Anion gap

Electrolytes · also called AG

What it measures

The anion gap is a calculated value representing unmeasured anions in plasma. It helps sort metabolic acidosis into high-gap causes (added acids) and normal-gap causes (bicarbonate loss).

Specimen and preparation

Specimen: Serum or lithium-heparin plasma
Tube: Gold top

Reference range

Group	Range
Adult (without K⁺)	8–16 mmol/L
Adult (with K⁺)	12–20 mmol/L

Reference ranges differ between laboratories, methods and populations. Always use the range printed on your own lab report.

Causes of high values

Lactic acidosis
Ketoacidosis (diabetic, alcoholic, starvation)
Kidney failure
Toxic alcohols (methanol, ethylene glycol)
Salicylate poisoning

Causes of low values

Hypoalbuminaemia
Multiple myeloma (cationic paraproteins)
Lithium or bromide toxicity
Laboratory error

Method

Calculated: Na⁺ − (Cl⁻ + HCO₃⁻)

Good to know

Each 1 g/dL fall in albumin lowers the gap by about 2.5 mmol/L; correct before interpreting. Local ranges depend on the analyser.

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