*Purpose: There lacks consensus regarding the appropriate timing of routine surveillance biopsies (SB) for pediatric kidney transplant recipients. We evaluated the yield of positive biopsy findings at various SB time points in the first 24 months post-transplant compared with indication biopsies (IB), and their impact on clinical management.

*Methods: Patients transplanted at a single institution between 2014-2019 were included with at least one SB scheduled at 1.5, 3, 6, 12, and 24 months post-transplant. Additional biopsies were done for indication (IB) or to follow-up treatment of rejection (FB). Histology was classified according to Banff criteria, and rejection (CMR) was considered present with ≥i1,t1. Retrospective chart review identified changes to immunosuppression and other clinical management based on biopsy findings.

*Results: Thirty eight patients had 147 biopsies total (SB=85, IB=27, FB=35), of whom 50 (34.0%) experienced at least one episode of CMR in the first 24 months. CMR detection was highest in IB (40.7%) and FB (60%; p<0.001). Among SBs, 21.2% demonstrated TCMR and 44.7% showed isolated tubulitis (i0,t1-2). Other SB findings included BKV nephropathy (3.5%), AKI (4.7%), chronic active CMR (1.2%), and acute AMR (1.2%). AKI was most common in IB (33.3%, p<0.001). The SB time point with greatest yield for CMR diagnosis was at 6 months (11/22; 50%), compared with 1/19 (5.3%), 4/20 (20%), and 2/17 (11.8%) at 1.5, 3 and 12 months, respectively (p=0.002). Isolated tubulitis is more common when CMR rates are lowest on early (63.2% at 1.5 months) and later SB (64.7% and 71.4% for 12, 24 months, respectively) compared with 3 and 6 month biopsies (20% and 27.3%, respectively, p=0.005). SB instigated increased immunosuppression in 27.1% cases and other changes in 3.5% cases. Changes in immunosuppression were more common in IB (59.3%) and FB (62.9%) cases (p<0.001). The 6 month SB resulted in the greatest number of changes in management (59.1%), compared with 1.5, 3, and 12 months (15.8, 20, and 23.5%, respectively; p=0.019).

*Conclusions: SBs continue to identify an important burden of subclinical rejection, with the greatest yield at 6 months post-transplant. The yield and utility was least at the 1.5 month biopsy time point. A higher than expected prevalence of isolated tubulitis was seen on early and later SBs, however the clinical significance of this finding is uncertain. The next steps of this project will involve comparing outcome data among groups and evaluating adverse treatment events.

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