Stratification of preventive measures for patients undergoing occasional transfusions. Preventive measures are recommended according to the risk of developing a new alloimmunization and DHTR, a risk that is itself evaluated from the patient's history of immunization and DHTR. Three situations are considered: (A) known history, with many transfusions in the past; (B) known history but with few transfusions in the past, and finally; (C) history unknown. In (C), the protocol may be based exclusively on the screening test result, with a risk that previous antibodies will be missed. *For (A) and (B), the cutoff between many and few transfusions is based on published data concerning the mean number of RBCs transfused at which the first antibody appears and on the risk being higher below 13 units. Few studies have addressed this issue, which remains a matter of debate. A count of 13 RBCs transfused is routinely used as the cutoff at our center.