Where:

Normal range: approximately 8-16 mEq/L (using the typical reference values of Na⁺ ~140 mEq/L, Cl⁻ ~100 mEq/L, and HCO₃⁻ ~24 mEq/L) - Henry's Clinical Diagnosis and Management by Laboratory Methods.

This represents the "unmeasured anions" in serum (things like phosphate, sulfate, and negatively charged proteins) since sodium is the major measured cation and chloride/bicarbonate are the major measured anions. Some versions of the formula also subtract potassium (Na⁺ + K⁺ − (Cl⁻ + HCO₃⁻)), but the potassium-excluding version above is more commonly used clinically.

Clinical use: An elevated anion gap points toward high-anion-gap metabolic acidosis (e.g., DKA, lactic acidosis, uremia, toxic ingestions like methanol/ethylene glycol/salicylates - remember "MUDPILES"), while a normal-gap acidosis suggests causes like diarrhea or renal tubular acidosis - ROSEN's Emergency Medicine.