The anion gap is one of the first calculations you run when a patient presents with metabolic acidosis. The formula is simple: Sodium minus the sum of Chloride and Bicarbonate. Normal range sits between 8 and 12 mEq/L. An elevated gap above 12 tells you unmeasured anions are present — and your job is figuring out why. The classic causes form the MUDPILES mnemonic: methanol, uremia, diabetic ketoacidosis, propylene glycol, isoniazid, lactic acidosis, ethylene glycol, and salicylates. Whether it’s high or normal fundamentally changes your diagnostic approach and the workup that follows in the emergency department or ICU.
Anion gap interpretation goes well beyond the number itself. In patients with low albumin — critically ill, malnourished, or those with liver disease — you must calculate the corrected anion gap, because hypoalbuminemia artificially lowers the result and can mask a true high anion gap acidosis hiding underneath. Add 2.5 mEq/L to the result for every 1 g/dL drop in albumin below 4. From there, the delta-delta ratio helps uncover mixed acid-base disorders layered beneath a primary anion gap acidosis. Online anion gap calculators handle these corrections instantly. But clinical context always drives interpretation — the number is your starting point, not your diagnosis.