Anion gap calculator

The anion gap is calculated as AG = Na − (Cl + HCO₃), where all three values are in mmol/L. It represents unmeasured anions in the blood — chiefly albumin, but also phosphate, sulphate, and organic acids — that aren't captured by the routine sodium, chloride, and bicarbonate measurements. A gap above the normal range suggests the presence of an unmeasured acid, such as lactate, ketones, uraemic acids, or a toxin (e.g. methanol, ethylene glycol, or salicylate).

Albumin is the largest single contributor to the normal anion gap, so hypoalbuminaemia lowers the observed gap and can mask a clinically significant elevation. This tool applies the Figge correction: corrected AG = observed AG + 0.25 × (40 − albumin), with albumin in g/L and 40 g/L taken as the normal reference value. The corrected figure is only shown once a serum albumin value has been entered.

This calculator uses the following bands: below 8 mEq/L is low, 8–16 mEq/L is normal, 17–20 mEq/L is elevated, and above 20 mEq/L is significantly elevated. A low anion gap is less common clinically and can reflect hypoalbuminaemia, paraproteinaemia, or a lab measurement issue. An elevated or significantly elevated gap warrants further workup for an underlying high anion gap metabolic acidosis.

The anion gap is a screening calculation, not a diagnosis — a normal gap does not exclude a metabolic acidosis, and reference ranges vary slightly between laboratories and analysers. This tool is for reference only and is not a substitute for clinical judgement — interpretation and management decisions remain with the treating clinician.