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As soon as it has been decided that a patient is likely to need a transfusion, blood typing should be performed. Determination of ABO blood groups is the most important pretransfusion compatibility test. If tests are done to insure that donor and recipient belong to the same ABO blood group, then even if no other tests are done, the donor's red blood cells will be compatible with the recipient's plasma in about 97% of cases. After A and B antigens, the Rh D antigen is the most important red cell antigen in transfusion practice. Rh typing is performed so that Rh positive red blood cells will not be given to an Rh negative recipient. This prevents Rh immunization in patients without pre-existing anti-D and prevents hemolytic transfusion reactions in patients who have already developed anti-D antibodies. ABO blood groups are found with varying frequencies in different U.S. populations.
About 85% of the US population is Rh positive and 15% is Rh negative. Historically, if a patient typed as Rh negative, additional testing was then performed to determine if they had Rh Du or weak D expression. In the past several years, weak D testing has been eliminated for all patients except obstetric patients. In December 2003, the American Association of Blood Banks decided that weak D testing is no longer necessary for obstetric patients. The main reason is that today's blood typing reagents are much more potent and most of the patients who were previously typed as weak D are now typed as Rh positive. All women are now typed as either Rh negative or positive. The clinical implication of this change is that a few women who actually have weak expression of the D antigen will now be classified as Rh negative and will be candidates for Rh immune globulin. Giving Rh immune globulin to these women is not harmful. Specimen requirement is one 15 mL red top tube of blood labeled with patient's full name and hospital number. |
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