Molecular Diagnostics Supporting a ≥35% Diffuse Peritubular Capillaritis Extent Threshold for Diagnosis of AMR—A Retrospective Dual Center Study

Peritubular capillaritis (ptc) is a hallmark lesion of antibody-mediated rejection (AMR), but the grading of its extent is historically based on arbitrary defined cut-offs. Molecular AMR diagnosis via intragraft gene expression measurements may provide evidence to challenge established ptc categories. We retrospectively included 38 renal allograft biopsies from clinical routine, performed because of suspicion of AMR. Biopsies were re-assessed by an experienced nephropathologist and intragraft gene expression was measured using the NanoString nCounter^® platform. Ptc categories were correlated with AMR gene expression to identify a ptc extent cut-off with optimal prediction of molecular diagnosis of AMR [gene expression levels above first quartile (AMR^Q>1)]. Finally, an independent validation cohort (<i>n</i> = 25, Erasmus MC, Rotterdam, The Netherlands) was included to reproduce the results. Re-assessment of biopsies revealed AMR in 26/68.4%, mixed rejection in 5/13.2%, and T-cell-mediated rejection in 3/7.9%. Biopsies with diffuse ptc had significantly higher AMR gene expression compared to biopsies with focal ptc and biopsies with no ptc (64.0/53.3-84.0 vs. 31.5/27.0-49.5, <i>p</i> = 0.023 and 27.0/14.3-31.8, <i>p</i> = 0.003, median/IQR). Sensitivity analysis revealed that a ≥35% ptc cut-off resulted in higher AUCs for predicting AMR^Q>1 compared to ptc^50% (AUC 0.78, 95% CI: 0.63-0.93, <i>p</i> = 0.009 versus AUC: 0.74, CI: 0.56-0.90, <i>p</i> = 0.03). In the validation cohort, only the ptc^35-, but not the ptc^50%, cut-off significantly predicted AMR^Q>1 (AUC 0.75, 95% CI: 0.54-0.96 <i>p</i> = 0.04 vs. AUC 0.69, CI: 0.46-0.93, <i>p</i> = 0.13). Using intragraft gene expression measurement, we identified a new ptc extent threshold with better prediction of molecular AMR. The newly proposed cut-off of ≥35% could potentially improve diagnostic evaluation and prognostication in cases with suspected or diagnosed AMR.