Antibody Screen (Indirect Antiglobulin Test)

Antibodies to blood group antigens must be detected and their clinical significance assessed. Clinically significant antibodies have been associated with red blood cell destruction, hemolytic transfusion reactions, and hemolytic disease of the fetus and newborn (HDFN). Commercially prepared 2-cell and 3-cell antibody screening sets are available and must meet licensing requirements. The FDA mandates that red blood cells used in antibody detection tests must express the following antigens: D, C, E, c, e, K, k, Fy^a, Fy^b, Jk^a, Jk^b, Le^a, Le^b, P₁, M, N, S, and s. Reagent red blood cells cannot possibly contain every blood group antigen. Low-incidence antigens such as V, C^w, Kp^a, or Js^a are not required because they rarely cause development of the corresponding antibody. 

The incidence of alloimmunization in all transfusion recipients is approximately 2% to 5% and significantly higher in multiply transfused populations. Typically, clinically significant antibodies react at 37°C and are detectable at the antiglobulin phase of testing. However, clinically significant antibodies in the early stages of development or that are primarily of the IgM isotype may be detected at lower temperatures such as in the room temperature phase of testing. Regardless of the phase in which the clinically significant antibody is detected, transfusion of antigen-negative red blood cells is the transfusion requirement. Transfusion requirements vary among facilities when clinically insignificant antibodies are detected. Most consider donor units that are crossmatch compatible by the indirect antiglobulin test (IAT) to be safe. The table below outlines common blood group antibodies according to their preferred phase of reactivity and clinical significance. 

Abbreviations: IAT, indirect antiglobulin test; RT, room temperature

Many testing methods are available, including tube tests with or without enhancement, column-agglutination technology, and solid-phase red cell adherence tests with effectiveness varying between each. Testing performed manually in tubes allows for the chemical modification of the patient’s red blood cells or plasma and the flexibility of testing at various phases. Column-agglutination technology and solid phase red cell adherence tests are available as automated testing platforms creating standardization and workload efficiency. 

If an antibody is clinically significant, the transfusion service needs to provide red blood cells that are antigen negative for the corresponding antibody. For example, if a patient has anti-Kell antibody, they will be transfused with Kell negative blood. When a patient has multiple clinically significant antibodies(i.e. anti-K, anti-E, anti-Fya), the task of finding compatible blood becomes more challenging. Many times a large number of units must be antigen typed to find a single compatible unit of blood. This may necessitate sending the specimen to the consultation lab of our local blood center which maintains a larger inventory of red blood cells. The time required to find a sufficient number of compatible units can range from  hours to days depending on the complexity of the case. 

If an emergent transfusion is required for a patient with one or more red cell antibodies, the transfusion service may not have time to supply compatible antigen-negative blood. In this situation, the risk to benefit ratio of transfusing incompatible blood to a patient with life threatening hemorrhage must be considered. Transfusion of incompatible blood is usually beneficial in these situations because most clinically significant antibodies do not destroy transfused red cells immediately.