ASK AN EXPERT: The accuracy of testing the JC virus

Beth writes:

Many of the DMARDS* have risk for PML. Can you discuss the accuracy of testing of the JC virus; and what is the time span from exposure to the virus to a positive JC antibody test? In other words, what is the average time from getting the virus to having enough antibodies to test?

LiveWiseMS.org consulting expert, Corey Ford, MD PhD, University of New Mexico responds:

The sensitivity of JCV Ab test is about 70% and specificity about 93%. I do not know how long antibodies appear after virus exposure but the recommended interval for testing negative patients on natalizumab is 6 months. So probably weeks to months. Conversion between tests at that interval seems to provide a reasonable window to consider changing to a different DMT.

*disease-modifying antirheumatic drugs

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