When transfusing red blood cells, the protocol is clear - you need to first take some of the patient's blood and check its blood type. Since the ABO system and the Rh antigen are the most likely by far to cause problems, these are essential to check. Provided the patient can wait an hour or two for the blood, the lab will also usually take the unit of blood about to be transfused and check it against the patient's blood. In this way, rare transfusion reactions that are due to other antigens can also be ruled out.
OK, but why not for platelet transfusions? I thought about attempting the answer myself, but then came across this wonderful answer from Robertson D. Davenport, associate professor of pathology and medical director of the Blood Bank and Transfusion Service at the University of Michigan. He was writing for Scientific American, and the bit reads:
The situation is a little different for transfusions of platelets rather than red cells, which occur with patients undergoing chemotherapy. [This is not the only reason for a platelet transfusion, but the rest still applies.] Because A and B antigens are weakly expressed on platelets, they are less important in this case. Although there are antigens that are specific to platelets, it is rare for people to make antibodies against them even after repeated transfusions.
But HLA antigens, which are critically important in transplantation, are strongly expressed on platelets (but only very weakly expressed on red cells). It is common for patients to make antibodies to HLA antigens in response to transfusions or pregnancy. When platelets are transfused to a patient with corresponding HLA antibodies they are very rapidly cleared from circulation, which is essentially immediate rejection of the transfusion. Usually there is not a clinically evident reaction, as in case of incompatible red cell transfusion, but the platelet transfusion is ineffective. Such a patient can become refractory to platelet transfusion (meaning that the platelet count does not rise and the patient experiences no benefit) and may be at risk for serious bleeding. Refractoriness to platelet transfusion is a serious problem in cancer chemotherapy and bone marrow transplantation.
Unfortunately, it is much more difficult to test for HLA and platelet antibodies than it is to test for red cell antibodies. Platelet compatibility testing is one for patients who do not have a successful response after several platelet transfusions.
I think that answers my question, don't you? (If you're wondering what HLA antigens are, I'll answer that in a future post.)