When would you consider checking JC virus prior to initiating biologic therapy?  

JC is a ubiquitous virus with sero-prevalence in the adult population of 60-70% in most studies. The concern is that in those who harbor latent JC are vulnerable to reactivation and ultimately the development of Progressive Multifocal Leukoencephalopathy (PML). The drug natalizumab used to treat MS ...