Here's a newish (2013 or 2014) article on Tysabri, PML, and the JCV antibody test.

http://www.medscape.com/viewarticle/820968_4

I need to ask my doctor about this study of 67 people where they checked blood with urine and said the false negative rate of the JCV antibody test was 37%. Okay that seems scary to me! Linda, I wonder if your doc wants you to test every 3 months because this article suggests testing every 3-6 months.

There is other useful info in here as well including stratifying risk based on titer levels. It's worth reading.