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Serum Anion Gap (with Albumin Correction)

Anion gap = Na − (Cl + HCO₃). A typical normal value is about 8–12 mmol/L (lab-dependent, often lower with modern analysers); higher suggests a high-anion-gap metabolic acidosis. Each 1 g/dL (10 g/L) fall in albumin lowers the gap by about 2.5, so correct for hypoalbuminaemia.

Serum sodium (mmol/L or mEq/L)
Serum chloride (mmol/L or mEq/L)
Serum bicarbonate (HCO₃⁻) (mmol/L or mEq/L)
Serum albumin (g/dL or g/L)

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Interpretation

Result	Meaning
< 3	Low
3–12	Normal (typical reference ~8–12; check local range)
> 12	Elevated: high anion gap

Formula

AG = Na⁺ − (Cl⁻ + HCO₃⁻). Albumin-corrected AG = AG + 2.5 × (4.0 − albumin g/dL) = AG + 0.25 × (40 − albumin g/L).

Clinical pearls

Use the local laboratory reference range: ion-selective electrode analysers often give normal gaps of 3–11.
Hypoalbuminaemia is common in hospital and can hide a significant gap acidosis unless corrected.
Potassium is omitted in the standard North American formula; adding it raises the normal range by ~4.

Examples

anion gap na 140 cl 100 hco3 10 ag 138 cl 102 bicarb 18 albumin 22 g/L what is the anion gap if sodium is 135, chloride 98 and bicarbonate 12

FAQ

What is a normal anion gap?

Traditionally 8–12 mmol/L (about 12 ± 4); many modern analysers report a lower normal range, so use your laboratory's reference interval.

How do you correct the anion gap for albumin?

Add 2.5 for every 1 g/dL that albumin is below 4.0 g/dL (0.25 per g/L below 40 g/L).

Evidence

Nephrology & Electrolytes · Reviewed 2026-10-14 · Methodology