Abstract 4144583: Beyond Acute Rejection Screening Following Pediatric Heart Transplant: In Patients Negative for Rejection, Elevated Donor-Derived Cell-Free DNA is Associated with Cardiac Allograft Vasculopathy (CAV) and Donor Specific Antibodies (D

Donor-derived cell-free DNA (dd-cfDNA) has been increasingly used to detect acute rejection (AR). We aimed to compare our institutional dd-cfDNA results to previously published adult and pediatric dd-cfDNA AR cutoffs. We also hypothesized that in the absence of AR, elevated dd-cfDNA would be associated with CAV and positive DSA. Patients (pt) < 18 years at transplant with > 1 dd-cfDNA between 2021-2023 were included. Using dd-cfDNA levels from this cohort, sensitivity, specificity, NPV, and PPV were calculated. False positives and false negatives (FN) were determined using published dd-cfDNA thresholds. AR was defined as decision-to-treat with increased immunosuppression, which was independent of dd-cfDNA in our cohort. In pt without AR, t -test was used to compare the means of dd-cfDNA levels in pt with and without DSA. χ 2 testing was then performed to evaluate the association between dd-cfDNA levels above and below 0.2% and the presence/absence of DSA and CAV. DSA was defined as allele-specific DSA identified by single antigen bead with mean fluorescence intensity >1000, and CAV as any disease by angiography. There were 379 samples among 163 pt, a median of 2 samples per pt, and 32 samples obtained at time of AR. Performance of dd-cfDNA in our cohort vs published dd-cfDNA thresholds is shown in Table 1. The FN rate ranged from 16 to 37% as the dd-cfDNA threshold increased. Mean dd-cfDNA was higher in patients with positive DSA versus those without (0.83% vs 0.19%, p<0.001). In patients that did not meet AR criteria, dd-cfDNA levels > 0.2% were associated with a higher prevalence of positive DSA (n=66) (48% vs 13%, p<0.001), and trended towards a higher prevalence of CAV (n=20) (15% vs 11%, p=0.08). Dd-cfDNA had high sensitivity and NPV for AR but the FN rate using published dd-cfDNA cutoffs was high in our validation cohort. The clinical utility of dd-cfDNA may extend beyond AR screening given increased frequency of DSA and CAV in pt with elevated levels that are negative for AR. We speculate that monitoring intra-pt dd-cfDNA variability will reduce the FN rate when screening for AR in addition to enhancing its clinical utility when monitoring for CAV and DSA.