Red Blood Cell (RBC) Antibody Identification

Red blood cell (RBC) antibody identification is used as a follow-up test to a positive RBC antibody screen or a positive direct antiglobulin test (DAT). It is used to identify the specific antibody detected by these screening tests to help identify the cause of a transfusion reaction, hemolytic disease of the newborn (HDN), or hemolytic anemia.

A DAT or RBC antibody screen is performed:

• On a pregnant woman during each pregnancy to determine whether the mother has developed any red blood cell (RBC) antibodies
• To confirm the presence and determine the cause of HDN
• Before transfusions of red blood cells as part of a "type and screen" or "type and crossmatch"
• In the investigation of a suspected transfusion reaction
• To help determine if hemolytic anemia may be caused by an autoantibody directed against a person's own red cells

The RBC antibody identification test is used to name the specific antibody or antibodies that are present to determine if they are likely to be clinically significant, i.e., if they are likely to cause a transfusion reaction of HDN. Some RBC antibodies are known to cause moderate to severe reactions while other less significant ones may cause a positive test but few to no symptoms or complications in the blood transfusion recipient or baby.

The antibody identification test may be ordered when an RBC antibody screen or a direct antiglobulin test is positive. The test may also be performed when a person has a transfusion reaction or when a mother has a baby with hemolytic disease of the newborn.

Results of RBC antibody identification will name the specific antibody or antibodies present in the blood of the person tested.

If the antibody identified is considered clinically significant, then it will need to be taken into account with each transfusion and/or pregnancy.

• If clinically significant RBC antibodies have been identified during pregnancy, the baby's condition will be monitored. Whether or not the antibodies will affect the baby's condition depends upon the antibody present and the RBC antigens that the fetus has inherited. Some antibodies can cross the placenta from mother to baby and cause hemolytic disease of the newborn (HDN).
• For blood transfusions, if one or more clinically significant RBC antibodies are identified, then donor blood that lacks the corresponding RBC antigens must be used for transfusion. When someone has a condition that requires recurrent transfusions, the person is exposed to many foreign RBC antigens and may develop multiple RBC antibodies over time, making the process of finding compatible blood increasingly challenging.

If an antibody is not considered clinically significant, then it is not likely to cause a transfusion reaction in the person or cause HDN. For blood transfusions, it is not necessary to find compatible blood if the antibody identified is not likely to cause a transfusion reaction (is not clinically significant).

Examples of RBC antibodies and their clinical significance are shown in the table below.

Some RBC antibodies are naturally-occurring; they do not require an initial exposure to the specific targeted antigen. These include antibodies that correspond to the major A and B red blood cell antigens.

Sometimes an RBC antibody may be present in such a small quantity that it does not cause a positive RBC antibody screen during pre-transfusion blood compatibility testing. Once a person has developed an RBC antibody, the person must always be matched with antigen-negative blood, even if the antibody is no longer detectable. This is because after the blood is given to the recipient, it can trigger renewed, rapid antibody production and cause a delayed hemolytic transfusion reaction several days later.

RBC antibodies are not the only things that can cause a transfusion reaction. The recipient's immune system may also react to someone else's white blood cells or platelets, or to drugs that the donor may have taken. Rarely, antibodies in the plasma of the blood donor may target the RBCs of the transfusion recipient if products with a large amount of plasma are transfused.

Some RBC antibodies may not target a specific RBC antigen but may react with a broad range of different red blood cell antigen types, including the person's own. These types of antibodies can occur in association with autoimmune disorders, lymphomas and chronic lymphocytic leukemia, certain viral or mycoplasma infections, and some medications.

An RBC antibody can occasionally be missed with antibody identification testing if the antibody is low titer or formed against a rare antigen. This is why the crossmatching process is important even in patients with no demonstrated antibodies. It evaluates the compatibility of the donor's red blood cells and recipient's serum for each unit of RBCs transfused (see Transfusion Medicine).

It is not necessary unless someone is pregnant or may need a transfusion. RBC antibodies do not otherwise affect the health of someone who has them. Sometimes a healthcare practitioner may test a woman after a pregnancy, especially if her baby had complications, to determine if there may be risks associated with a future pregnancy. Also, a healthcare practitioner may order a direct antiglobulin test (DAT) and RBC antibody screen if hemolysis or anemia due to autoantibodies is suspected.