Anion Gap Calculator

The anion gap approximates the unmeasured anions floating around in blood plasma (things like albumin, phosphate, and sulfate that routine labs don't measure directly) by subtracting the two main measured anions from the main measured cation: AG = Na⁺ − (Cl⁻ + HCO3⁻). It's mainly used to help narrow down the cause of a metabolic acidosis — a "high-anion-gap" pattern points toward one set of causes (like lactic acidosis, ketoacidosis, or toxic ingestions), while a "normal-anion-gap" acidosis points toward another (like diarrhea or renal tubular acidosis).

Potassium is also a measured cation, so some labs fold it in for a touch more precision: AG (with K⁺) = (Na⁺ + K⁺) − (Cl⁻ + HCO3⁻). This calculator reports both side by side; the plain, potassium-excluded version is the one most commonly cited against the 8-16 mEq/L classic teaching range used here.

That 8-16 mEq/L range reflects older flame-photometry lab methods. Many labs using newer ion-selective electrode methods report somewhat lower normal ranges (often closer to 3-11 mEq/L) — always check the reference range printed on the actual lab report rather than treating any single cutoff as universal.