Also Known As
Alloantibody Identification
Antibody ID, RBC
This article was last reviewed on
This article waslast modified on 30 January 2020.
At a Glance
Why Get Tested?

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)

When To Get Tested?

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.

Sample Required?

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’.

Test Preparation Needed?


On average it takes 7 working days for the blood test results to come back from the hospital, depending on the exact tests requested. Some specialist test results may take longer, if samples have to be sent to a reference (specialist) laboratory. The X-ray & scan results may take longer. If you are registered to use the online services of your local practice, you may be able to access your results online. Your GP practice will be able to provide specific details.

If the doctor wants to see you about the result(s), you will be offered an appointment. If you are concerned about your test results, you will need to arrange an appointment with your doctor so that all relevant information including age, ethnicity, health history, signs and symptoms, laboratory and other procedures (radiology, endoscopy, etc.), can be considered.

Lab Tests Online-UK is an educational website designed to provide patients and carers with information on laboratory tests used in medical care. We are not a laboratory and are unable to comment on an individual's health and treatment.

Reference ranges are dependent on many factors, including patient age, sex, sample population, and test method, and numeric test results can have different meanings in different laboratories.

For these reasons, you will not find reference ranges for the majority of tests described on this web site. The lab report containing your test results should include the relevant reference range for your test(s). Please consult your doctor or the laboratory that performed the test(s) to obtain the reference range if you do not have the lab report.

For more information on reference ranges, please read Reference Ranges and What They Mean.

What is being tested?

This test is designed to identify red cell antibodies that are present in the blood. A person may develop antibodies that react with antigens on red blood cells if they are exposed to ‘foreign’ red blood cells (e.g. through blood transfusion or during pregnancy). The red blood cell antibody identification test allows the laboratory to determine exactly which antibodies are present in the blood of a patient and this information can then be used to ensure that if they require blood or blood products, only those from a suitable donor (i.e. someone who does not have antigens on their cells that would react with the patient’s antibodies) are used.

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 ABO blood group system is the most important blood group system in 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 produces anti-B antibodies; a person with blood group B has B antigens on their red cells and produces anti-A antigens; a person of blood group O does not express A or B antigens on their own red cells and produces 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 does not produce antibodies to either of these. These naturally occurring antibodies are present in almost all by the age of 6 months and are very potent; if a person of blood group O was exposed to blood group A or B red cells, their antibodies would rapidly attack and destroy the foreign red cells. This destruction of red cells is called haemolysis.

Blood group Naturally occurring RBC antibodies that can be found in the blood
O Anti-A and anti-B
A Anti-B
B Anti-A
AB None

The second most important blood group system in blood transfusion is the Rhesus blood group system. If the Rhesus D antigen is present then this makes an individual Rh D positive; this is combined with the ABO blood group to describe an individuals’ basic blood group, e.g. A+, B-, O+, etc. The Rh D antigen is very important because, although antibodies against Rh D do not occur naturally in individuals who are Rh D negative, if these individuals are exposed to even a small amount of Rh D antigen there is a potent immune response with production of anti-D antibodies which are also capable of destroying red cells and causing haemolysis. It is estimated that a Rh D negative person needs to be exposed to only one or two drops of Rh D positive blood to trigger the production of anti-D antibodies.

The pattern of red cell antigens and therefore the blood group of an individual will be determined by their genetic make-up and so blood groups are inherited traits. 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. These other blood group systems are not associated with naturally occurring antibodies; as for the Rh system a person must be exposed to red cells carrying the foreign antigen before they mount an immune response producing 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 only matched units (i.e. antigen negative donor red cells) for cross-matching.

The presence of red cell antibodies may also be detected by a positive direct antiglobulin test (DAT).

It is very important to identify the presence of red cell antibodies. If a person with red cell antibodies is exposed to red cells which carry the antigenic target then antibodies will attach to the red cell antigen on the surface of the cells, coating the red cell and targeting it for destruction (haemolysis). Depending on the specific antibody-antigen reaction and the quantity of red cells that are affected, this can cause a reaction ranging from mild to severe and potentially life-threatening. Depending on the antigen and antibody involved and the quantity of RBCs affected, this can cause a reaction ranging from mild to severe and potentially life-threatening. It may happen immediately, such as during a blood transfusion, or take longer, from one to several days following a transfusion. When antibodies attach to antigens, the red blood cells can be destroyed, termed haemolysis. This can occur within the blood vessels or in the liver or spleen and cause symptoms and signs such as fever, chills, nausea, flank pain, low blood pressure, bloody urine, and jaundice.

Specific antibody identification tests that characterise antibodies directed against the minor red cell 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 the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

Accordion Title
Common Questions
  • 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.

  • 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.

  • 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)      
  • 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).

  • 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.

  • Do RBC antibodies go away?

    No. They may drop to low levels in the blood but once you have them, your body will keep a "library" of them so that they can be produced as needed.

  • 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.