Testing the blood periodically is a good idea if you are on one of the
MS drugs that can cause this. As far as I know, these are limited to Tecfidera,
Tysabri and Rituxan, the first being a pill and the others infusions done at an infusion center. Keep in mind that one's white count doesn't go from fine to low overnight, or even in a week or a month--it's a process. So if your doctor prescribes one of these, make sure you have your blood tested every few months.