Clinical meaning
Anion gap (AG) metabolic acidosis results from the accumulation of unmeasured organic acids in the blood, consuming bicarbonate as a buffer and lowering serum pH. The anion gap is calculated as AG = Na⁺ − (Cl⁻ + HCO₃⁻), with a normal value of 10-12 mEq/L (or 8-12 depending on the lab). An elevated AG indicates the presence of unmeasured anions such as lactate, ketoacids, uremic toxins, or toxic alcohol metabolites. The MUDPILES mnemonic organizes the differential: Methanol (formic acid accumulation causing retinal toxicity and blindness), Uremia (accumulation of sulfate, phosphate, and hippurate in renal failure), Diabetic ketoacidosis (beta-hydroxybutyrate and acetoacetate from uninhibited lipolysis), Propylene glycol (D-lactate and L-lactate from IV lorazepam/phenobarbital infusions), Isoniazid/Iron (INH blocks pyridoxine-dependent enzymes causing lactic acidosis and seizures; iron causes direct mitochondrial toxicity), Lactic acidosis (type A from tissue hypoperfusion or type B from mitochondrial dysfunction, medications, or malignancy), Ethylene glycol (glycolic and oxalic acid causing renal tubular obstruction with calcium oxalate crystals), and Salicylates (uncoupling oxidative phosphorylation causing mixed respiratory alkalosis and metabolic acidosis). The osmolar gap (measured serum osmolality minus calculated osmolality using 2Na + glucose/18 + BUN/2.8) is elevated >10 mOsm/kg in toxic alcohol ingestions before metabolism occurs, helping identify methanol and ethylene glycol early. The delta-delta ratio (ΔAG/ΔHCO₃) helps identify concurrent non-AG metabolic acidosis (ratio <1) or metabolic alkalosis (ratio >2) hiding within an AG acidosis.