Acid–Base Assessment

Anion Gap Calculator

Calculate the anion gap from sodium, chloride, and bicarbonate. Optionally calculate the albumin-corrected anion gap to unmask a high anion gap metabolic acidosis masked by hypoalbuminaemia.

Enter Values

Required

mmol/L
Please enter a valid sodium value.
mmol/L
Please enter a valid chloride value.
mmol/L
Please enter a valid bicarbonate value.

Optional — for albumin-corrected anion gap

Anion Gap
—
mmol/L
Worked Example
Given: Na 138 mmol/L, Cl 98 mmol/L, HCO₃ 14 mmol/L, Albumin 2.0 g/dL
Anion gap: 138 − (98 + 14) = 26 mmol/L (High)
Corrected AG: 26 + 2.5 × (4.0 − 2.0) = 26 + 5.0 = 31 mmol/L
Interpretation: High anion gap metabolic acidosis. Without albumin correction, the gap may appear less elevated, potentially understating the severity.
Formula
-- Anion gap --
AG = Na+ − (Cl− + HCO3−)
-- Albumin-corrected anion gap --
Corrected AG = AG + 2.5 × (4.0 − albuming/dL)
-- Albumin unit conversion --
g/L → g/dL: divide by 10
Interpretation (without potassium)
Low anion gap < 8 mmol/L
Normal anion gap 8–12 mmol/L
High anion gap > 12 mmol/L

Normal ranges vary by laboratory and analyser. Always check local reference ranges.

Clinical Notes
  • High anion gap: MUDPILES — Methanol, Uraemia, Diabetic ketoacidosis, Propylene glycol/Paracetamol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates.
  • Normal anion gap: hyperchloraemic acidosis — diarrhoea, RTA, saline excess.
  • Low anion gap: uncommon; consider hypoalbuminaemia, paraproteinaemia, or lab error.
  • Albumin is the main unmeasured anion — hypoalbuminaemia lowers the anion gap and can mask HAGMA.
Albumin correction is clinically important. Without it, a high anion gap metabolic acidosis may be missed in patients with hypoalbuminaemia (e.g., liver disease, malnutrition, critical illness).
Limitations
  • This formula excludes potassium; some laboratories include K+ (normal ~12–16).
  • Wide individual biological variation in normal anion gap exists.
  • Mixed acid–base disorders can cause a normal anion gap despite pathology.
  • Arterial blood gas values should be used alongside electrolytes for full acid–base interpretation.
References
  • Emmett M, Narins RG. Clinical use of the anion gap. Medicine (Baltimore). 1977;56(1):38–54.
  • Figge J et al. Serum proteins and acid-base equilibria. J Lab Clin Med. 1992;120(5):713–719.
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Educational use only. This calculator is intended for medical students and junior doctors. It does not replace clinical judgement, local guidelines, laboratory reference ranges, or senior and specialist advice.