At a Glance
Why Get Tested?
When to Get Tested?
When your doctor wants to find out the cause of your hemolytic anemia; when you have had a blood transfusion recently and are experiencing symptoms of a tranfusion reaction; or when a newborn shows signs of hemolytic disease of the newborn
A blood sample drawn from a vein in your arm
Test Preparation Needed?
The Test Sample
What is being tested?
The direct antiglobulin test (DAT) looks for antibodies attached to red blood cells (RBCs) circulating in the bloodstream. RBCs have structures on their surfaces called antigens. Each person has their own individual set of RBC antigens, 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 the D antigen in the Rh blood group system. If it is present on someone's red blood cells, that person's 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 but still clinically significant blood groups, such as Kell, Duffy, and Kidd.
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/or with some other conditions, such as lymphoma and chronic lymphocytic leukemia.
- With drug-induced anemia: Certain drugs can induce antibodies against red blood cell antigens and therefore cause hemolysis even without the presence of the drug. Sometimes, drugs may coat the surface of RBCs, causing antibodies to react with the RBCs. (This is relatively rare (about 1:1 million).) The drugs can induce antibodies to both the drug and the RBC itself, resulting in destruction of the RBC in the presence of drugs. This is seen with some antibiotics, such as IV penicillin, cephalosporins and pipercillin. Be sure to tell your doctor about any drugs you have been taking recently. If the doctor suspects drug-induced autoimmune anemia, the suspect medication will be discontinued. Symptoms typically resolve promptly after the drug is discontinued.
- 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. However, any baby may be affected by hemolytic disease of the newborn caused by antibodies to the ABO system. This generally is mild, which is fortunate, as it is the leading cause of maternal antibodies attaching to fetal RBCs today.
A mother will be screened for antibodies during pregnancy and again at delivery. A DAT performed on the blood of a baby born to an at-risk mother will determine if its mother's antibodies have attached to the baby's RBCs.
- Following a blood transfusion: Before receiving a blood transfusion, a person's ABO group and Rh type is matched with that of the donor blood to prevent a serious transfusion reaction from occurring. That is, the donor's blood must be compatible with the ABO group and Rh type of the person receiving the blood so that the recipient's antibodies do not react with and destroy the donor red blood cells.
If someone receives a blood transfusion, their body may also recognize other RBC antigens that it does not have, such as those from other blood groups (such as the Kell or Kidd blood groups), as foreign. The recipient may produce antibodies and they may become attached to these foreign antigens on the donor RBCs circulating in the bloodstream. People who have many transfusions are more likely to make antibodies to RBCs because they are exposed to more foreign RBC antigens. If someone shown symptoms of a reaction after transfusion, a A DAT will be performed to determine if those antibodies have attached to the transfused donor RBCs.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into 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.
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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
(© 1995-2012). Test ID: DCTM9008 Direct Coombs, Blood. Mayo Clinic Mayo Medical Laboratories [On-line information]. Available online at http://www.mayomedicallaboratories.com/test-catalog/Overview/9008 through http://www.mayomedicallaboratories.com. Accessed July 2012.
Devkota, B. (Updated 2012 May 25). Direct Antiglobulin Test. Medscape Reference [On-line information]. Available online at http://emedicine.medscape.com/article/2105623-overview#showall through http://emedicine.medscape.com. Accessed July 2012.
Wagle, S. and Deshpande, P. (Updated 2011 May 18). Hemolytic Disease of Newborn. Medscape Reference [On-line information]. Available online at http://emedicine.medscape.com/article/974349-overview through http://emedicine.medscape.com. Accessed July 2012.
Vorvick, L. (Updated 2012 February 7). Coombs' test. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003344.htm. Accessed July 2012.
Zieve, D. and Eltz, D. (Updated 2011 August 30). Rh incompatibility. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/001600.htm. Accessed July 2012.
(© 1996-2012). Rh Disease. The Children's Hospital of Philadelphia [On-line information]. Available online at http://www.chop.edu/healthinfo/rh-disease.html through http://www.chop.edu 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 307-308.
AABB Technical Manual, Pg 512. AABB, Bethesda, MD.
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.
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.