C3d Assay in Correlation With Single Antigen Bead Assay for Human Leukocyte Antigen Antibodies

Background. Single antigen bead assay (SAB) is a sensitive method for detecting HLA antibodies, but it does not specifically identify clinically relevant subsets. Recently, a new assay has been developed for detection of C3d bound to HLA antibody-antigen complex. We evaluated the C3d assay regarding its correlation with SAB in renal patients. Methods. A total of 138 serum samples from 109 sensitized patients were tested in parallel by SAB and C3d assay for detection of HLA class I antibodies. The relationship between C3d assay and SAB was analyzed for the numbers and median fluorescent intensity (MFI) values of the identified antibodies. Results. Of the 138 samples, 137 were positive on SAB; of the 137 SAB-positive samples, 76 were positive on C3d assay. A total of 3748 and 685 antibodies were identified by the SAB and the C3d assay, respectively. The maximal MFI values of the SAB in the 76 samples that were C3d assay-positive were significantly higher than those of the 61 samples that were C3d assay-negative (P < .05), with the median values of 17,057 and 6066, respectively. Only 11 (0.4%) of the 2905 antibodies with MFI < 10,000 on SAB vs 501 (59.4%) of the 843 antibodies with MFI > 10,000 on SAB were identified by C3d assay with MFI > 1000. Conclusions. The C3d assay positivity seems to be dependent on its MFI value on SAB. Further studies are needed to ascertain the clinical significance of C3d positivity by itself.