Anion Gap Calculator
Compute the serum anion gap from sodium, chloride and bicarbonate, with an albumin-corrected value to screen for metabolic acidosis.
How it works
Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻) · Albumin-corrected AG = AG + 2.5 × (4.0 − albumin g/dL)
The anion gap is the difference between measured cations and anions in serum, and it stands in for the unmeasured anions (mostly albumin, phosphate and organic acids). A raised gap points toward a high-anion-gap metabolic acidosis — the classic causes are captured by mnemonics like GOLD MARK (glycols, oxoproline, lactate, D-lactate, methanol, aspirin, renal failure, ketoacidosis). Because albumin is the dominant unmeasured anion, a low albumin lowers the gap and can hide an acidosis, which is why the albumin-corrected value adds about 2.5 mmol/L for every 1 g/dL that albumin falls below 4.0.
Worked example
With sodium 140, chloride 104 and bicarbonate 24 mmol/L, the anion gap = 140 − (104 + 24) = 12 mmol/L, at the upper edge of normal. If albumin were 2.0 g/dL, the corrected gap would be 12 + 2.5 × (4.0 − 2.0) = 17 mmol/L, revealing a hidden acidosis.
Sources: Anion gap
Frequently asked questions
What is a normal anion gap?
With modern ion-selective analysers the usual reference is roughly 8–12 mmol/L (some labs cite 3–11). Values above about 12 suggest a high-anion-gap metabolic acidosis and warrant a search for the cause.
Should potassium be included?
The simple, most common formula omits potassium. Some versions add K⁺ to the cations, which shifts the normal range up by about 4 mmol/L. This calculator uses the sodium-only convention.
Why correct for albumin?
Albumin is the largest unmeasured anion, so hypoalbuminaemia lowers the calculated gap and can mask a true acidosis. The correction adds about 2.5 mmol/L per 1 g/dL of albumin below the 4.0 g/dL reference.
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Educational estimate using standard published formulas — not medical advice, a diagnosis, or a substitute for clinical judgement. Dosing, diagnosis and treatment must be decided by a qualified clinician using validated tools and the full clinical picture.