ANION GAP - plasma or serum

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Specimen: 5 mL blood in lithium heparin or plain tube.

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

Some laboratories do not include K in the equation.

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

Application: Investigation of aetiology of metabolic acidosis.

Interpretation: An increased anion gap is seen in metabolic acidosis due to 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.

Reference: Emmett ME and Narins RG. Medicine 1977; 56: 38-54.

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