A commercial test has been developed for JC Virus (JCV), the virus responsible for the development of progressive multifocal leukoencephalopathy (PML), the often fatal disorder previously only seen in people with AIDS, but now recorded in many cases of people with MS taking Tysabri (natalizumab).
It has been difficult to estimate the exact risk of this in PwMS taking this drug, and it was thought to be around one in 1000, although more recent data indicate the overall risk as at February 2012 is around 1 in 500.
Current data show that this climbs dramatically to about one in 100 or higher for people who test JCV positive (actually 70-90% of the community is JCV positive), have been on the drug for two years or more, and have had previous immunosuppressant therapy (such as methotrexate).
For people who test negative, the risk is very much lower, at around one in 10,000, although it is important to remember that around 3% of people who test negative will actually be positive (that is the test result is a false negative; like all tests, this is not a perfect test).
There are also additional risks in stopping the drug, in that the immune reconstitution inflammatory syndrome (IRIS) can occur, and disease activity increases again to the level it was prior to starting therapy; careful planning with medical staff is required to manage this potentially serious risk of deterioration on stopping the drug.
Given these risks, and the availability of other medications with fewer potentially serious side effects, the decision to start taking Tysabri needs to be considered very carefully by PwMS in consultation with trusted medical advisors.