Direct Antiglobulin Test
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
A blood sample drawn from a vein in your arm
The direct antiglobulin test (DAT) looks for antibodies attached to red blood cells (RBCs) circulating in the bloodstream. The test may help to detect or identify conditions in which antibodies become attached to RBCs, causing them to break apart (hemolyze).
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:
- 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.
- 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 healthcare provider about any drugs you have been taking recently. If the healthcare provider suspects drug-induced autoimmune anemia, the suspect medication will be discontinued. Symptoms typically resolve promptly after the drug is discontinued.
- 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 shows symptoms of a reaction after transfusion, 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.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
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.
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
- Fever, chills
- Back pain
- Bloody urine
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.
Is there anything else I should know?
Can I get antibodies from donating blood?
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.
On This Site
Elsewhere On The Web
National Heart, Lung, Blood Institute: What is Hemolytic Anemia?
National Organization for Rare Disorders: Acquired Autoimmune Hemolytic Anemia
MedlinePlus Medical Encyclopedia: Rh incompatibility
KidsHealth from Nemours: Rh Incompatibility
March of Dimes Fact Sheet: Rh disease