Exchange transfusion in sickle cell disease

Sickle cell anemia is characterized both by chronic anemia and blood hyperviscosity. Transfusion may be aimed either to correct an aggravation of anemia, or to provide deformable red blood cells able to deliver oxygen to ischemic tissues in case of vaso-occlusion. In this later setting, attention must be paid not raise hematocrit above 35 %, in order not to increase hyperviscosity. A way to decrease HbS level without raising hematocrit is to associate phlebotomy and transfusion, which defines exchange transfusion. It may be performed manually, exchanging patient's whole blood with packed red cells, or using a cell separator, with reinfusion of the patient's plasma. Exchange transfusion may be prescribed in emergency, after an acute vaso-occlusive event (stroke, acute chest syndrome, acute organ failure). The most frequent indication of chronic transfusion program in children is the prevention of a cerebrovascular event, either of a recurrence after a first stroke, or of a first stroke in children with elevated velocities of arterial arteries on transcranial Doppler. In adults, indications of chronic transfusion are more frequently prevention of vaso-occlusive events (painful crises or acute chest syndromes), or progressive organ damages. In these conditions, hydroxyurea may also be discussed, given that there is no randomized trial allowing to determine which option is the best. Red blood cell allo-immunization represents a serious problem in countries where blood donors and recipients have different ethnic origins. The risk of transmitting viral infections has drastically decreased in developed countries. The burden of iron overload may be alleviated by the availability of a new oral chelator, deferasirox. Lastly, many patients undergoing chronic transfusions need insertion of central venous access devices, which represent an infectious risk especially in this population.