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 blood cell antibody identification is performed when a DAT or IAT antibody screening test is positive. Antibody screening tests are routinely performed in advance of a person receiving a blood transfusion as part of a ‘group and screen’ and they are also performed for all pregnant women as part of routine antenatal test. A positive DAT could indicate a possible transfusion reaction, autoimmune haemolytic anaemia (AIHA) or HDFN.
Antibody identification tests are performed on whole blood samples that are collected in EDTA anticoagulant. The blood sample is typically obtained by ‘venepuncture’ via a vein in the arm using a needle.
No test preparation is needed.
Antibody identification tests determine the type of red blood cell antibodies that are present in a person’s blood. Antibodies are produced by the body following exposure to foreign or ‘non-self’ red blood cells. These antibodies can bind to antigens which are present on the surface of foreign red blood cells causing clumping, or agglutination, of the cells. Exposure to non-self-antigens can occur if a person receives a blood transfusion or during pregnancy when the maternal and foetal blood mix. It is important for the blood transfusion laboratory to know exactly which antibodies are present in a person’s blood so that compatible blood products can be selected for transfusion. Donor blood selected for transfusion must be free of the red blood cell antigen to which a person has an antibody.
. 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, stimulating 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 RhD antigens that determine a person’s basic blood type (for more on this, see Blood Type and Blood Banking).
The ABO blood group system is the most important for blood transfusion because our bodies naturally produce antibodies against A and B antigens, depending on whether or not they are present on our own red blood cells. A person who is blood group A will have A antigens on their own red cells and they will produce anti-B antibodies; a person who is blood group B will have B antigens on their own red cells and they will produce anti-A antibodies; a person of blood group O does not express A or B antigens on their own red cells and thus they produce both anti-A and anti-B antibodies; a person with blood group AB has both A and B antigens on the surface of their red blood cells and therefore they do not produce antibodies to either A or B antigens. These antibodies are naturally produced and they develop by the age of 6 months. Antibodies to group A and B antigens are incredibly potent and they have the potential to cause rapid destruction of red blood cells should they encounter the corresponding antigen on a non-self red cell, known as haemolysis (intravascular haemolysis).
|Blood group||Naturally occurring RBC antibodies that can be found in the blood|
|O||Anti-A and anti-B|
The Rh blood group is also of importance to transfusion scientists. If an individual has RhD antigens on the surface of their red blood cells they are RhD positive, if they do not, they are RhD negative. A person’s blood group is therefore a combination of their ABO and RhD antigen status, e.g. A+, B-, O+, etc. The body does not naturally produce antibodies to the RhD antigen in RhD negative individuals, an immunising event must occur in which the person is exposed to these non-self antigens. Akin to anti-A and anti-B antibodies, RhD antibodies are capable of destroying red cells and causing haemolysis. It is estimated that an RhD negative person needs to be exposed to only one or two drops of RhD positive blood to trigger the production of anti-D antibodies.
A person will not usually develop antibodies against the blood group antigens present on their own cells but may develop antibodies against other blood group antigens that they are exposed to, e.g. if they receive blood from another individual during a blood transfusion. This is the reason why a ‘group and screen’ procedure is performed to identify the blood group and screen for the presence of any antibodies in blood during the blood transfusion screening process. If antibodies to red cells are identified during this process the red cell antibody identification test is performed to identify exactly which antibodies are present and allow for screening of donor blood and selection of compatible units (i.e. antigen negative donor red cells) for cross-matching.
It is important that transfusion laboratories identify red blood cell antibodies due to the potentially serious consequences of incompatible transfusion and haemolysis. Should a person with red cell antibodies be exposed to non-self red cells with the corresponding antigen, antibody will bind these cells, targeting them for destruction. Haemolysis can occur within the blood vessels or in the liver or spleen and cause symptoms such as fever, chills, nausea, flank pain, low blood pressure, bloody urine, and jaundice. Depending on the specific antibody-antigen reaction and the quantity of red cells that are affected, the reaction can range from mild to severe and potentially life-threatening. Such reactions may occur immediately following exposure to foreign red cell antigens, such as during a blood transfusion, or it may be delayed occurring several days after a transfusion (delayed haemolytic transfusion reaction).
Specific antibody identification tests that characterise antibodies directed against the minor blood group antigens are not routinely done on every patient, but these are performed when the presence of an antibody is detected through a positive IAT using a mixed panel of cells carrying all clinically significant antigens, or when antibodies are detected by a positive DAT.
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 any 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 determine the ABO and RhD 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, causing haemolysis in the recipient. The IAT used in the ‘group and screen’ involves mixing the patient serum / plasma (a cell-free fraction of blood) with commercial 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 one or more clinically significant red cell antibodies are identified in a patient requiring a blood transfusion then compatible antigen-negative donor blood units must be selected for transfusion. This can cause complications for individuals who are transfusion dependent as repeat exposure to donor red cells increases their exposure to foreign antigens, increasing their likelihood of developing antibodies. In turn, the process of finding compatible blood becomes increasingly challenging for such individuals.
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.
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 foetal red cells. 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. However, the gradual implementation of non-invasive pre-natal testing for RhD genotype will help to reduce the administration of unnecessary prophylactic anti-D in pregnant women.
When is it requested?
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 red cell 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?
Anti-A and anti-B antibodies are naturally occurring, they do not require an initial exposure to the target antigen during blood transfusion, pregnancy, etc. All other blood group antigens must be encountered by an individual’s immune system and recognised as foreign in order for antibodies to be produced.
Transfusion reactions are not exclusively caused by red cell antibodies, the recipient’s immune system can also be triggered when exposed to donor white blood cells and platelets. Very occasionally, antibodies in the plasma of the blood donor may target the red cells of the transfusion recipient. Additionally, a person may develop auto-antibodies that target their own red cell antigens.
Some antibodies may not target a specific red cell antigen but may react with a broad range of red blood cell antigen types, including the patient's own. Such antibodies can occur in association with autoimmune disorders, lymphomas and chronic lymphocytic leukaemia, certain viral or mycoplasma infections, and some medications.
Red cell antibodies 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 red cells transfused (see Blood Banking).
Should everyone have an IAT performed?
It is not generally necessary unless someone is pregnant or receiving a transfusion. Red cell 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. They may also test a person who has received multiple transfusions to evaluate whether red cell antibodies have developed since the last time they were tested.
Do RBC antibodies go away?
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 red cell antibodies in their plasma, however, then that plasma may not be acceptable for all transfusions.