Objective: To analyze the pathological characteristics of for-cause biopsies in pediatric kidney transplant recipients and to identify clinical and pathological risk factors associated with long-term graft survival. 
Methods: This was a retrospective study involving pediatric kidney transplant recipients who underwent for-cause allograft biopsy. Data on pathological scores, biopsy diagnoses, allograft function, and graft survival were collected. Impaired renal function (IRF) at one year post-biopsy was defined as a >20% decline in estimated glomerular filtration rate (eGFR) compared to the time of biopsy. Multivariate analysis was conducted to identify independent risk factors associated with poor graft outcomes.
Results: Between March 2012 and May 2023, a total of 477 pediatric kidney transplants were performed at our center, of which 96 recipients who underwent for-cause biopsies were included in this study. A total of 136 biopsy episodes were analyzed. The pathological diagnoses included rejection (54.3%), recurrent or de novo transplant nephropathy (25.7%), BK virus nephropathy (3.6%), and others (16.4%). Among the rejection cases, 16.5% were borderline, 21.8% TCMR, 12.8% caTCMR, 27.1% ABMR, 3.8% caABMR, and 18% mixed rejection. Among glomerular diseases, IgA nephropathy accounted for 48.5%, FSGS for 33.3%, and others for 18.2%. Figure 1 illustrates the distribution of biopsy-proven pathological diagnoses at various time points post-transplantation. 
Based on graft function one year post-biopsy, patients were divided into the IRF group (n=51) and non-IRF group (n=45). The graft loss rate was significantly higher in the IRF group than in the non-IRF group (19.6% vs. 2.2%). Kaplan-Meier analysis showed a significantly reduced long-term graft survival in the IRF group (Figure 2). Multivariate analysis confirmed that IRF was an independent risk factor for late graft loss (OR=8.93, p=0.03). Further subgroup analysis revealed that the IRF group had higher proportions of TCMR (31.4% vs. 17.8%, p=0.194), caTCMR (23.5% vs. 6.7%, p=0.05), and mixed rejection (25.5% vs. 8.9%, p=0.06). Multivariate analysis revealed chronic active T cell–mediated rejection (caTCMR) (OR = 4.73, p = 0.02) and non-adherance (OR = 3.81, p = 0.05) were independent risk factors for the development of impaired renal function (IRF).
Conclusion: Rejection is the leading pathological finding in for-cause biopsies among pediatric kidney transplant recipients. Impaired renal function at one year after for-cause biopsy significantly contributes to long-term graft loss. Chronic active T cell–mediated rejection (caTCMR) is a key driver of persistent graft dysfunction and inferior long-term outcomes.