*Purpose: Donor-derived cell-free DNA (dd-cfDNA) is a non-invasive measure of early allograft injury that has been clinically validated as a surveillance tool to detect antibody and/or cellular mediated rejection (ABMR/TCMR). Prior retrospective studies have yielded conflicting data as to whether dd-cfDNA values correlate with the magnitude of BK polyomavirus (BKPyV) viremia and/or the presence of BKPyV associated nephropathy (BKAN) in kidney transplant recipients (KTRs).

*Methods: Eligible KTRs with either a kidney transplant in the last 6 months (control group) or incident BKPyV infection (case group) were enrolled into this prospective study; those in the control group with later incident BKPyV infection were moved to the case group. All study participants underwent serial monitoring with serum BKPyV qPCR and dd-cfDNA (Allosure) assays. For this analysis, only those participants with >=3 dd-cfDNA results and without clinically evident ABMR/TCMR were included.

*Results: 49 KTRs were enrolled in the study, 39 are included in this analysis (9 excluded due to <3 dd-cfDNA results, 1 excluded due to TCMR). BKPyV infection was detected in 6 participants, 3/6 had BKPyV viremia of >10,000 cpy/mL, one of whom also had a biopsy confirming BKAN. Only one participant with BKPyV infection (peak viremia 3,000 cpy/mL) had dd-cfDNA >1%, although the timing of their BKPyV infection did not correspond to any increase in dd-cfDNA (elevated dd-cfDNA was instead correlated with culture/biopsy confirmed pyelonephritis). One additional participant with BKPyV infection had dd-cfDNA >0.5%, but the increased dd-cfDNA result again did not correspond to the timing of BKPyV infection. The three participants with BKPyV viremia of >10,000 cpy/mL did have a relative increase in dd-cfDNA as compared to their previous baseline which was temporally related to their BKPyV infection (144%, 238%, 333%), however, these relative increases were not significantly different from the (maximum) relative increase from baseline for participants in the control group without BKPyV infection (median 190%, IQR 115%).

*Conclusions: Among this group of KTRs, those with BKPyV infection had neither a corresponding elevated absolute dd-cfDNA value nor a corresponding relative increase in dd-cfDNA value significantly different from increases from baseline measured in the control group. This may suggest a potential role for dd-cfDNA to instead specifically detect concurrent or subsequent rejection associated with BKPyV infection and/or treatment (reduction in immunosuppression) for this infection.

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