*Purpose: Among adult kidney transplant recipients, donor-derived cell-free DNA (dd-cfDNA) has been validated as a highly sensitive biomarker for graft rejection. Unfortunately, few data are available for pediatric kidney transplant recipients, and whether stable pediatric kidney transplant patients have the same baseline dd-cfDNA levels as adults is unknown. The purpose of this study was to determine the baseline dd-cfDNA levels in stable pediatric kidney transplant recipients.

*Methods: From July-December 2019, 28 pediatric kidney transplant recipients were evaluated at the Children’s Hospital of the King’s Daughters. Twenty-six of these patients were considered stable with no clinical concern for rejection. dd-cfDNA was assessed at routine visits using AlloSure (CareDx Inc., Brisbane, CA).

*Results: Of the 26 stable pediatric kidney transplant recipients, 19 (73%) had a living-related donor, 6 (23%) a deceased donor, and 1 (4%) a living unrelated donor. The median age was 12.7 years (range 1.5-17.5) with a median post-transplant time of 1854 days (range 14-5133). The median serum creatinine was 0.95 mg/dl (IQR 0.6-1.1 mg/dl) with a median estimated glomerular filtration rate of 75 ml/min/1.73 m² (59-97 ml/min/1.73 m²). The median AlloSure dd-cfDNA observed was 0.20% (IQR 0.12-0.32%), with a coefficient of variation matching previously published results for adult kidney transplant recipients.

*Conclusions: Levels of dd-cfDNA in stable pediatric kidney transplant recipients share the same distribution as in stable adult kidney transplant recipients. Further investigation is warranted to determine the clinically relevant thresholds to capture clinically-relevant episodes of rejection in this population

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