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At A Glance
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Why get tested?To detect antibodies directed against red blood cell antigens, in preparation for a blood transfusion, or during pregnancy and at delivery When to get tested?When preparing for a blood transfusion or during pregnancy or at delivery, especially if you are Rh negative Sample required?A blood sample drawn from a vein in your arm
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The Test Sample
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What is being tested?The indirect antiglobulin test (IAT) looks for circulating antibodies directed against red blood cells (RBCs). RBCs normally have structures on their surface called antigens. You have your own individual set of antigens on your RBCs, determined by inheritance from your parents. If you have a blood transfusion, your body will recognize antigens that you do not have as foreign. Your plasma cells then may produce antibodies to attack these foreign antigens. People who have many transfusions are more likely to make antibodies to RBCs because they are exposed to more foreign RBC antigens. When a baby inherits antigens from its father that are not on its mother’s RBCs, the mother can produce antibodies against the foreign antigens on her baby’s RBCs inside the womb. This can cause hemolytic disease of the newborn, usually not affecting the first baby but subsequent children.
The IAT detects the presence of these circulating RBC antibodies in the plasma (fluid portion of the blood). The first time a person is exposed to a foreign antigen, by transfusion or pregnancy, their cells may recognize the need to produce an antibody directed against the foreign RBCs but does not usually have the time to make enough antibody to actually destroy the foreign cells. When the next transfusion or pregnancy occurs, the antibody may be strong enough to attach to the transfused RBCs or, if a woman is pregnant, to her baby’s RBCs, causing the red cells to have a shortened survival.
RBC antibodies that are detected with the IAT can be identified with an antibody identification test (see Blood Banking for more information).
How is the sample collected for testing?
A blood sample is drawn with a needle from a vein in your arm.
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The Test
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During pregnancy, the IAT is used to screen for antibodies that might cross the placenta and attack the baby’s red cells, causing hemolytic disease of the newborn. The most serious cause is antibody in a blood group system called the Rh system. An Rh negative mother will have an IAT 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 fetal RBC from the bloodstream to prevent the creation of Rh antibodies by the mother.
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, then another IAT test will be performed on the mother. If it is negative, an additional test (Kleihauer-Betke test) determines the number of fetal RBCs present in the mother’s blood and is used to determine how much RhIg is needed to prevent antibody production.
Immune-related hemolytic anemia also may be caused when a person produces antibodies against their own antigens. This can happen with some autoimmune disorders (such as systemic lupus erythematosus), with diseases such as chronic lymphocytic leukemia, and with infections such as mycoplasma pneumonia and mononucleosis. It can also occur in some people with the use of certain medications, such as penicillin, and in rare cases can be triggered by exposure to cold. If a person is experiencing symptoms suggesting hemolytic anemia, such as fatigue, dark urine, back pain, jaundice, paleness, or an enlarged spleen, then a doctor may order both a DAT and an IAT to help determine whether an RBC antibody is present.
An IAT test is performed as part of a “type 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 IAT is performed. The donor’s RBCs and the patient’s plasma are mixed and processed to see if there is any agglutination (clumping of RBCs) in the test tube that might indicate an incompatibility that would affect the patient if the blood was transfused to them. 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) that the body has become sensitized (produced antibodies) to.
Each blood transfusion that a person has exposes them to the combination of antigens on the donor’s RBCs. Whenever the transfused RBCs contain antigens foreign to the recipient’s RBCs, there is the potential to create 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.
If someone has an immediate or delayed reaction to a blood transfusion, the doctor will order both an IAT and DAT to help investigate the cause of the reaction. Another IAT may be run after the acute situation has resolved to see if the patient has developed any new antibodies.
When is it ordered?
An IAT is performed prior to any anticipated blood transfusion and as a follow-up to a transfusion reaction.
An IAT is performed as part of every woman’s pregnancy workup. In Rh-negative women, it is also done at 28 weeks, prior to giving an RhIg injection, and after delivery if the baby is found to be Rh positive. In Rh-negative pregnant women with known Rh antibodies, the IAT is sometimes ordered as a monitoring tool to roughly track the amount of antibody present.
What does the test result mean?
If an IAT is positive, then one or more antibodies are present. Some of these antibodies will be more significant than others. Part of the IAT test is conducted at room temperature and part at body temperature (37 degrees Celsius). The strength of the reaction and where it occurs in the testing process can give clues to the laboratorian about what antibodies may be present and their probable significance. A positive IAT indicates the need for an antibody identification test to see which antibodies are present.
If an Rh-negative mom has a negative IAT, then it is safe for a short window of time (72 hours) to give an RhIg injection to prevent antibody production. If she has a positive IAT, then the antibody or antibodies present must be identified. If there is an Rh antibody present, then the RhIg injection is not useful. If she has a different antibody, then the RhIg injection can still be given to prevent the Rh antibody.
Is there anything else I should know?
A circulating RBC antibody, once present, will never truly go away. If it has been many years since antigen exposure, circulating antibody levels may drop to undetectable levels. If the patient is exposed to the antigen again, however, production will kick quickly into gear and attack the RBCs.
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Common Questions
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1. What happened before the RhIg (RhImmune Globulin) injection was developed?
Prior to development of the injection, Rh-negative mothers would often become sensitized 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 injection has largely prevented these complications, although a small percent of women do still develop Rh antibodies.
2. Why would a blood typing be necessary on an Rh-negative woman’s husband?
If the woman’s husband is Rh negative, then all of their babies will be Rh negative and there will not be an Rh incompatibility. If the father is positive, then each baby may be either Rh positive or negative.
3. Can I get antibodies from donating blood?
No, you will not be exposed to anyone else’s blood while donating.
4. If I give my own blood prior to surgery (autologous donation) and receive my own blood back, do I need to worry about antibodies? ![]()
No, since you will not be exposed to foreign RBC antigens, your body will not be stimulated to produce RBC antibodies.
5. Do I need to tell a new doctor about an old, uneventful transfusion?
Yes. It is important for your doctor to have that information because there is a chance that you became sensitized to one or more antigens due to that transfusion. While this will not negatively affect your health, it will tell you doctor to be especially vigilant with any subsequent transfusions.
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Ask A Question
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If you still have a question about your test or need help
interpreting the results of your test, you can visit the ASCLS web site to
complete a lab testing information request form, and a certified clinical
laboratory scientist will gladly help you! Your communication will be kept
confidential. Go there now: http://www.ascls.org/labtesting/disclaimer.asp.
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