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Background and Aims Circulating donor-derived cell-free DNA (dd-cfDNA) has recently been proposed as an early non-invasive marker for renal allograft rejection (Bloom et al, J. Am. Soc. Nephrol. 2017;28, 2221–2232). Recent studies demonstrated its potential role in the detection of any kind of rejection with better performance in detecting antibody-mediated rejection (ABMR) (Huang et al, Am J Transplant. 2019;19:1663–1670). However, at the current cutoff value for a positive test, the test seems to be affected by low sensitivity for T-cell mediated rejection (TCMR) and false positive results in the setting of non-rejection related damage. In the present study, the performance of dd-cfDNA is assessed on a retrospective series of transplant renal biopsies. Method Sixty-three renal biopsies from 52 allograft recipients with available dd-cfDNA results, evaluated between November 2017 and December 2019 at Vanderbilt University Medical Center, Nashville, Tennessee, were reviewed. Clinical and histological features along with the final diagnosis were recorded. Renal allograft rejection was classified following Banff 2013 criteria. Pearson’s chi-squared test was used to compare cases positive and negative for dd-cfDNA. Receiving operator curve (ROC) analysis was used to assess the performance of the test in detecting any rejection and ABMR. A p-value of &amp;lt;0.05 was considered statistically significant. Results The patients were 53±12.5 years old, 43% were men, 51% and 49% were Caucasian and African-American, respectively. The median value of dd-cfDNA was 2.6% for ABMR and 2.4% for any kind of rejection, statistically different from the group without rejection (0.56%, p = 0.002 and 0.0003, respectively). TCMR failed to demonstrate a statistically significant difference in dd-cfDNA levels vs controls (1.8% vs 0.56%, p = 0.627). The comparison of groups with positive and negative dd-cfDNA test is reported in Figure (panel A). Positive patients demonstrated a higher rate of findings associated with ABMR such as DSA positivity, moderate microcirculation inflammation, and transplant glomerulopathy, in addition to diagnostic ABMR and chronic/active ABMR. Moreover, a significant difference was found between the two groups in the rate of any kind of rejection. ROC analysis for any ABMR (Figure, panel B) demonstrated an AUC of 0.76 (95% CI 0.61-0.91) with a sensitivity (Sn) of 92.3%, a specificity (Sp) of 60%, a positive predictive value (PPV) of 37.5% and a negative predictive value (NPP) of 96.8%. ROC analysis for any kind of rejection (Figure, panel C) demonstrated an AUC of 0.84 (95% CI 0.74-0.94) with a Sn of 94.1%, a Sp of 65.2%, a PPV of 50% and a NPP of 96.8%. Among the patients with positive dd-cfDNA and without rejection (n=16, 51%), allograft biopsy demonstrated borderline TCMR in 4, acute tubular injury in 8, chronic pyelonephritis in 1, mild interstitial fibrosis in 1, and 2 with minimal histological abnormalities. Notably, none of the cases with a positive test were diagnosed with polyomavirus nephropathy. Conclusion Similar to previous studies, dd-cfDNA represents a sensitive test for detection of ABMR, with high performance in any rejection setting with the Banff 2013 criteria. However, the high rate of false positive results and the presence of undetected T-cell mediated events (e.g. borderline cases) at the current recommended cut-off value for positivity limits its employment as a surrogate for the renal biopsy.

P1626LIQUID BIOPSY IN RENAL TRANSPLANT: THE ROLE OF DONOR-DERIVED CELL-FREE DNA TO DETECT REJECTION