To help diagnose the cause of hemolytic anemia as caused by autoimmune disease or induced by drugs; to investigate a blood transfusion reaction; to diagnose hemolytic disease of the newborn
Direct Antiglobulin Test
Direct Antiglobulin Test
Also Known As
DAT
Direct Coombs Test
Direct Anti-human Globulin Test
Formal Name
Direct Antiglobulin Test
This article was last reviewed
on
This article waslast modified
on November 1, 2017.
At a Glance
Why Get Tested?
When To Get Tested?
When your healthcare provider wants to find out the cause of your hemolytic anemia; when you have had a blood transfusion recently and are experiencing symptoms of a transfusion reaction; or 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
Accordion Title
Common Questions
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How is it used?
The direct antiglobulin test (DAT), also known as the direct Coombs test, is used primarily to help determine whether the cause of hemolytic anemia, a condition in which red blood cells (RBCs) are destroyed more quickly than they can be replaced, is due to antibodies attached to RBCs. This may occur in autoimmune-related hemolytic anemias, which are 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
- Infections such as mycoplasma pneumonia and mononucleosis
- With the use of certain medications, such as penicillin
A DAT may also 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 an Rh-positive baby is born to an Rh-negative mother. Formerly, antibodies to the Rh antigen were the most common cause of hemolytic disease of the newborn, but this condition is now rare due to preventive treatments given to the mother during and after each pregnancy. The most common cause of hemolytic disease of the newborn nowadays is an ABO incompatibility between a Group O mother and her baby. This type of fetal-maternal incompatibility is generally mild.
A DAT may also be used to investigate a suspected transfusion reaction. If a person being given blood develops a fever or other significant symptoms suggesting a potential for a hemolytic transfusion reaction, a DAT is done to determine if the person has made an antibody that has attached to the transfused RBCs. If the antibody is found coating the RBCs, then the RBCs may be destroyed (hemolyzed) or be removed from circulation faster than normal.
For more on these, see the "What is being tested?" section.
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When is it ordered?
The DAT may be ordered when someone has hemolytic anemia and the healthcare provider wants to determine the cause.
This test may be ordered when a newborn is born to an at-risk mother or exhibits signs of hemolytic disease of the newborn, in the absence of other causes of symptoms that may include:
- Pale appearance
- Jaundice, including elevated bilirubin
- Enlarged liver or spleen
- Swelling of the entire body
- Difficulty breathing
A DAT will be ordered when there are signs and symptoms of a blood transfusion reaction, such as:
- Fever, chills
- Back pain
- Bloody urine
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What does the test result mean?
A positive DAT means that 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, but this does not always equate to the severity of symptoms, especially if the RBCs have already been destroyed.
The DAT detects the presence of the antibody, but it does not tell the healthcare provider the cause or exact type of antibody or if it is causing the symptoms. A person'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 blood group incompatibility. A small percentage of the normal population will be DAT-positive and not experience hemolytic anemia.
A negative DAT means that antibodies are most likely not attached to RBCs and the signs and symptoms are due to another cause that requires further investigation.
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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 may be positive over a long period of time (chronically).
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Can I get antibodies from donating blood?
No, you will not be exposed to anyone else's blood while donating.
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If a mother has an incompatibility with one child, will she have them with all of her children?
It depends on whether the baby has the corresponding antigens for the mother's antibodies. A baby born to a blood group O mother may have hemolytic disease of the newborn in any pregnancy. When a mother is Rh-negative, she may develop antibodies against the red blood cells of her first Rh-positive child if she does not receive prophylaxis. 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, Rhogam) injections to prevent the development of Rh antibodies. Other antibodies may also recur in subsequent pregnancies and need to be discussed with the mother's healthcare provider.
