Diagnostic Importance of an Increased Serum Anion Gap

Abstract

Fifty-seven hospitalized patients with increased serum anion gaps, defined as [Na] − ([Cl] + [HCO₃]), were studied to explore the biochemical basis and the diagnostic importance of the anion gap. We found that an anion gap greater than 30 meq per liter was usually due to an identifiable organic acidosis (lactic acidosis or ketoacidosis). However, 10 of 35 patients (29 per cent) with anion gaps of 20 to 29 meq per liter did not have lactic acidosis or ketoacidosis. In 22 patients, the composition of the anion gap was studied in detail. Lactate and ketoanions accounted for 62 per cent of the increments in anion gaps, and changes in the equivalents of total proteins, phosphorus, potassium, and calcium accounted for 15 per cent. However, nine patients had an increment in anion gap of 5.5 meq per liter or more (average, 8.7 meq per liter) that was not explained by any of the factors measured here. The possibility of unidentified anions or unrecognized changes in the ionic equivalents of normal plasma constituents requires further consideration in such cases. (N Engl J Med. 1980; 303:854–8.)