The anion gap is the difference between measured cations and anions: AG = Na⁺ − (Cl⁻ + HCO₃⁻). The normal range is 3–12 mEq/L (varies by lab). An elevated anion gap indicates the presence of unmeasured anions, typically organic acids.

The classic mnemonic is MUDPILES: Methanol, Uremia, Diabetic ketoacidosis, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates. Each adds unmeasured acids that widen the gap between cations and measured anions.

Albumin is an unmeasured anion. Low albumin (common in hospitalized patients) can mask a true anion gap elevation. The correction adds 2.5 mEq/L to the AG for each 1 g/dL decrease in albumin below 4.0. This prevents missing significant acidosis.

A normal AG (hyperchloremic) metabolic acidosis has a normal AG with low bicarbonate and elevated chloride. Common causes include diarrhea, renal tubular acidosis, and normal saline infusion. The delta-delta calculation can help distinguish mixed disorders.