A small tube called a catheter will be placed in a vein in the child's arm and will stay in the vein while the child is in the hospital. From this tube, a very small blood sample will be drawn every 30 minutes during the hospital stay. After breakfast, blood samples will be taken about every 10 minutes for about an hour. For children who are at least 7 years old, blood samples will be taken about every 20 minutes during a period of exercise in the afternoon.