Donor-derived cell-free DNA (dd-cfDNA) is an emerging non-invasive biomarker of kidney allograft injury. Elevated dd-cfDNA has been associated with acute rejection, particularly antibody-mediated rejection; however, dd-cfDNA reflects graft-derived tissue injury rather than a specific alloimmune mechanism. Its interpretation may therefore be challenging in conditions characterized by endothelial and microvascular injury, such as atypical hemolytic uremic syndrome (aHUS). This retrospective observational study included 14 kidney transplant recipients who underwent their first kidney transplantation at Wroclaw Medical University between April and December 2022. Twelve non-aHUS recipients with stable renal function and two recipients with aHUS were assessed. Plasma samples were collected at four early post-transplant time points: day 7 (D7), day 14 (D14), month 1 (M1), and month 2 (M2). dd-cfDNA was measured using next-generation sequencing with the AlloSeq cfDNA assay. Donor-specific anti-HLA antibodies (DSA) were assessed using single-antigen bead testing. In non-aHUS recipients, median %dd-cfDNA declined from 0.72% [0.28%, 2.33%] at D7 to 0.12% [0.05%, 0.34%] by M1. In contrast, recipients with aHUS demonstrated persistently elevated dd-cfDNA values fluctuating around 1% [0.88%, 1.14%] during the first month of follow-up. No DSA were detected in any patient during the observation period. No apparent correlation was observed between %dd-cfDNA and serum creatinine in this small cohort. In this preliminary observational study, kidney transplant recipients with aHUS showed persistently elevated early post-transplant dd-cfDNA despite the absence of detectable DSA, absence of clinically documented rejection, and preserved graft function during follow-up. These findings suggest that universal dd-cfDNA thresholds may require cautious interpretation in aHUS. Larger biopsy-anchored studies integrating complement and endothelial injury markers are needed to clarify whether dd-cfDNA elevation in aHUS reflects alloimmune injury, complement-mediated microvascular injury, or overlapping mechanisms.

Donor-derived cell-free DNA (dd-cfDNA) is an emerging non-invasive biomarker of kidney allograft injury. Elevated dd-cfDNA has been associated with acute rejection, particularly antibody-mediated rejection; however, dd-cfDNA reflects graft-derived tissue injury rather than a specific alloimmune mechanism. Its interpretation may therefore be challenging in conditions characterized by endothelial and microvascular injury, such as atypical hemolytic uremic syndrome (aHUS). This retrospective observational study included 14 kidney transplant recipients who underwent their first kidney transplantation at Wroclaw Medical University between April and December 2022. Twelve recipients without aHUS and two recipients with aHUS were assessed at day 7 (D7), day 14 (D14), month 1 (M1), and month 2 (M2). Protocol biopsies were not systematically performed. In recipients without aHUS, median %dd-cfDNA declined from 0.72% (range, 0.26–2.33%) at D7 to 0.12% (range, 0.05–0.34%) at M2. In the two recipients with aHUS, values remained near or above 1% through M1 and declined at M2. No donor-specific antibodies against human leukocyte antigens (HLA) or clinically documented rejection occurred. Exploratory analyses did not identify a statistically significant association between %dd-cfDNA and serum creatinine (Spearman rho = 0.09, p = 0.541). These recipient-level observations suggest that early dd-cfDNA kinetics in the two recipients with aHUS differed from the declining pattern in the small comparison cohort, but they do not establish a group-level difference. Because protocol biopsies were not systematically performed, subclinical rejection could not be excluded. The findings should be considered hypothesis-generating and support cautious interpretation of universal dd-cfDNA thresholds in aHUS.