BK virus nephropathy: Clinical experience in a university hospital in Japan

Objectives: To review the medical records of patients with BK virus nephropathy (BKVN) following kidney transplantation in our institution. Methods: We screened patients for decoy cells using urine cytology, assessed serum creatinine levels, and conducted a graft biopsy, as well as assessed the presence of plasma BK virus DNA by quantitative real‐time polymerase chain reaction. The treatment of BKVN was based on the decreased use of immunosuppressants. Results: Overall, six male patients were studied (mean age 40.8 years, range 18–58; mean donor age 45.2 years, range 15–67). A positive urine cytology screen led to the subsequent detection of plasma BK virus DNA in the five patients with urine cytology results positive for decoy cells. In the four patients in whom plasma BK virus DNA was detected, a maximum value of DNA of ≥10 000 copies/mL was observed. Time elapsed from transplantation to BKVN diagnosis ranged from 3 to 62 months. Although the two cadaver grafts were lost, the loss was not due to any effects directly associated with BKVN. The other four grafts are still functioning with a mean creatinine level of 1.8 mg/dL. Most of the patients with BKVN were regarded as being in a state of heightened immunosuppression. BK virus transition to blood was prevented in one patient. Conclusions: Early diagnosis of BKV infection with reduction of immunosuppression may potentially counter BK viremia and retard progression of BKV nephropathy. Decoy cell screening by urine cytology as well as plasma BK virus DNA screening should be considered in addition to the required graft biopsy in kidney transplant recipients, particularly in those with impaired graft function.