General and clinical
Anion gap
The anion gap is the difference between measured cations and measured anions, and it separates the causes of a metabolic acidosis into two groups. Correcting it for albumin matters, because a low albumin hides a raised gap.
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This calculator is provided for reference and education. It does not replace clinical assessment, examination, current guidelines, local protocol, or professional judgment. Verify every result before acting on it. Nirogix accepts no liability for clinical decisions made using this tool.
The formula
- Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻)
- Albumin-corrected = anion gap + 2.5 × (4.0 − albumin g/dL)
Worked example
Na 140, Cl 104, HCO₃ 24 → 140 − 128 = 12 mEq/L. With albumin 2.0: 12 + 2.5 × 2.0 = 17 mEq/L, which is raised.
When to use it
- Working up a metabolic acidosis
- Any unexplained low bicarbonate
And when not to
- A normal-looking gap with a low albumin is often a raised gap in disguise. Correct it.
- Reference ranges differ between laboratories and between analyzers. Use your own.
- Potassium is included in some definitions; this one does not include it, which is the commoner convention.
References
- The anion gap — Emmett M & Narins RG, Medicine (Baltimore), 1977
- Albumin-corrected anion gap in critical illness — Figge J et al., Critical Care Medicine, 1998
Last reviewed 07/09/2026.
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