Should I Switch?

Here is My Question:
This September will be my 46th infusion of Tysabri. In October 2013 I continued to test negative for JCV. In March 2014 I tested positive, with a titer of .33. With every subsequent infusion we have tested for JCV. The titers have ranged from .27 to .43.

Now for my question...for my latest infusion (August 20th) my test came back negative. How can that be? Lab error? All tests have been done through Quest with a 92675 code.

I am seriously considering coming off Tysabri and starting Rituxan in October. I have done incredibly well on Tysabri and really hate to come off it, though I do have concerns. What are your thoughts? Can you offer an explanation? Thank you so much for your time. This is a wonderful service you provide.

Answer:
Your experience is very common and illustrates some important points. The bottom line is that I see little reason to stop Tysabri if you have done well. Here is why I say this:

1. The JC virus antibody index does not tell us if you’ve been exposed to the JC virus, nor does it tell us if the virus exists in a latent form in your body. In fact we and other groups have shown that the JC virus (in it’s harmless form, not the form that causes PML) can be found in people who are JC virus antibody negative.
2. The JC virus antibody index is, at present, only useful as a risk factor for the development of PML while taking Tysabri.  Why this test result is associated with the risk of PML is still a matter of speculation.
3. The JC virus antibody index results often fluctuate but tend to stay in either the negative/low, intermediate or high range. Generally those patients with index values less than 0.9 have a relative low long term risk of PML (less than 1 in 2500 treated patients)
4. To my knowledge no one with an antibody index less than 0.4 has developed PML and your highest test result has only been 0.43; This information could change with more testing and follow-up of Tysabri treated patients but is reassuring in your case. The fact that your most recent result is negative is even more reassuring.
   

Remember, all test results exhibit normal biologic variation (that due to natural changes in your body of no significance) and normal testing variation (that due to the normal variability of the actual test even if run on the same blood sample). You need to keep this in mind when interpreting results.

Rip Kinkel, MD

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