Anion gap

Keywords: A Gap

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

Application:

Investigation of aetiology of metabolic acidosis.

Interpretation:

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.

Reference:

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