The anion gap is a calculated value derived from the concentrations of major electrolytes in the blood: anion gap = sodium − (chloride + bicarbonate). It represents the difference between measured positively charged ions (cations) and negatively charged ions (anions). Normally this gap is 8–12 mmol/L, filled by unmeasured anions such as albumin, phosphate, and sulphate.

An elevated anion gap indicates the presence of excess unmeasured acid in the blood — a pattern seen in metabolic acidosis from causes including diabetic ketoacidosis, lactic acidosis, renal failure, and toxic ingestions (methanol, ethylene glycol, aspirin). It is a crucial tool in emergency medicine and in the evaluation of any patient with an unexplained metabolic disturbance.