I think they do every week, they take 2 vials of blood before every plex, I had low Iron levels and red blood cells were very low and started taking 600mg of Ferrous Fumarate daily. My red blood cell count went from 99 to 118 in the last 3 weeks, I still feel very SLUGGISH I'm weaning off preds I'm down to 1mg and hoping by this time next week to be off! My Rituxan hasn't improved things so far, I've had my last infusion may 10th my specialist told me it could take up to 8 weeks to kick in...