Study of the Serotype and Genotype of BK Virus in Kidney Transplant Recipients and Their Donors to Identify Individuals at Risk of Nephropathy

The aim of this observational study is to characterize the urinary replication of BK polyomavirus (BKV) in kidney transplant recipients. Although BKV reactivation after transplantation is well established, the origin of the replicating virus remains uncertain. Current evidence suggests that BKV detected in recipients may originate either from the transplanted kidney (donor-derived) or from viral reactivation in the recipient. The evaluation of new biomarkers to predict BKV replication are needed. This study seeks to address the following key questions: * Origin of the replicating virus: Is the BKV detected in the recipient identical to the virus originating from the donor kidney? * Host immune response and viral genotype: Is there an association between the recipient's immune response and the genotype of the replicating BKV? * Differences in immune response according to viral replication profile: Does the immune response differ between patients presenting isolated BKV viruria and those with both viruria and viremia? * Can new biomarkers help predict BKV replication and viremia? Patients will be grouped according to their BKV replication profile: Group 1: patients with BKV viruria without viremia Group 2: patients with both BKV viruria and viremia Comparisons between these two groups will help identify whether different viral genotypes or immune responses are associated with systemic dissemination (viremia). Kidney transplant recipients will be included if they present BKV viruria during their post-transplant follow-up. Additional blood samples will be collected during scheduled follow-up visits at the university hospital. These visits are part of routine clinical care, and no extra visits will be required specifically for the study.