Positive direct antiglobulin test and high serum immunoglobulin G values

To investigate the association between the positive direct antiglobulin test (DAT) and hypergammaglobulinemia, the authors prospectively studied 154 patients, classified into three groups: Group 1, 52 patients with a positive DAT result in pretransfusion samples; Group 2, 52 patients with a negative DAT result; and Group 3, 50 patients initially found to have an elevated serum immunoglobulin G (IgG) level. Serum protein electrophoreses and IgG quantifications were performed for all three groups. Serum haptoglobin and lactate dehydrogenase (LD) isoenzyme electrophoreses were also assayed for Group 1. Of 52 patients in Group 1, 17 (33%) had an elevated serum IgG level and nonreactive eluates. Clinical history, haptoglobin, and LD isoenzyme studies did not suggest increased red blood cell destruction. Only 2 (4%) of 52 patients in Group 2 had an elevated serum IgG level. Of 50 in Group 3, 25 (50%) had a positive DAT result with nonreactive eluates and did not have hemolytic diseases. Two of 10 patients (20%) with serum IgG levels ranging from 18 to 20 g/L (1.8-2.0 g/dL), 13 of 29 (45%) with serum IgG levels from 20 to 40 g/L (2.0-4.0 g/dL), 4 of 6 (67%) with serum IgG levels from 40 to 60 g/L (4.0-6.0 g/dL), and 6 of 6 (100%) with serum IgG levels from 60 to 80 g/L (6.0-8.0 g/dL) had a positive DAT result. The authors concluded there is a significant correlation between a positive DAT result and serum IgG concentrations and that the higher the elevated serum IgG, the more frequently the positive DAT result is observed. Elevated serum IgG levels may explain many positive DAT results in pretransfusion blood samples.