Red Blood Cell (RBC) Antibody Screen

A RBC antibody screen is used to screen an individual's blood for antibodies directed against red blood cell (RBC) antigens other than the A and B antigens. It is performed as part of a "group and screen" whenever a blood transfusion is anticipated or as part of routine antenatal testing. If an antibody is detected, then an antibody identification test is required to determine which antibodies are present. During a crossmatch, a variation of the RBC antibody screen is performed if clinically significant antibodies are present; this involves mixing the patient serum (a cell-free fraction of blood) with the donor red cells to check if the patient antibody causes agglutination by cross-linking antigens present on donor red cells. In the case of blood transfusions, RBC antibodies must be considered and donor blood must be found that does not contain the antigen(s) to which the person has produced antibodies, in other words, compatible blood.

If someone has an immediate or delayed reaction to a blood transfusion, a direct antiglobulin test (DAT) will be performed to establish if the patients antibodies are coating incompatible donor red cells. A DAT detects antibodies attached to RBCs. If the affected individual has a positive DAT an antibody screen will subsequently be performed to determine if any developed antibodies are incompatible with transfused donor red cells. Antibody identification will be performed to establish if the affected individual has developed any new antibodies.

During pregnancy, the RBC antibody screen is used to screen for antibodies in the blood of the mother that might cross the placenta and attack foetal red cells, causing haemolytic disease of the foetus and new-born (HDFN). The most serious cause is an antibody produced in response to the RBC antigen RhD. A person is considered to be RhD+ if the D antigen is present on their RBCs and RhD- if the D antigen is not present. A RhD- mother may develop an antibody when she is exposed to blood cells from a RhD+ foetus. Therefore, it is important that a RhD- mother has an RBC antibody screen performed early in her pregnancy, typically at booking, at 28 weeks gestation, and again at the time of delivery.

If there are no Rh antibodies present at 28 weeks, then the woman is given an injection of prophylactic anti-D (Rh immunoglobulin (RhIg) to clear any Rh-positive foetal RBCs that may be present in her bloodstream to prevent the production of Rh antibodies by the mother, effectively ‘mopping-up’ foetal red cells before they have a chance to stimulate an immune response.

At birth, the baby's RhD status is determined. If the baby is RhD-, then the mother does not require another prophylactic anti-D (RhIg) injection; if the baby is RhD+ and the mother is RhD-, the mother is given additional prophylaxis (RhIg).

This test may also be used to help diagnose autoimmune-related haemolytic anaemia in conjunction with a DAT. This condition may be caused when a person produces antibodies against their own RBC antigens. This can happen with some autoimmune disorders, such as systemic lupus erythematosus, with diseases such as lymphoma or chronic lymphocytic leukaemia, and with infections such as mycoplasma pneumonia and infectious mononucleosis. It can also occur in some people with the use of certain medications, such as penicillin.

• A RBC antibody screen is performed prior to any anticipated blood transfusion.
• A RBC antibody screen is performed as part of every woman's antenatal screening assessment. In Rh- women, it is also repeated at 28 weeks, prior to giving a RhIg injection, and after delivery if the baby is found to be Rh+. In Rh- pregnant women with known antibodies, the RBC antibody screen and a dilutional test (known as a titration) may be used as a monitoring tool to track the approximate amount of antibody present throughout the pregnancy.

If a RBC antibody screen is positive, then one or more RBC antibodies are present. Some of these antibodies will be more significant than others, in that they are more likely to cause haemolysis of donor red cells bearing the specific antigen if they are transfused into the body at 37^oC. When a RBC antibody screen is performed prior to a blood transfusion, a positive test indicates the need for an antibody identification test to accurately identify the antibodies that are present. Once the antibody has been identified, then donor blood must be found that does not contain the corresponding antigen(s) so that the antibody will not react with and destroy donor RBC antigens following a blood transfusion.

If an Rh- mother has a negative RBC antibody screen, then an Rh immune globulin injection is given within 72 hours to prevent antibody production. If she has a positive test, then the antibody or antibodies present must be identified. If an antibody to the RhD antigen has been actively formed by the mother, then the RhIg injection is not useful. If she has a different antibody, then the RhIg injection can still be given to prevent her from producing antibodies to the RhD antigen.

A circulating RBC antibody, once present, will never truly go away but may drop to undetectable levels. Upon re-exposure to the same red cell antigen, antibody is produced rapidly which could induce haemolysis. Therefore, historical RBC antibodies will always be treated as though they are present, even if they are undetectable by laboratory tests. This ensures that an individual with a RBC antibody will not be transfused antigen-positive donor blood.

Each blood transfusion that a person has exposes them to the combination of antigens on the donor RBCs. If transfused RBCs contain antigens foreign to the recipient's RBCs, there is the potential to produce an antibody. If someone has many blood transfusions over a period of time, they may produce antibodies against many different antigens. This can make finding compatible blood increasingly difficult.

Prior to development of the injection, RhD- mothers would often become sensitised from the blood of their first RhD+ baby and begin developing anti-RhD antibodies. Any subsequent RhD+ babies would experience some degree of haemolysis due to the mother's anti-RhD antibodies attacking the baby's RBCs. Miscarriages and stillborn babies were relatively common, and those babies who were born often needed immediate blood transfusions to survive. The immunoglobulin injection has largely prevented these complications, although a small percentage of women do still develop RhD antibodies.