Anion gap - plasma or serum

Last updated: Wednesday, 07, April, 2004

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Item Process
Specimen5 mL blood in lithium heparin or plain tube.

Calculated parameter: Anion gap = (Na + K) - (Cl + HCO3).

Some laboratories do not include K in the equation.

Reference Interval

8-16 mmol/L (4-13 mmol/L if K not included).


Investigation of aetiology of metabolic acidosis.


An increased anion gap is seen in metabolic acidosis due to unmeasured anions that accumulate in renal failure; ketoacidosis; lactic acidosis; drug (eg, salicylate), ethylene, glycol and methanol toxicity; and in any situation where increased accumulation of acidic anions occurs.

Metabolic acidosis with a normal anion gap is seen in renal tubular acidosis and in other causes of bicarbonate loss eg, diarrhoea.


Sirker AA et al. Anaesthesia 2002; 57(4): 348-356.