Objective: To evaluate the prevalence of BK virus (BKV) infection in renal transplant recipients based on the detection of urinary decoy cells. Methods: Fifty transplantation patients (56% males) aged 39.9 ± 11 years were screened for urinary decoy cells. The majority (86%) had received grafts from living donors. The inclusion criteria were graft dysfunction, hematuria, and/or leukocyturia. Creatinine and urea serum levels were monitored for 6 months. Results: Decoy cells were found in the urine of 12 (24%) patients 1–2 years after transplantation. The immunosuppression regimens most frequently adopted by BKV-positive patients were cyclosporine + azathiprine (50%) and FK + mycophenolate mofetyl + prednisone (25%). A renal biopsy revealed normal structures in 7 patients (58.4%), BKV nephropathy with lymphomononuclear infiltrate and fibrosis in 4 patients (33.3%), and acute cellular rejection with lymphomononuclear infiltrate without fibrosis in 1 patient (8.3%). In the latter patient, BKV-associated nephropathy was confirmed by immunohistochemistry. During the first month, creatinine and urea serum levels were higher among patients with urinary decoy cells. Creatinine levels decreased between the first and the sixth months. Conclusion: The finding of BKV-associated nephropathy in 33.3% of the patients with urinary decoy cells stresses the importance of screening for BKV with urinary cytology.

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