If it only happens at the end of the transfusion cycle, it is simply the transfused blood breaking down. This is normal. The only thing that may make it last a bit longer is using fresh blood, which in most cases means arranging a group of regular donors, so that the blood is always newly drawn from the donor. Electrophoresis should be done to see if the HbF level is increasing, as it is the method to determine if the hydroxyurea is having any impact.