Direct Antiglobulin Test
- When you have had a blood transfusion recently and are experiencing symptoms of a transfusion reaction, such as dizziness, back pain, dark urine, shortness of breath
- When a newborn shows signs of hemolytic disease of the newborn
- When you have symptoms of increased destruction of your red blood cells, which may include feeling tired more often than usual, shortness of breath, headaches, paleness and your healthcare practitioner wants to find out if this happens because your immune system mistakenly makes antibodies that destroy your red blood cells
A blood sample drawn from a vein in your arm
The direct antiglobulin test (DAT) determines whether your red blood cells (RBCs) circulating in the bloodstream are covered with antibodies. The antibodies that are attached to the surface of the RBCs are responsible for their destruction.
There are a few reasons why RBCs may be attacked by antibodies:
- Following a blood transfusion: If someone receives a donor's blood that does not fully match their own type, their body will make antibodies that recognize the donor's RBC as foreign. (For more on blood types, see the article on Blood Typing.) These antibodies will attack donor’s RBCs and destroy them. People who have many blood transfusions are more likely to make antibodies to RBCs because they are exposed to more foreign RBC. If someone shows symptoms of a reaction after a transfusion, a DAT will be performed to determine if those antibodies have attached to RBCs.
- Mother/baby blood type incompatibility: A mother and baby may have different blood types if the baby inherits a blood type from the father. During pregnancy or labor, the mother may be exposed to the baby's RBCs. These RBCs may be recognized by the mother’s immune system as foreign and she will produce antibodies directed against the baby's RBC. A baby’s RBCs might be covered with antibodies that cross the placenta from the mother’s blood into the baby’s circulation. These antibodies will destroy the baby's circulating RBCs and cause hemolytic disease of the newborn (HDN). A DAT is performed on the blood of a baby to determine if the antibodies have attached to the baby's RBCs.
This may occur when an Rh-positive baby is born to an Rh-negative mother. This type of incompatibility usually does not affect the first baby but affects subsequent children. (For more on blood types and pregnancy, see the article on Blood Typing and RBC Antibody Screen). Formerly, antibodies to the Rh protein were the most common cause of hemolytic disease of the newborn, but this condition is now rare due to preventive treatments given to the Rh-negative 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.
- Autoimmune diseases and other conditions: Some people make antibodies that target their own RBCs (autoantibodies). These antibodies are produced because the immune system mistakenly recognizes their own RBCs as foreign. Some examples of conditions that cause this include:
- Drug-induced anemia: Certain drugs can induce antibodies against RBCs and therefore cause their destruction (hemolysis). This is seen with some antibiotics, such as penicillin, cephalosporins and piperacillin. 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.
How is the test used?
The direct antiglobulin test (DAT) is used primarily to help determine whether the cause of hemolytic anemia is due to antibodies attached to RBCs. Hemolytic anemia is a condition in which red blood cells (RBCs) are destroyed more quickly than they can be replaced.
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.
A DAT may also be used to investigate a suspected transfusion reaction.
For more on these, see the "What is being tested?" section.
When is it ordered?
The DAT may be ordered when you have hemolytic anemia and your 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. Your medical history and a clinical examination are needed to determine the cause of a positive DAT.
Some examples of conditions that can cause a positive DAT include:
- Transfusion reaction
- Autoimmune disorder, such as lupus
- Lymphoma or other malignant disease
- Infection, such as mycoplasma pneumonia and mononucleosis
- Medication, such as penicillin
- Baby-mother blood group incompatibility
For more information about these, see the "What is being tested?" section.
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 your RBCs and the signs and symptoms are due to another cause that requires further investigation.
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.
Is there anything else I should know?