Background: Donor-derived cell-free DNA (dd-cfDNA) may reliably detect allograft rejection in pediatric kidney transplant (KT) recipients. This study evaluates the utility of dd-cfDNA for monitoring treatment response amongst pediatric KT recipients suffering graft rejection. 
Methods: Fifty-eight pediatric KT recipients were enrolled between April 2018 and March 2020 to undergo dd-cfDNA testing to monitor for rejection. Patients with dd-cfDNA scores >1.0% and biopsy-proven rejection formed the study cohort. Treatment for rejection consisted of intravenous immunoglobulin and rituximab, with or without pulse steroids or thymoglobulin. Results of dd-cfDNA, serum creatinine (SCr), biopsy results, and treatment and outcomes were evaluated. Standard statistical analyses were applied. 
Results: Nineteen of 58 (31%) patients had dd-cfDNA score >1.0%, of which 18 (94.7%) were found to have rejection on biopsy. Mean dd-cfDNA value was 2.57±1.74% and biopsy results showed 11 patients (61.1%) with antibody mediated rejection (ABMR), 2 patients (11.1%) with T-cell mediated rejection (TCMR) and 5 patients (27.7%) with Mixed ABMR/TCMR. SCr at time of biopsy was 1.28±1.09 mg/dL. Following treatment, dd-cfDNA scores decreased for all types of rejection but remained >1.0% in both ABMR (1.97±1.13%) and Mixed (2.32±2.03%) groups. Mean dd-cfDNA value was lower for patients with TCMR (0.28±0.10%). SCr showed minimal change between pre- and post-treatment levels, regardless of rejection subtype. 
Conclusions: Measurement of dd-cfDNA represents a potential method for early detection of rejection in pediatric KT recipients. Patients with TCMR may be reliably followed by dd-cfDNA; however, it remains unclear whether persistently elevated dd-cfDNA levels in patients with ABMR reflects ongoing subclinical rejection or an inherent limitation of the assay.