What does it mean that my JC virus test came back positive?

Here is My Question:
I have been on Tysabri for over ten years. My JC virus test have come back negative this whole time........until now. My neuro called me today to tell me that my test came back positive. We now need to decide if I stay on my current medicine or change to a different therapy. She did give me a number of .4 as the reading, but I don't know what that means truly. Please explain.

Answer:
The decision to stay on or change from Tysabri in this situation is a very personalized issue – even with a positive blood test each patient’s decision is specific to their history. The number associated with the JCV Ab is referred to as an index. Data suggests that the highest risk of PML occurs in patients with an index above 1.5.
 
Benjamin M. Greenberg, MD, MHS
Vice Chair of Translational Research and Ambulatory Care
Department of Neurology and Neurotherapeutics
Director, Transverse Myelitis, Neuromyelitis Optica Programs
Co-Director, Pediatric CONQUER Program
UT Southwestern Medical Center
Childrens Health
Dallas, Texas

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