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RBC Antibody Screen

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Also known as: Indirect Antiglobulin Test; IAT; Indirect Coombs Test; Indirect Anti-human Globulin Test; Antibody Screen
Formal name: Red Blood Cell Antibody Screen
Related tests: Direct Antiglobulin Test; Blood Typing; RBC Antibody Identification; Type and Screen; Crossmatch

At a Glance

Why Get Tested?

To detect antibodies directed against red blood cell antigens

When to Get Tested?

When preparing for a blood transfusion; during pregnancy and at delivery

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?


The Test Sample

What is being tested?

The RBC antibody screen looks for circulating antibodies in the blood directed against red blood cells (RBCs). The primary reason that a person may have RBC antibodies circulating in their blood is because they have been exposed, through blood transfusion or through pregnancy, to RBCs other than their own (foreign RBCs).

RBCs normally have structures on their surface called antigens. Each person has their own individual set of antigens on their RBCs, determined by inheritance from their parents. The major antigens or surface identifiers on human RBCs are the O, A, and B antigens, and a person's blood is grouped into an A, B, AB, or O blood type 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 a person's red blood cells, their blood type is Rh+ (positive); if it is absent, the blood is type Rh- (negative). (For more on these antigens, 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 someone may produce antibodies against 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, a person's ABO group and Rh type are matched with that of donor blood to prevent a serious transfusion reaction from occurring. That is, the donor's blood must be compatible with the recipient's so that their antibodies do not react with and destroy donor blood cells.

    If someone receives a blood transfusion, their body may also recognize other RBC antigens from other blood groups (such as Kell or Kidd) that they do not have as foreign. The recipient may produce antibodies to attack these foreign antigens. People who have many transfusions make antibodies to RBCs because they are exposed to foreign RBC antigens with each transfusion.

  • With fetal-maternal 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 outside of the ABO blood group.

The first time a person is exposed to a foreign RBC antigen, by transfusion or pregnancy, they may begin to produce antibodies but their 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. Antibodies to the ABO antigens are naturally-occurring so do not require exposure to foreign RBCs. 

The RBC antibody test screens for the presence of RBC antibodies (other than ABO antibodies) in the blood. RBC antibodies that are detected can be identified with an antibody identification test.

How is the sample collected for testing?

A blood sample is drawn with a needle from a vein in the 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

Common Questions

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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

Wagle, S. and Deshpande, P. (Updated 2011 May 18). Hemolytic Disease of Newborn. Medscape Reference [On-line information]. Available online at through Accessed July 2012.

Vorvick, L. (Updated 2012 February 7). Coombs’ test. MedlinePlus Medical Encyclopedia [On-line information]. Available online at Accessed July 2012.

(© 1996-2012). Rh Disease. The Children’s Hospital of Philadelphia [On-line information]. Available online at through Accessed July 2012.

Pagana, K. D. & Pagana, T. J. (© 2011). Mosby's Diagnostic and Laboratory Test Reference 10th Edition: Mosby, Inc., Saint Louis, MO. Pp 309-310.

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 through

Triulzi, D. (2000 October). Indirect and Direct Antiglobulin (Coombs) Testing and the Crossmatch. Transfusion Medicine Update [On-line information]. Available online at through

Grund, S., Updated (2004 August 16, Updated). Coombs' test – direct. MedlinePlus Medical Encyclopedia [On-line information]. Available online at

Grund, S., Updated (2004 August 16, Updated). Coombs' test – indirect. MedlinePlus Medical Encyclopedia [On-line information]. Available online at

(2001 March).Rh Disease. March of Dimes Fact Sheet [On-line information]. Available online at through

(1995-2005). Autoimmune Hemolytic Anemia. The Merck Manual of Diagnosis and Therapy. Anemias Caused By Excessive Hemolysis. [On-line information]. Available online at through

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.

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 Accessed on 10/01/08.

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

Levin, M. (2007 March 13, Updated). Transfusion Reaction. MedlinePlus Medical Encyclopedia. Available online at Accessed on 10/04/08.

(© 2008) Hemolytic Disease of the Newborn. Lucille Packard Children's Hospital at Stanford. Available online at through Accessed October 2008.