Most doctors only give platelet transfusions in cases where the patient is actively bleeding and the blood won't clot. The reason is that this treatment is of limited benefit and only lasts a very short time. The problem has to do with the life of the cells. Unlike red blood cells, which on average live about 120 days, platelets on average live only eight days. When you get a transfusion of blood from someone else, some of those cells are very old and some are brand-new, so you have to assume that the average platelet's going to last only four days.