Anion Gap Calculator

Anion gap: unmeasured anions in metabolic acidosis

The anion gap (AG) is a measure of the anions in plasma that routine panels don't directly measure. You calculate it with AG = Na⁺ − (Cl⁻ + HCO₃⁻), all in mEq/L. The normal range sits around 8–12 mEq/L, though it shifts with the assay and runs lower on newer chloride-selective electrodes. When the AG is elevated in metabolic acidosis, it tells you unmeasured anions are piling up. To remember the usual culprits, clinicians lean on MUDPILES: Methanol, Uremia, DKA (diabetic ketoacidosis), Paraldehyde/Propylene glycol, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates. A normal-AG (hyperchloremic) acidosis points elsewhere: diarrhea, renal tubular acidosis (RTA), urinary diversion, early CKD. Example: Na 140, Cl 105, HCO₃ 24 → AG = 11 mEq/L, which is normal.

Clinical context

You'll see the AG everywhere acidosis gets triaged. The emergency department and the ICU use it to sort out metabolic acidosis. Endocrinology follows it during DKA management, watching the gap close as ketones clear with insulin. Toxicology reaches for it with salicylate, methanol, or ethylene glycol, and nephrology uses it for uremic acidosis and the RTA workup. One thing worth doing every time is correcting for albumin: corrected AG = AG + 2.5 · (4 − albumin g/dL). Albumin is the dominant unmeasured anion in healthy plasma, so a raw AG will understate the gap when a patient is hypoalbuminemic.

FAQ

Should I include potassium? Most clinicians leave K⁺ out. Its plasma concentration is small and fairly stable, so it adds little. If you do include it (AG = Na + K − Cl − HCO₃), expect the reference range to climb by roughly 4.

What is the delta ratio? It's ΔAG/ΔHCO₃, and it flags mixed disorders. A value near 1 fits a pure high-AG acidosis. Below 1, you're probably looking at a coexisting normal-AG acidosis; above 2, a coexisting metabolic alkalosis.

Can the AG be low? It can. Hypoalbuminemia narrows it, and so do multiple myeloma (with its cationic paraproteins), lithium toxicity, and severe hypercalcemia.