Anion Gap

The anion gap represents the difference between measured cations (Na⁺) and measured anions (Cl⁻ + HCO₃⁻). It reflects unmeasured anions in the blood (albumin, phosphate, sulfate, organic acids). An elevated anion gap indicates accumulation of unmeasured acids.

• MUDPILES mnemonic for high AG acidosis: Methanol, Uremia, DKA, Propylene glycol, INH/Iron, Lactic acidosis, Ethylene glycol, Salicylates
• HARDUPS for normal AG acidosis: Hyperalimentation, Acetazolamide/Addison's, RTA, Diarrhea, Ureterosigmoidostomy, Pancreatic fistula, Saline infusion
• Correct for hypoalbuminemia: For every 1 g/dL ↓ albumin, add 2.5 to expected AG
• Normal AG = 12 ± 4 (varies by lab; some use 8-12)
• Delta-delta ratio = ΔAG / ΔHCO₃ → helps identify mixed disorders

• Classify metabolic acidosis: High AG vs Normal AG (hyperchloremic)
• Identify underlying cause: MUDPILES causes produce unmeasured acids
• Monitor treatment response: AG should normalize with appropriate therapy
• Detect mixed disorders: Use delta-delta ratio