Anion Gap (Blood Gas)

Diagnostic Use

Calculation: Anion Gap = (Na + K) – (Cl + [HCO3])

Albumin Corrected Anion Gap = Anion Gap + [0.25 x (40 – albumin)]

The anion gap is a mathematical calculation of the difference between the measured cations and anions, as shown in the above equation.

It is recommended that the anion gap is monitored using the same platform/device. This is due to the small differences in electrolyte measurement between the blood gas analyser and the main laboratory analyser.

The anion gap is increased in metabolic acidosis due to ketones, lactate, and other organic acids. As well as in poisoning by ethylene glycol, paraldehyde, methanol, and salicylate.

The anion gap is normal in renal tubular acidosis and in acidosis due to bicarbonate loss (e.g. diarrhoea).

The anion gap may be decreased in dilutional states, with some paraproteins, hypoalbuminaemia and where there is a marked increase in unmeasured cations (e.g. lithium, magnesium, calcium, etc.).