It is not clear exactly how transfusion prevents stroke, but it has several effects on the body. These effects may stabilize or reverse the conditions in the abnormal brain arteries detected by TCD and prevent the development of stroke. The blood supply in the US and Canada is very safe but risk can never be totally eliminated. Patients in the trial were monitored for evidence of infections known to be transmitted by blood. As of the close of trial, there was no indication of any infection having been caused by transfusion in STOP in over 1500 transfusions.
Transfusion also causes a build-up of iron in the body which can affect several vital organs.
When red blood cells break down, the iron that is part of each cell does not leave the body but is stored in the heart, liver, and other organs. In a transfusion, the new blood is broken down more rapidly and iron overload becomes a health concern. Some of the children who received transfusion in STOP are being treated with a drug to remove the iron through a process called chelation. When transfusions are used to prevent the second stroke, they are usually continued at least until the child grows to adulthood. Even then it is not clear how safe it is to stop transfusions.