Printed on 2/13/2026
For informational purposes only. This is not medical advice.
The anion gap is a calculated value from routine electrolytes that helps differentiate causes of metabolic acidosis. It represents the difference between measured cations (sodium) and measured anions (chloride + bicarbonate). An elevated anion gap suggests the presence of unmeasured acids (e.g., lactate, ketoacids, toxins). The albumin-corrected anion gap adjusts for low albumin, which can mask a true elevation.
Formula: AG = Na⁺ − (Cl⁻ + HCO₃⁻); Corrected AG = AG + 2.5 × (4.0 − Albumin)
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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.