To detect antibodies directed against red blood cell antigens
Red Blood Cell (RBC) Antibody Screen
When preparing for a blood transfusion; during pregnancy and at delivery.
The test is performed on a sample of blood obtained from a vein in the arm using a needle. This is a process which may be referred to as ‘venepuncture’.
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How is it used?
A red cell 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. If an antibody is detected, then an antibody identification test must be done 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 taken into account and donor blood must be found that does not contain the antigen(s) to which the person has produced antibodies.
If someone has an immediate or delayed reaction to a blood transfusion, a healthcare professional will request a direct antiglobulin test (DAT) to help investigate the cause of the reaction. (The DAT detects RBC antibodies attached to red blood cells). A RBC antibody screen will be performed to see if the affected person has developed any new antibodies if the DAT is positive.
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 the baby's red cells, causing haemolytic disease of the foetus and newborn (HDFN). The most serious cause is an antibody produced in response to the RBC antigen called the "D antigen" in the Rh blood group system. A person is considered to be Rh-positive if the D antigen is present on their RBCs and Rh-negative if the D antigen is not present. A Rh-negative mother may develop an antibody when she is exposed to blood cells from a Rh-positive foetus. To prevent this, a Rh-negative mother should have a RBC antibody screen performed early in her pregnancy, at 28 weeks, 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 Rh immune globulin (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 Rh status is determined. If the baby is Rh-negative, then the mother does not require another RhIg injection; if the baby is Rh-positive and the mother's antibody status is negative for anti-D, the mother is given additional RhIG.
This test may 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.
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When is it requested?
- 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-negative women, it is also done at 28 weeks, prior to giving a RhIg injection, and after delivery if the baby is found to be Rh-positive. In Rh negative 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 roughly track the amount of antibody present throughout the pregnancy.
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What does the test result mean?
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 37oC. 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-negative 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 D 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 D antigen.
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Is there anything else I should know?
A circulating RBC antibody, once present, will never truly go away but may drop to undetectable levels. If the person is exposed to the antigen again, production will kick quickly into gear and attack the RBCs so the antibody will be honoured (treated as though it is present) even when not detectable, meaning that a person with an identified antibody should never be transfused blood from a donor who carries that specific antigen.
Each blood transfusion that a person has exposes them to the combination of antigens on that donor's RBCs. Whenever the 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.
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What happened before the RhIg (Rh immune globulin) injection was developed?
Prior to development of the injection, Rh-negative mothers would often become sensitised from the blood of their first Rh-positive baby and begin developing anti-Rh antibodies. Any subsequent Rh-positive babies would have some degree of Rh disease, due to the mother's anti-Rh 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 immune globulin injection has largely prevented these complications, although a small percent of women do still develop Rh antibodies.
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I’m blood type O. Do I have a chance of having a baby with ABO haemolytic disease of the foetus and neonate?
Yes. Haemolytic disease of the foetus and neonate may occur when there is an ABO incompatibility between mother and baby, especially with mothers who are blood group O. However, the RBC antibody screen is not useful in this situation because our bodies naturally produce antibodies against the A and B antigens we do not have on our red blood cells. A mother who is blood type A will naturally have antibodies directed against the B surface antigens on red blood cells, and a mother who is type B will have anti-A antibodies, and so on. Generally, this is a clinically mild haemolytic disease that is easily treatable.
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Can I get antibodies from donating blood?