*Purpose: Donor derived cell-free DNA (dd cf-DNA; Allosure, CareDx) is increasingly employed for non-invasive kidney transplant monitoring. The goal of this study was 1) to examine the association between dd cf-DNA and for-cause biopsy findings and 2) describe changes in dd cf-DNA and creatinine following rejection treatment.

*Methods: We included patients who 1) received a kidney transplant between May 2017 and July 2020 at our center, 2) underwent for-cause kidney transplant biopsy, 3) had ≥ 1 dd cf-DNA prior to biopsy and 4) had ≥ 2 dd cf-DNA levels measured during the entire study period. Patient demographic and clinical data were abstracted from medical records.

*Results: 41 patients were stratified into 4 groups based on their most recent pre-biopsy dd cf-DNA: >1.0% (Group 1), 0.50-0.99% (Group 2), 0.21-0.49% (Group 3), and <0.21% (Group 4). Baseline characteristics are shown in table 1. The prevalence of rejection was greatest in the highest dd cf-DNA stratum (Group 1) and decreased across the 4 dd cf-DNA strata (table 2). Severe rejection (per Banff Criteria) was seen more frequently in Group 1. Only 1 patient (out of 11) in the lowest dd cf-DNA stratum (Group 4) had rejection and that was a ‘borderline’ T cell mediated rejection diagnosed on a background of prominent tubular injury and delayed graft function. Nearly all patients had a decrease in dd cf-DNA after treatment of rejection. Post-rejection decrease in creatinine (measured at the same time as dd cf-DNA) was less pronounced (figure 1).

*Conclusions: Our study suggests that kidney transplant recipients with low dd cf-DNA (<0.21%) who undergo for-cause biopsy are unlikely to have rejection. Monitoring of dd cf-DNA may prove useful in assessing response to rejection treatment.

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