*Purpose: BK polyomavirus (BKPyV) is a causative agent of BKPyV-associated nephropathy (BKPyV-AN) and hemorrhagic cystitis, which occurs subsequent to BKPyV reactivation. Intravenous immunoglobulin is presumed to have significant neutralizing activity in vitro; therefore, a high-antibody titer against BKPyV should be effective in controlling BKPyV-associated diseases. This study aimed to measure the preoperative anti-BKPyV antibody titer in both kidney transplant recipients and living donors and investigate the significance of a preoperative anti-BKPyV antibody titer as a clinical marker.

*Methods: From July 2013 to December 2016, 68 consecutive kidney living-donor-recipient pairs underwent kidney transplantation at Akita University Hospital. First, we obtained urine and blood samples of the kidney transplant recipients before and 1, 3, 6, and 12 months after kidney transplantation. Next, we measured the DNA copy number of BKPyV by quantitative real-time polymerase chain reaction. Anti-BKPyV antibody titers of the serum of both recipients and living donors were evaluated preoperatively by neutralizing activity using enzyme-linked immunosorbent assay.

*Results: In this study, nine kidney transplant recipients (14.3%) developed viremia and three (4.8%) developed BKPyV-AN. Almost all the BKPyV-DNA subtypes among the detectable specimens were I/c (94.4%), which is the major subtype in Japan. In patients with BKPyV-AN, we detected serum BKPyV-DNA by monitoring BKPyV-DNA several months before the patients developed symptomatic BKPyV-AN. Multivariate analysis revealed that potent immunosuppressive protocol (odds ratio [OR] = 12.7; p = 0.044) and non-high titer of anti-BKPyV antibody of recipients or living donors (OR = 11.5; p = 0.049) were significant risk factors of viremia and/or BKPyV-AN.

*Conclusions: Anti-BKPyV antibody titers of kidney transplant recipients and living donors may be a useful clinical marker for the potential risk of BKPyV-AN.

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