A sensitive immunoradiometric assay (IRMA) was used to quantitate the amount of red blood cell-associated IgG (RBC-IgG). The mean RBC-IgG of 91 healthy volunteers was 33±13 (SD) molecules per one RBC. The correlation between the agglutination strength in antiglobulin test and the number of IgG were very satisfactry. We also tested the amount of RBC-IgG of 27 patients with immune hemolytic anemia of warm type (AIHA). (AIHA, n=17; RBCs with positive antiglobulin test, n=2; RBCs with negative antiglobulin test, n=8; eluates from patients' RBCs). The mean RBC-IgG of 17 patients with antiglobulin test positive AIHA was 998±930 (SD) molecules per one RBC. It was ranged from 257 to 3000 molecules per one RBC. Two patients with antiglobulin test negative AIHA (7.4%, 2/27) had 138 and 172 IgG molecules per one RBC, respectively. The indirect antiglobulin tests of RBCs sensitized with 8 eluates showed positive. The RBC-IgG of sensitized RBCs was higher than that of RBCs from healthy volunteers. Using this assay, quantitative studies of RBC-IgG was extremely useful for the detection of small amount of IgG on RBC in case of the antiglobulin test negative AIHA.
