PML and progressive multifocal leukoencephalopathy: Recently, Bloomgren et al. proposed a clinical flowchart based on three different risk factors, all associated to an increased risk of PML: positive status with respect to anti-JC virus antibodies, prior use of immunosuppressants and increasing duration of natalizumab treatment (prolonged natalizumab treatment likely increased PML risk, but some studies found no evidence of JCV reactivation in natalizumab-treated MS patients of 18 month follow-up study).