Anion gap - Investigations and procedures

Anion gap

Anion gap calculation is useful in metabolic acidosis, to differentiate causes. It is a measure of the unestimated anions (phosphate, ketones, lactate) and cations. The formula used to calculate the anion gap varies from source to source, and the normal range depends on the methods by which the laboratory measures each electrolyte. For example:

In metabolic acidosis, an increased anion gap occurs due to raised acid levels:

·        Lactic acidosis (exercise, shock, hypoxia, liver failure).

·        Acid ingestion (aspirin, ethylene glycol).

·        Ketoacidosis.

·        Uraemia.

In metabolic acidosis, a normal anion gap indicates that there is failure to excrete acid or loss of base:

·        Failure to excrete acid occurs in renal tubular disease and Addison’s disease.

·        Loss of base (bicarbonate) occurs in diarrhoea, proximal renal tubular acidosis, or iatrogenically, due to carbonic anhydrase inhibitor or ureterosigmoidostomy.