculator

Use paired laboratory measurements and the laboratory's matched equation to inspect a discrepancy between measured and estimated total osmolality. Suspected toxic-alcohol exposure requires urgent toxicology assessment. A calculator result must not delay specific testing or treatment.

Inputs used

  • Measured serum osmolality in mOsm/kg
  • Sodium, glucose and urea in mmol/L; glucose/BUN alternatives in mg/dL
  • Ethanol in mg/dL, including an explicit zero when absent
  • Chosen ethanol coefficient convention

Clinical interpretation

A positive gap means measured osmolality exceeds the chosen estimate; a negative gap is retained rather than clamped. Neither direction identifies a particular solute. Changing the ethanol equation can change the gap even when the same sample is used.

Worked example

Sodium 140, glucose 5 and urea 5 with ethanol zero give a calculated estimate of 290. Measured osmolality 320 produces a gap of 30; measured 290 produces zero. Ethanol 46 with the ideal convention adds 10 to the estimate, reducing the first gap to 20.

Limitations and safety notes

Equation and assay variation, sampling time, other solutes and alcohol metabolism affect the gap. A normal or negative result does not exclude poisoning, especially after metabolism. No universal diagnostic cutoff, antidote trigger or dialysis decision is assigned. Urea and ethanol remain part of total osmolality, not simply effective tonicity.

Frequently asked questions

Can I enter urea mg/dL in the BUN field?

No. BUN expresses nitrogen mass, not urea mass. Confirm the analyte as well as its units.

Why does the ethanol choice matter?

The empirical and ideal conventions estimate different ethanol contributions. Interpretation requires the same convention as the laboratory reference interval.

References

  • Lynd et al. primary diagnostic evaluation of osmolal gap and equation dependence; not a stand-alone poisoning exclusion. https://pubmed.ncbi.nlm.nih.gov/18442409/
  • Purssell et al. primary empirical ethanol contribution study supporting mg/dL divided by 3.7, distinct from ideal molar 4.6 convention. https://pubmed.ncbi.nlm.nih.gov/11719745/

Editorial review and citation methodology

Maintained by the Quick Medical Calculator Editorial Team. Content record date: September 9, 2026. A content date is not evidence of independent clinical review. Individual clinical sign-off is not recorded on this page.

  • Prefer original validation studies for scoring systems and prediction tools.
  • Use current specialty society guidance, transplant allocation policy, public health guidance, or regulator resources when they govern clinical use.
  • Include limitations and safety notes when a calculator is population-specific, context-dependent, or unsuitable as a standalone decision tool.

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