Direct Antiglobulin Test

Also known as: DAT; Direct Coomb's test; Direct Anti-human Globulin test

Formal name: Direct Antiglobulin Test

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.