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Indirect Antiglobulin Test

Also known as: IAT; Indirect Coomb's test; Antibody screen; Indirect Anti-human globulin test
Formal name: Indirect Antiglobulin Test
Related tests: Direct antiglobulin test; Blood typing; Antibody Identification; Type and screen; Crossmatch

At a Glance

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; during pregnancy or at delivery, especially if you are Rh-negative

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?

None

The Test Sample

What is being tested?

The indirect antiglobulin test (IAT) looks for circulating antibodies in the blood directed against red blood cells (RBCs). The primary reasons that you may have RBC antibodies circulating in your blood are because you have been exposed through blood transfusion or through pregnancy to RBCs other than your own (foreign 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. The two major antigens or surface identifiers on human RBCs are the A and B antigens, and your blood is grouped 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 your red blood cells, your blood type is Rh+ (positive); if it is absent, your blood is type Rh- (negative). (For more on these, see the article on Blood Typing). In addition, there are many other types of RBC antigens that make up lesser known blood groups such as Kell, Lewis, and Kidd blood groups.

There are a few reasons why you may begin to produce antibodies against certain 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. Before receiving a blood transfusion, your ABO group and Rh type is matched with that of donor blood to prevent a serious transfusion reaction from occurring. That is, the donor’s blood must be the same ABO group and Rh type as yours so that your antibodies do not react with and destroy donor blood cells. However, if you receive a blood transfusion, your body may recognize antigens from other blood groups (such as Kell or Kidd) that you do not have as foreign. You 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.

With mother-baby blood type incompatibility
A baby may inherit antigens from its father that are not on its mother’s RBCs. 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 hemolytic disease of the newborn, 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 hemolyze them. An IAT can help determine if the mother has produced RBC antibodies.

The first time you are exposed to a foreign RBC antigen, by transfusion or pregnancy, you may begin to produce antibodies but your cells do not usually have the time during the first exposure to make enough antibodies to actually destroy the foreign RBCs. When the next transfusion or pregnancy occurs, the immune response may be strong enough for enough antibodies to be produced, attach to and hemolyze the transfused RBCs or the baby’s RBCs.

The IAT screens for the presence of RBC antibodies (other than ABO antibodies) in your blood. 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.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

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

No test preparation is needed.

The Test

How is it used?

An Indirect Antiglobulin Test (IAT) is used to screen your blood for antibodies directed against red blood cell (RBC) antigens other than the A and B antigens. It 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. Donor’s RBCs and your plasma are mixed and processed to see if there is any clumping of RBCs (agglutination) in the test tube that might indicate an incompatibility that would affect you if the blood was transfused into you. In the case of blood transfusions, RBC antibodies that are present must be taken into account and donor blood must be found that does not contain the antigen(s) to which you have produced antibodies.

If you have an immediate or delayed reaction to a blood transfusion, your doctor will order both an IAT and Direct Antiglobulin Test (DAT) to help investigate the cause of the reaction. (The DAT detects RBC antibodies attached to red cells.) Another IAT may be run after the acute situation has resolved to see if the patient has developed any new antibodies.

During pregnancy, the IAT 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 hemolytic disease of the newborn (HDN). 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. An Rh-negative mother may develop an antibody when she is exposed to blood cells from an Rh-positive fetus. To prevent this, an Rh-negative mother may 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 RBCs that may be present in her bloodstream to prevent the production 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 the test is negative, the mother is given additional RhIG.

This test may rarely be used to help diagnose autoimmune-related hemolytic anemia. 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 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.

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When is it ordered?

An IAT is performed prior to any anticipated blood transfusion and as a follow-up to a transfusion reaction.

Signs and symptoms of a blood transfusion reaction may include:

  • Fever, chills
  • Rash
  • Back pain
  • Bloody urine
  • Feeling faint or dizzy

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.

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What does the test result mean?

If an IAT is positive, then one or more RBC antibodies are present. Some of these antibodies will be more significant than others. When an IAT is used to screen prior to a blood transfusion, a positive IAT indicates the need for an antibody identification test to 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 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 her from producing Rh antibodies.

