Definition (1): organisms exist in a state of electroneutrality with major (sodium) and minor cations (potassium, calcium, magnesium) balanced by similar anions (major: chloride, bicarbonate; minor: phosphates, sulphates, organic anions such as proteins). The anion gap is an artificial disparity between the concentrations of the major plasma cations and anions routinely measured.

(Patho)physiology (1): a decreased anion gap is usually caused by hypoalbuminaemia or severe haemodilution. Less commonly, it occurs as a result of an increase in minor cation concentrations such as in hypercalcaemia or hypermagnesaemia. Increased anion gap acidosis is caused by dehydration and by any process that increases the concentrations of the minor anions, lactate, ketones, and renal acids, along with treatment with drugs given as organic salts such as penicillin, salicylates, and with methanol and ethylene glycol. Rarely an increased anion gap may result from decreased minor cation concentrations such as calcium and magnesium.