Comparison of C4d Detection on Erythrocytes and PTC‐C4d to Histological Signs of Antibody‐Mediated Rejection in Kidney Transplantation

C4d on erythrocytes (EC4d), C4d peritubular capillary deposition (PTC‐C4d) staining and histology were compared in a cross‐sectional cohort of 146 renal allograft biopsies (132 patients). EC4d levels paralleled PTC‐C4d staining, but were more predictive of peritubular capillaritis (PTC). Donor‐specific antibodies (DSA), PTC‐C4d, EC4d and PTC were analyzed in an independent longitudinal follow‐up cohort (96 biopsies, 76 patients). Seventy‐six samples were PTC and EC4d concordant, 11 positive and 65 negative, 7 PTC‐EC4d+ and 13 PTC+EC4d?. EC4d levels were related to DSA occurrence. With ABMR defined by PTC and DSA, all apparently discordant patients, EC4d negative, were correctly reassigned comparing EC4d level curves with rejection kinetics, with positive EC4d samples predating biopsy or late biopsies compared with ABMR flare‐ups. All EC4d‐positive patients without PTC or DSA had permanent high EC4d levels unrelated to rejection. EC4d was more abundant in PTC‐positive (mean = 108.5%± 3.4; n = 50) than PTC‐negative samples (mean = 88.1%± 1.3; n= 96; p < 0.0001). Sensitivity, specificity, positive predictive value and negative predictive value of PTC‐C4d and EC4d for PTC were, respectively, 75%, 79%; 64%, 76% (p < 0.05); 28%, 46% (p < 0.05) and 93%, 94%. Values were similar for DSA. A noninvasive blood test, EC4d, and particularly longitudinally monitoring EC4d levels, may increase surrogate ABMR testing options.