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Direct Antiglobulin Test

Also known as: DAT; Direct Coomb's test; Direct Anti-human Globulin test
Formal name: Direct Antiglobulin Test
Related tests: Indirect antiglobulin test; Blood typing; Antibody identification

At a Glance

Why Get Tested?

To help diagnose hemolytic anemia caused by autoimmune disease or induced by drugs; to investigate a transfusion reaction; to diagnose hemolytic disease of the newborn

When to Get Tested?

When you have symptoms suggesting hemolytic anemia, such as unusual tiredness, unexpected difficulty breathing when exercising, and unusually dark urine, or when your hemoglobin and/or hematocrit are unexpectedly low, or when you have had a blood transfusion recently and are experiencing any of the above symptoms; when a newborn shows signs of hemolytic disease of the newborn

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?

None

The Test Sample

What is being tested?

The direct antiglobulin test (DAT) looks for antibodies attached to red blood cells (RBCs) circulating in your bloodstream. RBCs have structures on their surfaces 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 call 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 antibodies may become attached to antigens on RBCs:

With autoimmune diseases and other conditions
Some people make antibodies directed against their own RBC antigens. These autoantibodies may be produced in autoimmune diseases and with some other conditions, such as lymphoma and chronic lymphocytic leukemia. Sometimes, drugs may coat the surface of RBCs, causing antibodies to react with the RBCs.

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 to the foreign antigens on her baby’s RBCs during pregnancy or at delivery 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. A DAT performed on the baby’s blood will determine if a mother’s antibodies have attached to a baby’s RBCs.

Following a blood transfusion
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 that you do not have as foreign (such as Kell or Kidd blood groups). You then may produce antibodies, and they may become attached to these antigens on the donor RBCs circulating in your bloodstream. People who have many transfusions are more likely to make antibodies to RBCs because they are exposed to more foreign RBC antigens. A DAT will determine if you have produced antibodies against RBC antigens and if those antibodies have attached to transfused donor RBCs.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into 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?

The direct antiglobulin test (DAT) is used primarily to help determine if the cause of hemolytic anemia, a condition in which red blood cells (RBCs) are being destroyed more quickly than they can be replaced, is due to antibodies attached to RBCs. This may occur in autoimmune-related hemolytic anemias, which may be caused by a person producing antibodies against their own RBC antigens (autoantibodies). Examples of this include autoimmune disorders such as systemic lupus erythematosus, malignant diseases such as lymphoma and chronic lymphocytic leukemia, and infections such as mycoplasma pneumonia and mononucleosis. It can also occur in some people with the use of certain medications, such as penicillin.

A DAT may be used to help diagnose hemolytic disease of the newborn (HDN) due to an incompatibility between the blood types of a mother and baby. When a baby is born, the mother may be exposed to the foreign antigens on the baby’s RBCs and may produce antibodies directed against the baby’s RBC antigens. This may occur when there is a baby-mother Rh incompatibility. Formerly, this was the most common cause of hemolytic disease of the newborn, but this condition is now rare due to preventative treatments given to the mother during and after each pregnancy. The most common cause of hemolytic disease of the newborn is an ABO incompatibility between mother and baby, especially with mothers that are blood group O.

A DAT may also be used to investigate a suspected transfusion reaction. If you are being transfused and have a fever or other significant symptoms suggesting a potential for a hemolytic transfusion reaction, a DAT is done to determine if you have made an antibody to the transfused RBCs. If the antibody is found coating the RBCs, then the RBCs may be destroyed (hemolyzed) or be removed from your circulation faster than normal.

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When is it ordered?

The DAT may be ordered when you are experiencing symptoms suggesting hemolytic anemia, such as:

  • Fatigue
  • Dark urine
  • Back pain
  • Jaundice
  • Paleness
  • An enlarged spleen

This test may be ordered when a newborn exhibits signs of hemolytic disease of the newborn, including:

  • Pale appearance
  • Jaundice
  • Enlarged liver or spleen
  • Swelling of the entire body
  • Difficulty breathing

A DAT may be ordered when there are signs and symptoms of a blood transfusion reaction, such as:

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

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What does the test result mean?

If the DAT is positive, then there are antibodies attached to the RBCs. In general, the stronger the DAT reaction (the more positive the test), the greater the amount of antibody bound to the RBCs. The DAT detects the presence of the antibody, but it does not tell the doctor the cause or exact type of antibody. A patient’s medical history and a clinical examination is needed to determine if a positive DAT is due to a transfusion reaction, autoimmune reaction, an infection, a medication, or a baby-mother Rh incompatibility. A small percentage of the normal population will be DAT positive and not experience hemolytic anemia.

If a DAT is negative, then antibodies are not attached to RBCs and the signs and symptoms may be due to some other cause.

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Is there anything else I should know?

If a DAT is positive due to a transfusion reaction, an infection, or drug, it will remain positive for 48 hours to 3 months. If it is positive due to an autoimmune condition, it will often be chronically positive.

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Common Questions

1.  Can I get antibodies from donating blood?

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

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2.  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 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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3.  If a mother has an Rh-incompatibility with one child, will she have them with all of her children? 

It depends on each baby’s Rh type.  Rh status is part of a person’s blood type.  A person will have a blood type “O,” “A,” “B,” or “AB” and be either “Rh-positive” or “Rh-negative” – referred to as positive or negative.  An example of this is “O positive” or “O negative.” When a mother is Rh-negative, she may develop antibodies against the red blood cells of her first Rh-positive child.  Any subsequent Rh-positive children may then be affected by the mother’s Rh antibodies.  Fortunately, this is now relatively rare as Rh-negative mothers are tested during and after their pregnancy and are given RhIg (RhImmune Globulin) injections to prevent the development of Rh antibodies.

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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.

Wagle, S. and Deshpande, P. (2008 September 2, Updated). Hemolytic Disease of the Newborn. eMedicine [On-line information]. Available online at http://www.emedicine.com/ped/TOPIC959.HTM through http://www.emedicine.com. Accessed on:  10/01/08.

Direct Antiglobulin Test (DAT) Information Sheet. United Blood Services [On-line information]. PDF available for download at http://www.unitedbloodservices.org/forms/BS_950.pdf through http://www.unitedbloodservices.org. Accessed on 10/01/08.

(2005 November, Revised). Autoimmune Hemolytic Anemia. Merck Manual for Healthcare Professionals [On-line information]. Available online at http://www.merck.com/mmpe/print/sec11/ch131/ch131b.html through http://www.merck.com. Accessed on 10/01/08.

(© 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.