The anion gap is a concept that helps to understand metabolic acidosis produced by adding non-volatile acids to the body fluid.

In normal condition the neutrality condition in the body fluids is stated as follow:

[Na+] + [Others Cations] = [Cl-] + [HCO3-] + [Other Anions]

That equation implies that all the cations (positive charges) are in balance whit all the anions (negative charges). In normal conditions,

The anion gap = AG = [Na+] – ([Cl-] + [HCO3-]) is about 8 to 16 mEq/L.

If non-volatile acids are added to the body fluids the pH will decrease and the [HCO3-] will also decrease. If the major acid that has been added is not containing as the principal anion Cl- the anion gap will increase. The calculation of the anion gap helps to identify the cause of metabolic acidosis. The anion gap is increased in metabolic acidosis associated with renal failure, diabetes -ketoacidosis-, lactic acidosis and the ingestion of large amount of anions like aspirin.