Urine cytology as a screening method for polyoma virus active infection

Polyoma virus nephropathy (PVN) occurs in 3% to 4% of renal transplants, causing graft loss in about 50% of cases. The presence of viral cytopathic changes in graft epithelial cells is the only diagnostic tool for PVN. However, identification of cells with viral inclusions (decoy cells) in urine can be used as a screening tool for viral replication of or for active infection with PV. The aim of the present study was to identify the occurrence of PV active infection in renal transplant recipients. Two hundred forty urine cytology samples, collected from 80 transplant patients with stable renal function, were collected on a monthly basis and stained with the Pap smear for decoy cells. Active infection with polyoma virus was confirmed by urine immunostaining. All samples were analyzed blindly and classified as negative or positive (>1 decoy cell/sample). Among 240 urine cytologies collected from 48 men and 32 women, decoy cells were identified in 37.5%. No differences were observed in serum creatinine or immunosuppressive regimen between patients with positive versus negative cytology. No graft losses occurred secondary to PVN in the present study setting. The incidence of decoy cells in this series (37.5%) was consistent with previous reports (20% to 40%), suggesting that active infection may be confirmed by PV immunohistochemistry. The absence of PVN in this group may be attributed to the low doses of immunosuppressive drugs in the late posttransplant transplant period, but also to the unknown incidence of polyoma virus infection in Brazil.