The anion gap is a calculated value used to help assess metabolic acidosis and identify the presence of unmeasured anions in the blood.

It is commonly calculated as: Sodium minus the sum of Chloride and Bicarbonate. Some formulas also include Potassium.

A normal anion gap usually depends on the laboratory and formula used, but a common reference range without potassium is around 8 to 12 mEq/L.

It helps clinicians evaluate acid-base disorders, especially high anion gap metabolic acidosis such as diabetic ketoacidosis, lactic acidosis, and toxic ingestions.