Can you tell me about having multiple sclerosis and being tested for the jc virus before starting tysabri (natalizumab)?

SEQUENCE: Tysabri (natalizumab) is most potent MS drug on market, but risk of PML approaches 1 in 90, if 2 yrs of usage, prior chemotherapeutic agents, and positive anti-JCV index test. However, in many others with negative testing, risk is less than 1 in 40,000 at 2 yrs. Positive test does NOT exclude Tysabri (natalizumab), but maybe stop at 18-24 months, and switch to Gilenya at that point.