Comparison Between Total IgG, C1q, and C3d Single Antigen Bead Assays in Detecting Class I Complement-Binding Anti-HLA Antibodies

Background Complement-binding donor-specific antibodies (DSAs) are associated with antibody-mediated rejection and allograft loss. Novel single antigen bead (SAB) assays—that is, complement component 1q (C1q) and complement component 3d (C3d) assays—have been developed to specifically detect complement-binding DSA, but it remains unclear whether these assays have an improved ability to detect complement-binding DSA as compared with using the total IgG SAB assay with a high mean fluorescence intensity (MFI) cutoff. The aim of this study was to compare the ability of the total IgG, C1q, and C3d SAB assays in detecting complement-binding anti-HLA antibodies. Methods Twenty sera known to have complement-binding anti-HLA antibodies (serologic class I HLA typing by complement-dependent cytotoxicity method) were tested with 3 different SAB assays: total IgG (undiluted and 1:8 dilution), C1q, and C3d. Serologic anti-HLA specificities were compared with those obtained by IgG, C1q, and C3d SAB assays. Results IgG SAB was more sensitive in detecting complement-binding antibodies (sensitivity 24 of 24 = 1, odds ratio infinity). Pearson correlation showed the association between (1) C1q and IgG SAB assays (cutoff C1q SAB 1000 MFI, cutoff IgG SAB 5000 MFI: r = 0.347, P <.0001) and (2) C3d and IgG SAB assays (cutoff 500 MFI C3d SAB, 5000 MFI for IgG SAB: r = −0.173, P =.279). Conclusions For class I anti-HLA antibodies, IgG SAB (cutoff MFI > 5000) was more sensitive in detecting complement-binding antibodies when compared with C1q and C3d SAB assays.