Renal & electrolytes
Anion gap calculator
Sodium minus chloride minus bicarbonate — plus the albumin correction that stops a low albumin hiding the acidosis.
How to read the result
Anion gap (mmol/L) falls into these published bands.
An ICU patient with sodium 138, chloride 105 and bicarbonate 22 mmol/L — and an albumin of 20 g/L.
Anion gap 11 mmol/L (reads normal); albumin-corrected gap 16 mmol/L — elevated
The raw gap conceals the problem: correcting for the low albumin unmasks a high-anion-gap acidosis, and the work-up turns to ketones, lactate, toxins, and renal failure.
Who this is for
For clinicians working up metabolic acidosis — ketoacidosis, lactic acidosis, toxic ingestions, renal failure — and anyone reading a gas in ICU, where hypoalbuminaemia is routine. Each 10 g/L of albumin below 40 lowers the apparent gap by about 2.5 mmol/L, so the Figge-corrected gap is the one that keeps a significant acidosis from reading as unremarkable.
How it works
What it calculates
The anion gap — sodium minus chloride and bicarbonate — and, when albumin is supplied, the Figge-corrected gap that unmasks a raised gap hidden by hypoalbuminaemia.
Interpreting the result
A raised gap points to unmeasured anions: ketoacidosis, lactic acidosis, toxins (methanol, ethylene glycol, salicylates), and renal failure. A normal-gap acidosis instead suggests bicarbonate loss — diarrhoea or renal tubular acidosis.
Worth remembering
Each 10 g/L of albumin below 40 lowers the apparent gap by about 2.5 mmol/L — in the hypoalbuminaemic ICU patient an 'unremarkable' gap can conceal a significant acidosis. Reference: Figge 1998.
Built on published evidence
- AG = Na − (Cl + HCO₃). Reference interval varies by analyser (≈ 4–12 mmol/L)
- Corrected AG = AG + 0.25 × (40 − albumin g/L) — low albumin hides a raised gap. Reference: Figge 1998
Codification last reviewed 2026-08-28, and checked against published reference values in automated tests on every release — see the full evidence ledger.
Questions clinicians ask
What causes a raised anion gap?
Unmeasured anions: ketoacidosis, lactic acidosis, toxins (methanol, ethylene glycol, salicylates), and renal failure. A normal-gap acidosis instead suggests bicarbonate loss — diarrhoea or renal tubular acidosis.
Why correct the gap for albumin?
Albumin is the major unmeasured anion in health, so hypoalbuminaemia lowers the apparent gap — about 2.5 mmol/L per 10 g/L below 40. The Figge correction (AG + 0.25 × (40 − albumin), Figge 1998) restores the gap a normal albumin would show.
What counts as a normal anion gap?
The reference interval varies by analyser — roughly 4–12 mmol/L on modern chemistry platforms — so anchor to your own laboratory's range. A low gap is rare and worth a second look: hypoalbuminaemia, paraproteinaemia, or laboratory error.
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