*Purpose: Biomarkers such as donor-derived cell free DNA (cfDNA) and gene expression profiling are utilized in adult transplant patients (pts), but these tests lack sensitivity and have not been validated in pediatric pts. QSant™ is a urine-based custom assay of six markers (cfDNA, methylated cfDNA, culsterin, total protein, creatinine and CXCL10) that provides a quantitative score (Qscore) for prediction of rejection in both adult and pediatric kidney transplant pts.

*Methods: We reviewed charts of pediatric kidney transplant pts in whom a QSant™ assay was obtained April-October 2021. The associated QScore (scaled from 0-100) with an immune quiescence threshold of 32 [Yang, STM 2020] was used to determine those at low risk versus high risk for rejection. Pts with complex urologic anatomy were excluded.

*Results: 90 QSant results were obtained 28-5446[IQR2062] days in 56 pediatric renal transplant pts (51% female, 66% deceased donor, 3-24[IQR8] yrs). Induction: basiliximab 40 pts and rabbit ATG 17 pts. 11 pts had QSant study within ~3wks of an allograft biopsy. QSant™ had a 100% sensitivity and specificity for detection of rejection (Table). 4 pts had T cell rejection, 1 pt had antibody rejection and 4 pts had both. 9 pts developed circulating DSA. All nine had Qscore >32. Serial Qscore monitoring revealed distinct patterns of alloimmune injury across pts (Figure 1). Over first 2 serial timepoints (n=23): 35% had a decrease, 30% an increase, and 35% no change in Qscore. Over timepoints 1 and 3 (n=10): 30% had a decrease, 30% an increase, and 40% no change in Qscore.

*Conclusions: This is the largest study of the urine based QSant™ in pediatric pts. Our results are promising primary data for the validation of this test. Future studies will focus on the utility of biomarker assays to serially monitor graft immune health.

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