Definition. It is the awakening, in immunosuppressed subjects, of the BK virus (BKV) within the cells of the uroepithelium.
In kidney transplant recipients, this event is favoured by the immunesuppressive treatment, and can lead to “BKV nephropathy”, with possible graft loss.

The reactivation of BKV is heralded by the finding in the urine sediment of the so-
called “decoy cells”, which are infected cells with well defined nuclear morphological
changes (described in: Singh HK, et al. Urine cytology findings of polyomavirus infections. Adv Exp Med Biol 2006; 577: 201-12).

Decoy cells are at best identified in alcohol-fixed, Papanicolaou-stained urine cytology
specimens. However, they can also be seen in routine, unstained, urine sediment samples.