*Purpose: Although BK virus nephropathy (BKVN) is a major complication after kidney transplant leading to graft loss, it is often difficult to diagnose and treat. Our post-transplant strategy is monthly monitoring of renal function within 1 year after transplant. We investigated the incidence and clinical outcome of BK virus nephropathy in our hospital.

*Methods: We retrospectively analyzed the clinical records of the patients who developed BKVN between January 2007 and April 2018. The timing of diagnosis, treatment, and prognosis were reviewed.

*Results: We identified 8 recipients with BKVN among 188 kidney recipients during the period. In those BKVN cases, 3 were men and median age at transplantation was 47 (range, 22-62). Four cases were ABO incompatible. All cases received basiliximab as an induction, with maintenance triple immunosuppressants comprising CNI (tacrolimus in 6 and cyclosporin A in 2), mycophenolate mofetil (MMF), and methylprednisolone. The median time at diagnosis was 6.5 (2-15) months after transplant. Three cases had received anti-rejection treatment before BKVN diagnosis. All 8 recipients were diagnosed with SV40T antigen that was immunopositive in kidney graft biopsy. The reasons for biopsy were elevated serum creatinine in 7 cases, and protocol biopsy after anti-rejection therapy in 1 patient, respectively. All cases were positive for BKV-DNA in blood (median 1.9 x 10³, range 1.4 x 10² – 1.2 x 10⁴ copies/mL). Most cases were diagnosed in early BKVN (A in 7 cases and B in 1 case, respectively, Banff 2009 criteria). All cases were treated with reduction of CNI and/or MMF, and 3 cases had additional everolimus. BKV-DNA in blood became negative in all cases in 5 (2-13) months and the resolution of BKVN was also confirmed pathologically. Renal function reversed to the baseline in all cases. No cases experienced another allograft rejection episode during or after reduction of immunosuppression. Currently, 7 (1-12) years after posttransplant, all grafts are surviving with stable function.

*Conclusions: Our analysis shows that reduction in immunosuppression therapy results in resolution of BKVN with good long-term outcome. Monthly monitoring for early detection of BKVN within 1 year post-transplant may enable us preservation of kidney with simple reduction of immunosuppression.

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