barb, that's a bummer.... but... I've read that it should be okay to keep taking it.

did they tell you how many times you have to test positive for the antibodies before they will be wanting to discontinue the Tysabri?

Approximately 9% of patients receiving TYSABRI developed detectable antibodies at least once during treatment.

Persistent antibody-positivity was associated with a substantial decrease in the effectiveness of TYSABRI.

Antibodies detected early in the treatment course (within the first 6 months) may be transient and disappear with continued dosing.

http://www.rxlist.com/cgi/generic/tysabri_ad.htm