Absolute and Fractional Donor‐Derived Cell‐Free <scp>DNA</scp> in the Follow‐Up of Kidney Allograft Rejection

Donor-derived cell-free DNA (dd-cfDNA) is an emerging tool for noninvasive kidney allograft pathology diagnostics. However, its utility in anti-rejection treatment follow-up remains to be established. We measured dd-cfDNA in 41 kidney transplant recipients (KTRs) at biopsy demonstrating rejection and then weekly for 3 weeks (155 samples). Biopsies were assessed by a pathologist (Banff 2022) and by Molecular Microscope Diagnostic System (MMDx), all KTRs received antirejection therapy. We found that dd-cfDNA fraction decreased in most KTRs after treatment. However, absolute dd-cfDNA decreased significantly only in those treated for T cell-mediated rejection (77 copies/mL [cp/mL] at biopsy, 35 cp/mL at week 3), while there was no significant decrease (93 cp/mL at biopsy, 73 cp/mL at week 3) in patients with antibody-mediated rejection (ABMR). Furthermore, we found that absolute dd-cfDNA did not decrease in KTRs treated with plasma exchange. Finally, there was an unanticipated increase of total cell-free DNA (cfDNA) in all KTRs after treatment. In conclusion, dd-cfDNA follow-up assessments may be confounded by treatment-induced increase of total cfDNA. Thus, absolute dd-cfDNA concentrations, in addition to fraction, might provide more complete information about the response to anti-rejection treatment.