Method of assessing risk of PML

A method of treating a subject in need thereof with an anti-VLA-4 therapy, the method comprising:

• a) determining an anti-JC virus (JCV) antibody titer in two or more biological samples obtained from the subject over a period of time, wherein the titer is determined to be at or above a pre-determined level in a first biological sample of the two or more biological samples and below the pre-determined level in a second subsequent biological sample of the two or more biological samples;
• and
• b) administering the anti-VLA-4 therapy to the subject, thereby treating the subject with the anti-VLA-4 therapy, wherein the subject suffers from multiple sclerosis, a relapsing form of multiple sclerosis or inflammatory bowel disease.