Anion Gap Calculator
Calculate the serum anion gap from sodium, chloride, and bicarbonate. With optional albumin correction for hypoalbuminaemia.
How to use this tool
- Enter sodium (na⁺), chloride (cl⁻), bicarbonate (hco₃⁻) and albumin (optional) in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your anion gap and the full breakdown beneath it.
Educational estimate — not medical advice. Consult a clinician.
The anion gap (AG) estimates unmeasured anions in plasma: AG = Na⁺ − (Cl⁻ + HCO₃⁻). An elevated AG suggests an anion-gap metabolic acidosis (e.g., lactic acidosis, ketoacidosis, uraemia, toxins). Always interpret in clinical context.
Formula
Anion Gap (AG) = Na+ − (Cl− + HCO3−)
Albumin-corrected AG = AG + 2.5 × (4.0 − Albumin)
All values in mEq/L; albumin in g/dL.
How it works
The anion gap represents the difference between measured cations and measured anions in serum; unmeasured anions (lactate, ketoacids, toxins, phosphate) widen the gap above the normal range of 8–12 mEq/L. This calculator applies the classic three-variable formula (Na minus Cl plus HCO3) and optionally corrects for hypoalbuminaemia using the Figge correction, adding 2.5 mEq/L for each 1 g/dL that albumin falls below 4.0 g/dL. Because the formula omits potassium, results may differ slightly from laboratories that use a four-variable version; the albumin correction is especially important in critically ill patients where low albumin masks a true high-AG acidosis.
Worked example
Worked example
- Inputs: Na+ = 140 mEq/L, Cl− = 102 mEq/L, HCO3− = 24 mEq/L, albumin = 4.0 g/dL.
- AG = 140 − (102 + 24) = 140 − 126 = 14 mEq/L.
- Albumin deficit = 4.0 − 4.0 = 0 g/dL, so corrected AG = 14 + 0 = 14 mEq/L.
Anion Gap = 14 mEq/L, Corrected AG = 14 mEq/L — Mildly elevated (13–20 mEq/L).
Key terms
- Anion gap
- The calculated difference between measured cations (Na+) and measured anions (Cl− + HCO3−), reflecting unmeasured anions in plasma.
- High-AG metabolic acidosis
- A form of metabolic acidosis caused by accumulation of unmeasured acids (e.g. lactate, ketoacids, salicylate), widening the anion gap above 12 mEq/L.
- Figge correction
- An albumin-based correction that adds 2.5 mEq/L to the anion gap for each 1 g/dL drop in albumin below 4 g/dL, unmasking a high-AG acidosis in hypoalbuminaemic patients.
- Hypoalbuminaemia
- Abnormally low serum albumin (below ~3.5 g/dL), which artificially lowers the anion gap and can conceal a true high-AG acidosis.
- Normal anion gap
- Typically 8–12 mEq/L using the three-variable formula; values in this range suggest hyperchloraemic (non-gap) acidosis rather than accumulation of unmeasured acids.
Frequently asked questions
- What is a normal anion gap?
- With modern analysers (which include K⁺), normal AG is typically 8–12 mEq/L. Older literature sometimes uses 12–16 (K excluded). This calculator uses the standard Na−(Cl+HCO₃) formula.
- Why correct for albumin?
- Each 1 g/dL fall in albumin below 4.0 g/dL lowers AG by ~2.5 mEq/L. A hypoalbuminaemic patient with a seemingly normal AG may actually have a masked high-AG acidosis; the corrected AG unmasks it.
- What are common causes of a high anion gap?
- The MUDPILES mnemonic: Methanol, Uraemia, Diabetic ketoacidosis, Propylene glycol / Paracetamol, Isoniazid, Lactic acidosis, Ethylene glycol, Salicylates.