Anion Gap
Definition
Sodium minus the sum of chloride and bicarbonate in serum. A widened anion gap (above 16) in a comatose patient with unexplained acidosis is the bedside clue to methanol or ethylene glycol.
- Formula
- Na - (Cl + HCO3)
- Alert threshold
- Above 16 is widened
- Clinical use
- Bedside screen for toxic alcohol ingestion
- Sample
- Serum electrolytes
Common questions
Why does a widened anion gap point toward methanol or ethylene glycol rather than another cause?+
Many conditions widen the anion gap, but in a comatose patient with unexplained acidosis and no obvious cause such as diabetic ketoacidosis or renal failure, toxic alcohol metabolites accumulating as unmeasured acids become a leading suspect that prompts specific confirmatory testing.
Can the anion gap alone confirm methanol poisoning?+
No, it is a rapid screening clue, not a confirmatory test. Definitive identification requires measuring methanol and its toxic metabolite formate directly, typically by gas chromatography, since other conditions can also widen the gap.
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