[Urine cytology pitfalls due to Polyomaviruses].

Specific cytopathologic changes represent an important tool for the identification of a viral infection. After primary infection, generally during childhood, BK and JC polyomaviruses often remain latent within the urinary tract and can reactivate along life. These reactivations are usually encountered in immunosuppressed patients. In renal transplanted recipients, BK virus may cause a polyomavirus nephropathy inducing sometimes graft loss. A good morphologic sign of reactivation is characterized by the shedding in urine of viral-infected cells called decoy cells. The latter are easily identified in urine from renal transplanted patients but in other circumstances, they may be misdiagnosed as high-grade urothelial carcinoma cells. Correct cytological identification of decoy cells, confirmation of the diagnosis by urine PCR analysis and use of immunocytochemistry with anti-SV40 antibody are of good value for differential diagnosis in most cases. However, polyomavirus reactivation and urothelial carcinoma cells may be observed in the same urine specimen. The possible involvement of BK or JC virus in the pathogenesis of human urogenital tumors has been suggested by some studies but is not yet conclusively resolved.