Lyn,
I just read about a new test that can possibly stratify PML risk even better than the JCV antibody test. What this means is that positive antibody test people can be identified as greater or lesser risk than titers show.
I can't link to the article myself but it's posted in this forum thread.
http://www.msworld.org/forum/showthread.php?t=134636
Perhaps ask your doctor about this? This could be good news and help you in your decision if you can access the test.