RBC Antibody Screen
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’.
The RBC antibody screen looks for circulating antibodies in the blood directed against red blood cells (RBCs). The primary reason that a person may have RBC antibodies circulating in their blood is because they have been exposed, through blood transfusion or through pregnancy, to RBCs other than their own (foreign RBCs).
Red cells carry many different proteins and substances on their cell membrane surface that can act as antigens. An antigen is any substance that may be recognised by the immune system and stimulate an immune response that generates antibodies. The combination of antigens present on the surface of red blood cells determines your blood type. The major red cell antigens include the A, B and Rhesus (Rh) antigens that determine a person’s basic blood types (for more on this, see Blood Type and Blood Banking).
The major antigens or surface identifiers on human RBCs are the O, A, and B antigens, and a person's blood is grouped into an A, B, AB, or O blood type according to the presence or absence of these antigens. Another important surface antigen is Rh factor, also called D antigen. If it is present on a person's red blood cells, their blood type is Rh+ (positive); if it is absent, the blood is type Rh- (negative). (For more on these antigens, see the article on Blood Typing). The major blood group systems (ABO and Rhesus) represent only two of the 33 currently recognised blood group systems. These other blood group systems include the Kell, Duffy, Kidd and Lutheran groups to name a few.
If there are RBC antibodies present and RBC bearing the corresponding antigens on their surface are introduced into the bloodstream (by transfusion or during pregnancy and delivery) the antibodies rapidly attack and destroy the foreign red cells. This destruction of red cells is called haemolysis.
There are a few reasons why someone may produce antibodies against RBC antigens.
- Following blood transfusions: Antibodies directed against A and B red cell antigens are naturally occurring; we produce them without having to be exposed to the antigens. These naturally occurring antibodies are present in almost all by the age of 6 months and are very potent. Before receiving a blood transfusion, a person's ABO group and Rh type are matched with that of donor blood to prevent a serious transfusion reaction from occurring. That is, the donor's blood must be compatible with the recipient's so that their antibodies do not react with and destroy donor blood cells.
If someone receives a blood transfusion, their body may also recognise other RBC antigens from other blood groups (such as Kell or Kidd) that they do not have on their own RBCs as foreign. The recipient may produce antibodies to attack these foreign antigens. People who have many transfusions make antibodies to RBCs because they are exposed to foreign RBC antigens with each transfusion.
- With foetal-maternal blood type incompatibility: A baby may inherit antigens from its father that are not present on its mother's RBCs and are therefore recognised as ‘foreign’ by the mother’s immune system. The mother may be exposed during pregnancy or at delivery to the foreign antigens on her baby's RBCs when some of the baby's cells enter the mother's circulation as the placenta separates. The mother may begin to produce antibodies against these foreign RBC antigens. This can cause haemolytic disease of the foetus and neonate, usually not affecting the first baby but affecting subsequent children when the mother's antibodies cross the placenta, attach to the baby's RBCs, and haemolyse them. An IAT can help determine if the mother has produced RBC antibodies other than the expected naturally occurring anti-A and anti-B antibodies.
The first time a person is exposed to a foreign RBC antigen, by transfusion or pregnancy, they may begin to produce antibodies but their cells do not usually have the time during the first exposure to make enough antibodies to actually destroy the foreign RBCs. This process may be referred to as ‘sensitisation’. However, when the next transfusion or pregnancy occurs, the immune response may recognises the foreign antigen and upregulates the immune response, stimulating production of more antibodies direct against the foreign antigen. This reaction may be strong enough and produce sufficient antibodies to attach to and stimulate haemolysis (destruction) of the transfused RBCs or the baby's RBCs causing clinical symptoms. Antibodies to the ABO antigens are naturally-occurring so do not require exposure to foreign RBCs.
The RBC antibody test screens for the presence of RBC antibodies (other than ABO antibodies) in the blood. RBC antibodies that are detected can be identified with an antibody identification test.
How is the sample collected for testing?
A blood sample is taken with a needle from a vein in the arm.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
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.
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.
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.
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.
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.
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.
Can I get antibodies from donating blood?