Protocol biopsy of kidney allograft enables early detection of BK virus nephropathy to preserve kidney allograft function

Background The Banff Working Group has updated the histological classification of BK virus nephropathy (BKVN), highlighting the importance of early detection. However, an early detection strategy for BKVN using biopsy has not yet been established. Our investigation aimed to assess the efficacy of protocol biopsy for the diagnosis of BKVN. Methods We performed a retrospective cohort study of 314 patients who had undergone kidney transplantation between 2006 and 2021. Kidney allograft biopsies were performed as part of a protocol biopsy at 3 months and 1 year post‐transplantation. Following the diagnosis of BKVN, the immunosuppressant dose was reduced. Results Twelve patients (3.8%) were diagnosed with BKVN by biopsy. Most diagnoses are established during the early stages of BKVN (polyomavirus nephropathy class 1 in six, class 2 in five, and class 3 in one). Following the reduction in immunosuppressant dose, kidney allograft function did not deteriorate in any patients. Additionally, test for BK virus DNA in the blood was negative. All but one patient demonstrated histological resolution of BKVN, and the other had a very slight positivity for the simian virus 40 large T antigen. The median follow‐up time after BKVN diagnosis was 6 years. One patient developed de novo donor‐specific antibody and subclinical acute antibody‐mediated rejection that was successfully cured. Conclusions Our analysis indicates that protocol biopsy may enable the early detection of BKVN, resulting in the preservation of kidney function. In the post‐transplant management of protocol biopsies, most diagnoses were made in the early stages of BK virus nephropathy. According to the Banff 2019 classification, the distribution of polyomavirus nephropathy was class 1 in 50%, class 2 in 42%, and class 3 in 8%.