Due to the increasing importance of BKV infection on the survival of kidney transplants, a better knowledge, in the pretransplant phase, of the risk factors leading to viral reactivation could allow the follow-up physician to be more vigilant and better prevent this pathology. There are no commercial tests for BKV serology. In the virology laboratory, the investigators have developed the technique for producing "Virus-like particles" (VLP) that mimics the antigenic structure of the BK virus. The investigators plan to evaluate seroreactivity to the five BK virus serotypes in the recipient of a kidney transplant as a pretransplant and to compare this data with the detection or not of viral reactivation in the recipient during the first post-transplant year.

Inclusion Criteria:

• adult kidney transplanted patients
• Renal transplant patients with regular virological follow-up (blood and urine for BKV) during the first year after transplantation.

Exclusion Criteria:

• Renal transplant patients with no follow-up for one year after transplantation (transplant failure, return to dialysis, death, follow-up outside the transplant centre)

Renal transplant patients with regular virological follow-up (blood and urine for BKV) during the first year after transplantation