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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. However, if the patient is exposed to the antigen again, production will kick quickly into gear and attack the RBCs.

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 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.

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Common Questions

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.

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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. However, if the father is positive, then each baby may be Rh-positive and therefore incompatible with an Rh-negative mother.

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3.  I’m blood type O and I have a chance of having a baby with ABO hemolytic disease of the newborn.  Should I have an IAT done while I’m pregnant?

Hemolytic disease of the newborn may occur when there is an ABO incompatibility between mother and baby, especially with mothers who are blood group O. However, the IAT 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.  An IAT is not used to screen in cases of ABO hemolytic disease of the newborn because it is already known that the antibodies are present in the mother’s blood.

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4.  Can I get antibodies from donating blood?

No, you will not be exposed to anyone elses blood while donating.

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5.  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.

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6.  Do I need to tell a new doctor about a prior, uneventful transfusion?

Yes. It is important for your doctor to have that information because there is a chance that you produced antibodies to one or more antigens due to that transfusion. While this will not negatively affect your health, it will tell your doctor to be especially vigilant with any subsequent transfusions or pregnancies.

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Article Sources

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Sources Used in Current Review

Pagana, Kathleen D. & Pagana, Timothy J. (© 2007). Mosby’s Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO.pp 307-308.

Wu, A. (2006).  Tietz Clinical Guide to Laboratory Tests, Fourth Edition. Saunders Elsevier, St. Louis, Missouri. Pp 126-129.

Cutler, C. (2006 September 11, Updated). Coombs’ test. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003344.htm. Accessed on 10/01/08.

Sandler, S.G. and Johnson, V. (2008 September 25, Updated). Transfusion Reactions. EMedicine [On-line information]. Available online at http://www.emedicine.com/med/TOPIC2297.HTM through http://www.emedicine.com. Accessed on 10/01/08.

Levin, M. (2007 March 13, Updated). Transfusion Reaction. MedlinePlus Medical Encyclopedia. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/001303.htm. Accessed on 10/04/08.

(© 2008) Hemolytic Disease of the Newborn. Lucille Packard Children’s Hospital at Stanford. Available online at http://www.lpch.org/DiseaseHealthInfo/HealthLibrary/hrnewborn/hdn.html through http://www.lpch.org. Accessed October 2008.

Sources Used in Previous Reviews

Thomas, Clayton L., Editor (1997). Taber’s Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].

Pagana, Kathleen D. & Pagana, Timothy J. (2001). Mosby’s Diagnostic and Laboratory Test Reference 5th Edition: Mosby, Inc., Saint Louis, MO. Pgs 286-289.

Dhaliwal, G. et. al. (2004 June 1). Hemolytic Anemia. American Family Physician [On-line journal]. Available online at http://www.aafp.org/afp/20040601/2599.html through http://www.aafp.org.

Triulzi, D. (2000 October). Indirect and Direct Antiglobulin (Coombs) Testing and the Crossmatch. Transfusion Medicine Update [On-line information]. Available online at http://www.itxm.org/TMU2000/tmu10-2000.htm through http://www.itxm.org

Grund, S., Updated (2004 August 16, Updated). Coombs’ test – direct. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003344.htm.

Grund, S., Updated (2004 August 16, Updated). Coombs’ test – indirect. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003343.htm.

(2001 March).Rh Disease. March of Dimes Fact Sheet [On-line information]. Available online at http://www.marchofdimes.com/professionals/681_1220.asp through http://www.marchofdimes.com/.

(1995-2005). Autoimmune Hemolytic Anemia. The Merck Manual of Diagnosis and Therapy. Anemias Caused By Excessive Hemolysis. [On-line information]. Available online at http://www.merck.com/mrkshared/mmanual/section11/chapter127/127d.jsp through http://www.merck.com.

Suzanne H. Butch, MA, CLDir. Chief Technologist. Blood Bank and Transfusion Service. University of Michigan Hospitals and Health Centers, Ann Arbor, Michigan.

Julie Brownie MBA, CLS(NCA), SBB(ASCP). Coral Blood Services. Bangor, Maine.