Last updated: 2026-06-21

Anion Gap

The serum anion gap is a calculated value that estimates the anions in blood that routine laboratory panels do not measure. It is derived from sodium, chloride, and bicarbonate and is used mainly to narrow the causes of a metabolic acidosis. This calculator returns the gap and places it in a low, normal, or high band.

The idea behind it

Blood must be electrically neutral, so the total positive charge from cations equals the total negative charge from anions. Laboratories routinely measure one major cation, sodium, and two major anions, chloride and bicarbonate. The "gap" between them stands in for everything not measured — chiefly albumin, along with phosphate, sulphate, and organic acids.

The formula

$\mathrm{AG} = \mathrm{Na}^+ - (\mathrm{Cl}^- + \mathrm{HCO_3}^-)$

Some clinicians add potassium to the cation side; the potassium-free version shown here is the more common convention, with a normal range of roughly 8 to 12. For the monovalent ions sodium, chloride, and bicarbonate, the conventional unit (mEq/L) and the SI unit (mmol/L) are numerically identical, so the same figures apply in either system.

Worked example

For sodium 140, chloride 100, and bicarbonate 12 mEq/L:

A gap of 28 mEq/L is markedly raised and, together with the low bicarbonate, points to a high-anion-gap metabolic acidosis such as lactic acidosis or ketoacidosis.

Interpretation and limits

A high gap suggests added unmeasured acids; a normal gap in an acidosis points instead to bicarbonate loss. Because albumin is the largest unmeasured anion, a low albumin lowers the gap and can hide a true elevation, so the value is sometimes corrected for albumin. The gap is one piece of an acid-base assessment, not a diagnosis. See also the Corrected Calcium Calculator for another albumin-sensitive laboratory adjustment.