Blood tests

Most countries nowadays apply the “Type and Screen” policy, consisting of a systematic ABO/D grouping and "rare" antibodies (i.e. antibodies other than anti A or anti B) screening for all patients potentially in need of a transfusion. The blood crossmatch is then optional and usually reserved for patients at risk for having alloantibodies such as pregnant women or polytransfused patients.

Blood grouping

Blood grouping consists in the determination of the specificity of the most immunogenic antigens present at the red cell surface.

ABO grouping refers to two kinds of tests: the cellular test, consisting in phenotyping the A and B antigens at the red cell surface, and the serological test, searching for anti-A and anti-B antibodies in plasma or patient’s serum.
RH phenotyping refers primarily to tests determining D antigen (RH1) on the RHD protein. Other antigens from the RHCE protein are now also regularly tested: C (RH2), E (RH3), c (RH4), e (RH5), usually in combination with KEL1 phenotyping (Kell antigen). RH-KEL1 phenotyping is recommended in women under age of 50, in order to prevent feto-maternal allo-immunization complications (hemolytic disease of the newborn).
Extended phenotyping: this facultative test consists in typing the main immunogenic antigens in transfusion, such as the ones in the Duffy, Kidd and MNS systems, respectively Fy^a-Fy^b, Jk^a-Jk^b, S and s.

Antibody screening

Antibody screening consists of searching for unexpected (irregular) antibodies directed to red blood cell alloantigens in the patient’s plasma or serum. These antibodies may cause severe immune hemolytic reactions when red blood cells carry the respective antigens.
This screening is performed with the indirect antiglobulin testing (IAT) method (indirect Coombs technique) at 37°C, and needs 30 to 45 minutes to be done. After the transfusion, the result is valid only for a short period of time; after 72 hours, the patient needs to be tested again if red cells have to be transfused again.

Cross match (direct compatibility testing): This test consists of mixing the patient’s plasma (or serum) with a small amount of the red cell unit to be transfused. This compatibility testing is performed with the indirect antiglobulin testing method, at 37°C. This technique confirms ABO compatibility and might detect antibodies not revealed with the routine "rare" antibodies screening test. Because this technique is less sensitive than the antibody routine screening method, the “Type & Screen” approach is preferred.
Direct antiglobulin test (DAT) or direct Coombs technique: This critical test in transfusion medicine is commonly used in situations such as diagnosis of hemolytic disease of the newborn, diagnosis and follow-up of auto-immune and medication-related hemolytic anemia's in some other auto-immune disorders, and in the characterization of transfusion reactions.