The invention provides a method of assessing the risk of occurrence of progressive multifocal leukoencephalopathy (PML) in a subject as well as a method of stratifying a subject undergoing alpha4-integrin and/or VLA-4 blocking agent treatment for suspension of the treatment and a method of stratifying a subject undergoing Highly Active Antiretroviral Therapy ( HAART) for alteration of HAART. These methods comprise detecting the level of P-selectin glycoprotein ligand- 1 (PSGL-1) expressing T cells in a sample from the subject.
