*Purpose: Surveillance biopsies provide the opportunity to identify subclinical rejection and other pathologic changes, yet their utilization varies across pediatric kidney transplant centers. The purpose of this study was to evaluate subclinical rejection on surveillance biopsies performed at a large pediatric kidney transplant center.

*Methods: A single-center, retrospective review of pediatric patients transplanted between 2012 and 2016 was performed to assess pathologic findings on surveillance biopsies at 6, 12, and 24 months post-transplant. Individual Banff scores were collected from pathology reports and reviewed by a single pathologist blinded to the original diagnosis using Banff 2017 criteria.

*Results: A total of 131 patients met inclusion criteria. During the study period, 275 surveillance biopsies were performed; 105 at 6 months, 105 at 12 months, and 65 at 24 months post-transplant. Histologic findings of rejection were found on 21% of 6 month, 27% of 12 month, and 28% of 24 month biopsies. Borderline rejection was the most common grade of acute cellular rejection (ACR) (Table 1). There were no cases of ACR Grade 2 or above. Approximately 9% of biopsies at any time point met full criteria for antibody mediated rejection (AMR). Interestingly, 25% of the surveillance biopsies had some histologic features of AMR without meeting full Banff criteria. Concurrent ACR and AMR were noted in 3.3% (n=9) of biopsies. On average 20-30% of the biopsies at any time point showed mild and 13% showed moderate or severe interstitial fibrosis (IF) and tubular atrophy (TA).

*Conclusions: Subclinical rejection was common on surveillance biopsies at all time points assessed post-transplant. Borderline ACR was the most common finding followed by subclinical AMR. Further studies are needed to evaluate the effect of treatment of subclinical rejection on long term allograft survival. The clinical significance of partial histologic features of AMR needs further evaluation.

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