A Rapid Noninvasive Assay for the Detection of Renal Transplant Injury

Background. The copy number of donor-derived cell-free DNA (dd-cfDNA) in blood correlates with acute rejection (AR) in heart transplantation. We analyzed urinary dd-cfDNA as a surrogate marker of kidney transplant injury. Methods. Sixty-three biopsy-matched urine samples (41 stable and 22 allograft injury) were analyzed from female recipients of male donors for chromosome Y (donor)-specific dd-cfDNA. All biopsies were semiquantitatively scored by a single pathologist. Standard statistical measures of correlation and significance were used. Results. There was baseline scatter for urinary dd-cfDNA/mu g urine creatinine across different patients, even at the time of stable graft (STA) function (undetected to 12.26 copies). The mean urinary dd-cfDNA in AR (20.5+/-13.9) was significantly greater compared with STA (2.4+/-3.3; P<0.0001) or those with chronic allograft injury (CAI; 2.4+/-2.4; P=0.001) but no different from BK virus nephropathy (BKVN; 20.3+/-15.7; P=0.98). In AR and BKVN, the intrapatient drift was highly significant versus STA or CAI patients (10.3+/-7.4 in AR; 12.3+/-8.4 in BKVN vs. -0.5+/-3.5 in STA and 2.3+/-2.6 in CAI; P<0.05). Urinary dd-cfDNA correlated with protein/creatinine ratio (r=0.48; P<0.014) and calculated glomerular filtration rate (r=-0.52; P<0.007) but was most sensitive for acute allograft injury (area under the curve=0.80; P<0.0006; 95% confidence interval, 0.67-0.93). Conclusion. Urinary dd-cfDNA after renal transplantation has patient specific thresholds, reflecting the apoptotic injury load of the donor organ. Serial monitoring of urinary dd-cfDNA can be a surrogate sensitive biomarker of acute injury in the donor organ but lacks the specificity to distinguish between AR and BKVN injury.