To identify the specific antibody present when a direct antiglobulin test (DAT) or indirect antiglobulin test (IAT) is positive; to help identify the cause of a transfusion reaction or the cause of haemolytic disease of the foetus and neonate (HDFN)
Red Blood Cell (RBC) Antibody Identification
Red cell antibody identification will be performed when an antibody screen using the IAT is found to be positive during a routine ‘group and save’ investigation which may be performed as a routine antenatal screening tests, prior to a blood transfusion or prior to surgery where a blood transfusion may be required; when a person has a positive DAT following a suspected transfusion reaction or is suspected to have an autoimmune haemolytic anaemia; when a newborn has HDFN.
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?
Antibody identification is used as a follow-up test to a positive indirect antiglobulin test (IAT). The IAT is performed as part of a routine ‘group and screen’ or ‘group and crossmatch’ testing panel in a blood transfusion laboratory. The ‘group and screen’ test is part of a routine antenatal test performed during each pregnancy to identify whether the mother may carry ant red cell antibodies that may affect the foetus. This test is also performed on any individuals who require, or may require, a blood transfusion in the immediate future. The ‘group and screen’ test procedure is used to identify the ABO and Rhesus D blood group and to screen for the presence of any ‘clinically significant’ antibodies, i.e. any antibodies that would react with donor red cells carrying the target antigen and cause haemolysis in the recipient. The IAT used in the ‘group and screen’ involves mixing the patient serum (a cell-free fraction of blood) with screening red cells which are known to carry antigens of all the clinically significant blood groups. Thus the fraction of the patient’s blood which contains antibodies is mixed with foreign red cells (screening cells) to see if there is any antigen-antibody reaction. If this test is positive, i.e. there is an antibody in the patient’s blood that reacts with the screening red cells then the red cell antibody identification test must be performed to identify exactly which antibody/antibodies are present.
If an antibody is identified in a pregnant mother, then the father of the baby is tested to predict whether the foetus is likely to carry an antigen that the mother’s antibody may target causing haemolytic disease of the foetus and neonate (HDFN). The most important antibody causing HDFN is anti-D; however, a screening and prophylactic anti-D administration programme is used in this country to reduce the risk of the mother becoming ‘sensitised’ to the RhD positive foetus and producing any antibodies against the foetus. If a clinically significant antibody is identified in a pregnant woman then the pregnancy will be closely monitored by her healthcare team who will formulate a care plan based on the nature of the antibody, the presence of the antigen in the foetus and the clinical history.
If one or more clinically significant RBC antibodies are identified in a patient who requires a blood transfusion, then donor blood that lacks the corresponding RBC antigens must be used for transfusion. When someone has a condition that requires recurrent transfusions, they are exposed to many foreign RBC antigens and may develop multiple RBC antibodies over time, making the process of finding compatible blood increasingly challenging.
An IAT and antibody identification test may also be used as part of an investigation if a person has a transfusion reaction. Very rarely, an RBC antibody may be present in such a small quantity that it does not cause a positive IAT during pre-transfusion blood compatibility testing. But after the blood is given to the recipient, it can trigger renewed, rapid antibody production and cause a delayed haemolytic transfusion reaction several days later.
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When is it requested?
The antibody identification test may be requested whenever an IAT or a DAT is positive and may be repeated when a person has a transfusion reaction or when a mother has a baby with HDFN.
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What does the test result mean?
When a RBC antibody is identified, it means that an antibody that specifically targets an RBC antigen or antigen group is present in the blood. If the antibody is considered clinically significant, then it will need to be taken into account with each transfusion and/or pregnancy. If it is not considered clinically significant, then it is not likely to cause a transfusion reaction in the patient or haemolytic disease of the foetus and neonate.
Examples of RBC antibodies and their clinical significance are shown in the table below.
Clinically Significant Sometimes Clinically Significant Usually not Significant Not Considered Significant Rh (C, E, c, e) MNS (U, Vw, Mur) Lutheran (Lua, Lub) Chido/Rodgers (Cha, Rga) Kell (K, k, Ku) Vel Lewis (Lea, Leb) JMH Duffy (Fya, Fyb, Fy3) Ge MNS (M, N) Bg Kidd (Jka, Jkb, Jk3) Hy A1 Csa Diego (Dia, Dib, Wra) Yta P1 Xga MNS (S, s) ABO -
Is there anything else I should know?
Some RBC antibodies are naturally occurring; they do not require an initial exposure to the specific targeted antigen during blood transfusion, pregnancy, etc. These include antibodies that correspond to the major A and B red blood cell antigens.
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, platelets, and may sometimes form auto antibodies that target their own red blood cells. Rarely, antibodies in the plasma of the blood donor may target the RBCs of the transfusion recipient.
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 patient's own. These types of antibodies can occur in association with autoimmune disorders, lymphomas and chronic lymphocytic leukaemia, certain viral or mycoplasma infections, and some medications.
An RBC antibody can occasionally be missed with antibody identification testing. There are many RBC antigens and some of them are quite rare. Testing evaluates the most common and clinically significant ones. An example of when this might occur is with a person who receives multiple recurrent transfusions and may have a variety of clinically significant and insignificant RBC antibodies. This is why the crossmatching process is important. It evaluates the compatibility of the donor's red blood cells and recipient's serum for each unit of RBCs transfused (see Blood Banking).
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Should everyone have an IAT performed?
It is not generally necessary unless someone is pregnant or receiving a transfusion. RBC antibodies do not otherwise affect the health of someone who has them. Sometimes a doctor 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. Or they may test a person who has received recurrent transfusions to evaluate which RBC antibodies have developed since the last time the person was tested.
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Do RBC antibodies go away?
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Do RBC antibodies affect blood donation?
They do not affect the safety of the person donating and will not affect the processing of red blood cells for transfusion. If someone has potent RBC antibodies in their plasma, however, then that plasma may not be acceptable for all transfusions.