Anion gap
Classification of metabolic acidosis
Sources last checked: 2026-10-13
Anion gap
Classification of metabolic acidosis
Potassium is not included here. The 12 mmol/L cut-off is conventional — reference ranges differ between laboratories and analysers.
Interpretation
| Normal / low | ≤ 12 mmol/L — in metabolic acidosis, a gap in this range fits hyperchloraemic (normal-gap) acidosis. |
|---|---|
| High | > 12 mmol/L — high-anion-gap acidosis (lactate, ketones, renal failure, toxins). |
What is the Anion gap?
The anion gap is the difference between measured cations and anions in serum — sodium minus chloride and bicarbonate — and reflects unmeasured anions such as albumin, lactate and ketoacids. It is used to classify metabolic acidosis into high-gap and normal-gap (hyperchloraemic) forms. Because albumin is the main unmeasured anion, Figge and colleagues (1998) showed that each 1 g/dL fall in albumin lowers the gap by about 2.5 mmol/L and proposed a correction.
When to use it
- Any metabolic acidosis (low bicarbonate, low pH), to separate high-gap from normal-gap causes.
- Suspected toxic ingestion (methanol, ethylene glycol, salicylates), diabetic or alcoholic ketoacidosis, or lactic acidosis.
- Critically ill or hypoalbuminaemic patients, in whom an uncorrected gap can hide a high-gap acidosis.
When not to use it
- Without a matching blood gas or clinical context — an isolated gap value does not diagnose an acid-base disorder.
- When sodium, chloride and bicarbonate come from different samples or analysers.
How it is calculated
AG = Na⁺ − (Cl⁻ + HCO₃⁻) Albumin-corrected AG = AG + 2.5 × (4.0 − albumin in g/dL) = AG + 0.25 × (40 − albumin in g/L)
Clinical pearls and limitations
- Potassium is not included here. The 12 mmol/L cut-off is conventional — reference ranges differ between laboratories and analysers.
- This calculator uses 4.0 g/dL (40 g/L) as normal albumin; Figge's correction uses the laboratory's own normal value, so results can differ by about 1 mmol/L.
- European laboratories report albumin in g/L: divide by 10 for g/dL.
- The upper limit of normal depends on the analyser — with modern ion-selective electrodes many laboratories quote roughly 3–11 mmol/L without potassium. Use local ranges.
- In high-gap acidosis compare the rise in gap with the fall in bicarbonate (delta ratio) to detect a coexisting normal-gap acidosis or metabolic alkalosis.
- A low or negative gap suggests laboratory error, hypoalbuminaemia, paraproteinaemia, or bromide or lithium intoxication.
Frequently asked questions
What is a normal anion gap?
Conventionally about 8–12 mmol/L without potassium, but modern analysers give lower values and ranges vary between laboratories. Interpret against your local reference range and the albumin level.
How do I correct the anion gap for albumin?
Add 2.5 mmol/L for every 1 g/dL (10 g/L) that albumin is below normal: corrected AG = AG + 2.5 × (4.0 − albumin in g/dL), or AG + 0.25 × (40 − albumin in g/L).
What causes a high anion gap metabolic acidosis?
Lactate, ketoacids (diabetic, alcoholic, starvation), advanced kidney failure, and toxins such as methanol, ethylene glycol and salicylates. Calculating the osmolar gap helps when toxic alcohols are suspected.
Evidence and guidelines
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