The anion gap (AG) represents the difference between measured cations (Na) and measured anions (Cl + HCO₃). It helps differentiate causes of metabolic acidosis.
Formula
AG = Na − (Cl + HCO₃)
Albumin-corrected: Corrected AG = AG + 2.5 × (4 − Albumin g/dL)
Normal Range
8–12 mEq/L (lower with newer ion-selective electrodes)
High Anion Gap Causes (MUDPILES)
- Methanol
- Uremia
- Diabetic ketoacidosis
- Propylene glycol
- Iron, isoniazid, ibuprofen
- Lactic acidosis
- Ethylene glycol
- Salicylates
Why Albumin Correction Matters
Albumin is a major unmeasured anion. In hypoalbuminemia, the AG decreases by ~2.5 mEq/L for every 1 g/dL drop in albumin. Without correction, a "normal" AG may mask a true high-AG acidosis.