Anion Gap Calculator
The serum anion gap is a simple calculation from the basic metabolic panel that helps clinicians classify metabolic acidosis. It represents the difference between the principal measured cation (sodium) and the principal measured anions (chloride and bicarbonate). An elevated anion gap above 12 mEq/L indicates the presence of unmeasured anions, pointing to causes such as lactic acidosis, ketoacidosis, uremia, or toxic ingestions. A normal anion gap acidosis (hyperchloremic) suggests bicarbonate loss from the GI tract or kidneys. The optional albumin-corrected AG adjusts for hypoalbuminemia, which can mask a true high AG. This tool is for educational reference only.
Anion gap formula
AG = Na - (Cl + HCO3)
Albumin-corrected AG = AG + 2.5 x (4 - albumin)
All values in mEq/L (or mmol/L). Normal AG with modern analyzers is approximately 6-12 mEq/L. The albumin correction uses 4.0 g/dL as the normal albumin reference value.
Anion gap interpretation
- AG below 6 mEq/L: Low gap. Consider hypoalbuminemia, multiple myeloma, or lab error.
- AG 6-12 mEq/L: Normal. If acidosis present, likely hyperchloremic (normal AG) type.
- AG above 12 mEq/L: Elevated. Suggests unmeasured anions; investigate cause (MUDPILES).
- AG above 20 mEq/L: Marked elevation, highly specific for organic acidosis.
Frequently asked questions
What is the anion gap?
The anion gap (AG) represents the difference between measured cations and anions in serum. It is calculated as sodium minus the sum of chloride and bicarbonate. A normal AG reflects unmeasured anions (albumin, phosphate, sulfate, organic acids) that balance unmeasured cations.
What is a normal anion gap?
With modern analyzers, normal AG is approximately 6-12 mEq/L (some labs report 3-11 mEq/L). An elevated AG above 12 mEq/L in the context of metabolic acidosis suggests accumulation of unmeasured anions such as lactate, ketones, or uremic acids.
What causes a high anion gap metabolic acidosis?
The MUDPILES mnemonic covers common causes: Methanol, Uremia, Diabetic ketoacidosis, Propylene glycol/Paraldehyde, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates. Each adds unmeasured anions that widen the gap.
What is the albumin-corrected anion gap?
Since albumin is the main unmeasured anion, low albumin reduces the expected AG. The corrected AG = measured AG + 2.5 x (4 - albumin in g/dL). This correction prevents missing a high AG acidosis in hypoalbuminemic patients.
What is a normal anion gap metabolic acidosis?
When metabolic acidosis occurs with a normal AG (hyperchloremic), bicarbonate loss is compensated by chloride retention. Common causes include diarrhea, renal tubular acidosis, and carbonic anhydrase inhibitors. The delta ratio can help distinguish mixed disorders.
Official sources
- NIH National Library of Medicine, MedlinePlus: CO2 Blood Test (Bicarbonate).
- NIH National Institute of Diabetes and Digestive and Kidney Diseases: Chronic Kidney Disease and Acid-Base.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.