Changes in the plasma anion gap during chronic metabolic acid-base disturbances

A basic premise in the utilization of the plasma anion gap in the assessment of acid-base disorders is that this parameter remains constant during hyperchloremic metabolic acidosis and metabolic alkalosis. Experimental data under in vitro conditions, however, cast serious doubt on this premise. The purpose of the present study was to characterize the plasma anion gap, estimated as (Na + K) - Cl + HCO3), in two large groups of dogs with graded degrees of chronic, HCl-induced metabolic acidosis or chronic, diuretic-induced metabolic alkalosis. The data indicate that the plasma anion gap decreases significantly in HCl acidosis and increases significantly in metabolic alkalosis; the predicted mean anion gap in animals with a plasma bicarbonate concentration of 10, 21 (normal), and 40 meq/liter approximated 13, 18, and 26 meq/liter, respectively. The observed variation in the plasma anion gap is interpreted as originating mainly from directional changes in the net negative charge of plasma proteins; these changes result from the titration process secondary to the altered plasma acidity and, in the case of metabolic alkalosis, from the additional effect of an increased plasma protein concentration.