Anion Gap Calculator

The formula

The serum anion gap estimates the unmeasured anions in plasma using routinely measured electrolytes:

AG = Na⁺ − (Cl⁻ + HCO₃⁻)

All three inputs are in milliequivalents per litre (mEq/L). Some labs and textbooks include potassium — AG = (Na⁺ + K⁺) − (Cl⁻ + HCO₃⁻) — which shifts the reference range up by roughly 4 mEq/L. This calculator uses the more common potassium-free version.

Worked example

Suppose a basic metabolic panel reports Na⁺ = 140, Cl⁻ = 104 and HCO₃⁻ = 24 mEq/L:

AG = 140 − (104 + 24) = 140 − 128 = 12 mEq/L

A value near 12 sits in the usual reference band. Now suppose the bicarbonate falls to 10 with chloride unchanged: AG = 140 − (104 + 10) = 26 mEq/L → a high anion gap, suggesting added acid.

Reference ranges

The common high-gap causes are often taught with the mnemonic GOLD MARK (glycols, oxoproline, lactate, D-lactate, methanol, aspirin, renal failure, ketoacidosis).

Pitfalls

• Albumin matters. Albumin is the dominant unmeasured anion, so low albumin lowers the gap; a corrected gap adds about 2.5 mEq/L for every 1 g/dL drop below 4 g/dL.
• Units must be mEq/L. Mixing mmol/L conventions or entering total CO₂ as something else skews the result.
• A “normal” gap is not reassuring on its own — a normal-gap (hyperchloraemic) acidosis still needs a cause.
• Lab variation. Reference ranges differ between analysers and between the K⁺-inclusive and K⁺-free formulas, so always read your own lab’s range.