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The most conservative estimate of the risk of PML with a JCV antibody index of 0.4 is < 1 in 2500 after 2 years of treatment. However, most of this risk occurs in people with an index value between 0.6 and 0.9. Until someone (Biogen with the approval of the FDA) decides to raise the cut off value for a positive test, I think it is best to consider this low a positive result.
We have no information on the relationship between JCV antibody index and PML risk after Rituximab treatment. The estimated risk of PML after rituximab treatment (taken from patients treated for lupus, rheumatoid arthritis and Sjogren’s syndrome) is less than 1 in 25,000 in all treated patients. This would suggest a 10 fold safety benefit over Tysabri treatment for the risk of PML, if the same risk applies to the treatment of people with MS.
Approximately 20 % of people with MS on Tysabri who develop PML do not survive. Outcome is variable in the remaining 80 % and seems to be improving with more careful case selection and monitoring and earlier identification. Perhaps 20 to 30 % are now recovering with minimal residual problems and the remaining patients have significant residual problems. Poor prognostic factors (people who tend to have a worse outcome if they develop PML) include longer duration between onset of PML symptoms and initiation of treatment, greater involvement of the brain on MRI, involvement of the brainstem or cerebellum, higher amounts of the JC virus in the CSF at time of diagnosis, older age and patients previously treated with immunosuppressants before starting tysabri.
There is growing consensus that patients at risk of PML (JCV antibody positive) who switch from monthly treatments after 12 months to treatment every 6 to 8 weeks, have a much lower risk of PML
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