ABO typing of red blood cells is a two-step process. Forward typing detects antigens on the patient’s red blood cells. Separate tubes with the patient’s red blood cell suspension are mixed with a solution of antibodies against group A antigen, group B antigen, or Rh antigen. The presence of agglutination or hemolysis indicates that antibody has bound to the red blood cells in that tube, and that the antigen of interest is present. Reverse typing detects antibodies in the patient’s plasma against antigens on the red blood cell surface. In reverse typing, it is the patient’s plasma that is used rather than the patient’s red cells. The plasma is mixed with cells known to have an A antigen or a B antigen. As in forward typing, agglutination or hemolysis indicates that antibody is present.
BLOOD COMPONENT PREPARATION
Whole blood is collected from a donor into a bag which contains an anticoagulant that mixes with the blood as it enters the bag to prevent clotting. A predetermined weight of blood is collected. After sealing, the blood is first separated into red blood cells and platelet-rich plasma by centrifugation. The packed red blood cell component is then available for testing prior to transfusion. The platelet-rich plasma is transferred to a second bag, which is centrifuged to separate the plasma from the pellet of platelets. The plasma is removed from the platelets and is available for testing prior to freezing and subsequent transfusion. The platelets are resuspended to create a “random donor” platelet concentrate. “Single donor” platelets are collected by apheresis. In this way, packed red blood cells, fresh frozen plasma, and random donor platelet concentrates are prepared from a single unit of whole blood.
The crossmatch test for ABO incompatibility identifies clinically significant patient antibodies to the red blood cells of a potential red blood cell donor. To identify antibodies to donor red blood cells, the patient’s serum is mixed with a small volume of the donor’s red blood cells, and the red blood cell suspension is then checked for agglutination or hemolysis. The presence of agglutination or hemolysis indicates that the cross match is incompatible, and that the tested donor unit is not suitable for transfusion.
DIRECT ANTIGLOBULIN TEST (DAT)
To determine if a red blood cell has IgG immunoglobulin or the complement component C3d bound to its surface, a suspension of red blood cells from a patient is mixed with antibodies against IgG or C3d. If agglutination of the red cells is observed, the test is positive, indicating the presence of IgG or C3d on ...