Anion Gap Calculator

The anion gap (AG) is a calculated value that reflects the difference between measured cations and anions in serum. It helps clinicians identify the cause of metabolic acidosis — an elevated AG suggests the presence of unmeasured anions such as lactate, ketones, or toxins. It is a fundamental tool in diagnosing conditions like diabetic ketoacidosis, lactic acidosis, and renal failure.

The standard formula is: AG = Na⁺ − Cl⁻ − HCO₃⁻. An alternative formula includes potassium: AG = Na⁺ + K⁺ − Cl⁻ − HCO₃⁻. The standard formula is more commonly used in clinical practice since potassium contributes minimally and can be left out for simplicity.

Using the standard formula (without K⁺), a normal anion gap is typically 8–12 mEq/L. When potassium is included, the normal range shifts to approximately 10–14 mEq/L. Values above these thresholds suggest a high anion gap metabolic acidosis.

A high anion gap is caused by an increase in unmeasured anions. Common causes are remembered using the mnemonic MUDPILES: Methanol, Uremia, Diabetic ketoacidosis, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, and Salicylates. Each condition introduces acid that consumes bicarbonate while raising the gap.

A normal anion gap metabolic acidosis (also called hyperchloremic acidosis) is caused by conditions such as diarrhea, renal tubular acidosis, or excessive saline administration. In these cases, bicarbonate is lost and replaced by chloride, keeping the anion gap within normal limits. A very low anion gap may suggest hypoalbuminemia or laboratory error.

Yes, in patients with low albumin levels, the anion gap should be corrected. Albumin is a major unmeasured anion, so hypoalbuminemia can mask a true high anion gap. The corrected AG = measured AG + 2.5 × (4 − albumin in g/dL). This correction is especially important in critically ill patients.

The potassium-inclusive formula (AG = Na⁺ + K⁺ − Cl⁻ − HCO₃⁻) is used in some European countries and certain laboratory conventions. In most clinical settings in North America, potassium is omitted because its contribution is small and relatively constant. Always confirm which formula your institution uses to ensure consistent interpretation.

Yes. Serial anion gap measurements are useful for monitoring the response to treatment in conditions like diabetic ketoacidosis and lactic acidosis. As the underlying cause is corrected and acid levels fall, the anion gap should normalize. A persistently elevated or rising anion gap despite treatment may indicate an ongoing or additional source of metabolic derangement.