The anion gap (AG) is a key calculation in acid-base evaluation. Elevated AG indicates the presence of unmeasured anions, guiding the differential diagnosis of metabolic acidosis. Albumin correction is essential when hypoalbuminemia is present.
Anion Gap Calculator
How to Use the Anion Gap Calculator
This anion gap calculator computes the standard anion gap (Na − [Cl + HCO₃]) and the albumin-corrected anion gap for evaluating metabolic acidosis in clinical settings.
When to Use the Corrected AG
Always correct for albumin when the serum albumin is below 4.0 g/dL. Critically ill patients commonly have hypoalbuminemia, which falsely lowers the AG by 2.5 mEq/L for every 1 g/dL decrease in albumin. A patient with AG of 14 and albumin of 2.0 g/dL has a corrected AG of 14 + (4−2) × 2.5 = 19 — significantly elevated and requiring further workup.
Using the Delta-Delta Ratio
When AG is elevated, calculate the delta-delta ratio: (AG − 12) / (24 − HCO₃). A ratio below 1 suggests a mixed AG and non-AG metabolic acidosis. A ratio above 2 suggests a concurrent metabolic alkalosis hidden by the AG acidosis. This provides insight into complex mixed acid-base disorders.
FAQ
Is this anion gap calculator free?
Yes, completely free with no signup required.
What is the anion gap?
The anion gap (AG) is the difference between measured cations and anions in serum: Na+ − (Cl− + HCO3−). An elevated AG indicates the presence of an unmeasured anion — often a sign of metabolic acidosis. The normal range is approximately 8–12 mEq/L (without albumin correction).
Why correct the anion gap for albumin?
Albumin is a negatively charged protein that contributes to the anion gap. Low albumin (hypoalbuminemia) lowers the AG, masking an elevated gap. For every 1 g/dL decrease in albumin from normal (4 g/dL), add 2.5 mEq/L to the calculated AG. The corrected AG should be used when albumin is below normal.
What does MUDPILES stand for?
MUDPILES is a mnemonic for causes of elevated anion gap metabolic acidosis: Methanol, Uremia, Diabetic ketoacidosis (DKA), Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates. These conditions produce unmeasured anions that widen the AG.
What is a normal anion gap?
Normal anion gap is approximately 8–12 mEq/L using the Na − (Cl + HCO3) formula. With albumin correction and modern analyzers, many centers use 3–11 mEq/L as normal. An AG above 20 mEq/L is almost always pathological; 12–20 requires clinical context.