In cases of progressive multifocal leucoencephalopathy (PML) and associated tumors, where the reactivation of polyomavirus is suspected, PCR is used on a biopsy of the tissue or cerebrospinal fluid to amplify the polyomavirus DNA.
In cases of progressive multifocal leucoencephalopathy (PML), a cross-reactive antibody to SV40 T antigen (commonly Pab419) is used to stain tissues directly for the presence of JC virus T antigen. PCR can be used on a biopsy of the tissue or cerebrospinal fluid to amplify the polyomavirus DNA.