Donor-derived cell-free DNA in solid organ transplantation: analytical considerations, diagnostic performance, and clinical interpretation.

Donor-derived cell-free DNA (dd-cfDNA) has emerged as a valuable noninvasive biomarker for detecting allograft injury in solid organ transplantation. It is released into the bloodstream from the transplanted organ as a result of cell injury and immune activation, with baseline levels influenced by organ type, tissue turnover, and posttransplant physiological changes. Several analytical platforms are available, including quantitative polymerase chain reaction (PCR), digital droplet PCR, and next-generation sequencing, each differing in sensitivity, throughput, and reporting format. Commercial assays have been clinically validated across multiple organs. dd-cfDNA can be quantified as a percentage of total cell-free DNA or as an absolute concentration, with diagnostic thresholds varying by platform and organ type. Although dd-cfDNA demonstrates high negative predictive value and can reduce the need for unnecessary biopsies, it is not specific to rejection and may be elevated in the setting of infection, ischemia, or inflammation. Preanalytical and technical factors can also affect test performance. Therefore, dd-cfDNA should be interpreted with careful consideration of biological variation, assay characteristics, and the patient's clinical context. Future efforts should focus on defining organ-specific thresholds, improving interlaboratory standardization, expanding validation in underrepresented graft types, and assessing cost-effectiveness and clinical impact to support broader clinical adoption.