Anion Gap, Plasma

Diagnostic Use

The anion gap is a calculated parameter that gives information about the number of unmeasured anions in the plasma.

Anion gap = Na+ + K+ - Cl- - HCO3-

The principle of electroneutrality states that the number of positively charged ions (cations) and the number of negatively charged ions (anions) in plasma must be equal. Measured cations include sodium and potassium, whereas measured anions include chloride and bicarbonate.

Under normal circumstances, subtracting the total number of measured anions from the total number of measured cations results in a "gap" of approximately 10-20mmol/L (accounted for by anions such as albumin, sulphates and phosphates).

The anion gap increases in metabolic acidosis occuring due to an increase in the amount of organic acid present, whereas if there is a metabolic acidosis due to loss of bicarbonate, the anion gap would be expected to remain in the normal range (the lost bicarbonate is replaced by retained chloride). Because of this principle, we can measure anion gap in patients with a metabolic acidosis, to assist in determining the underlying cause.


Core Biochemistry

Delphic Registration Code



AG Calculation

Turnaround Time

3 hours

Test Code