Direct Antiglobulin Test
When your doctor wants to find out the cause of your haemolytic anaemia; when you have had a blood transfusion recently and are experiencing symptoms of a transfusion reaction; or when a newborn shows signs of haemolytic disease of the foetus and neonate (HDFN).
The test is performed on a sample of blood obtained from a vein in the arm using a needle. This is a process which may be referred to as ‘venepuncture’.
The direct antiglobulin test (DAT) is a laboratory test that can be used to identify whether red blood cells have antibodies attached to their surface; this can provide important information that can identify the cause of haemolysis.
Red cells carry many different proteins and substances on their cell membrane surface that can act as antigens. An antigen is any substance that may be recognised by the immune system and stimulate an immune response that generates antibodies. The combination of antigens present on the surface of red blood cells determines your blood type. The major red cell antigens include the A, B and Rhesus (Rh) antigens that determine a person’s basic blood types (for more on this, see Blood Type and Blood Banking).
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). The major blood group systems (ABO and Rhesus) represent only two of the 33 currently recognised blood group systems. These other blood group systems include the Kell, Duffy, Kidd and Lutheran groups to name a few.
The DAT test is used to check if there are antibodies attached to circulating red blood cells.
Listed below are some of the common conditions associated with RBCs becoming coated with antibodies:
- With autoimmune diseases and other conditions: Some people make antibodies directed against their own RBC antigens (autoantibodies). These autoantibodies may be produced in autoimmune diseases and/or with some other conditions, such as lymphoma and chronic lymphocytic leukaemia.
- With drug-induced anaemia: Certain drugs can induce antibodies against red blood cell antigens and therefore cause haemolysis even without the presence of the drug. Sometimes, drugs may coat the surface of RBCs, causing antibodies to react with the RBCs. This condition is relatively rare (only affecting about 1:1 million patients). 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 anaemia, 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 haemolytic disease of the foetus and neonate, 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 haemolyse them. However, any baby may be affected by haemolytic disease of the foetus and neonate 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 foetal 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 recognise 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 as part of the routine panel of investigations to determine if those antibodies have attached to the transfused donor RBCs.
How is it used?
The direct antiglobulin test (DAT) is used primarily to help determine if the cause of haemolytic anaemia, a condition in which red blood cells (RBCs) are being destroyed more quickly than they can be replaced, is due to antibodies attached to RBCs. This may occur in autoimmune-related haemolytic anaemias, 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 leukaemia, and infections such as mycoplasma pneumonia and infectious mononucleosis. It can also occur in some people with the use of certain medications, such as penicillin.
A DAT may be used to help diagnose haemolytic disease of the foetus and neonate (HDFN) 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 these 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 haemolytic disease of the neonate, but this condition is now rare due to preventive treatments given to the mother during and after each pregnancy. The most common cause of haemolytic disease of the foetus and neonate nowadays is an ABO incompatibility between a Group O mother and her baby. This type of foetal-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 haemolytic transfusion reaction, a DAT is performed as part of the panel of investigative tests 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 (haemolysed) or be removed from circulation faster than normal.
When is it requested?
The DAT may be requested for the investigation of the cause of a haemolytic anaemia or as a routine test when a newborn is born to an at-risk mother or exhibits signs of haemolytic disease of the foetus and neonate, in the absence of other causes.
Signs and symptoms of haemolysis of the foetus and neonate include;
- Pale appearance
- Anaemia, with characteristic blood film appearances (spherocytes, polychromasia and nucleated red blood cells)
- Jaundice, including elevated bilirubin
- Enlarged liver or spleen
- Swelling of the entire body
- Difficulty breathing
A DAT will also be requested when there are signs and symptoms of a blood transfusion reaction, such as:
- Fever, chills
- Back, abdominal or flank pain
- Bloody urine
- Pain at the venepuncture site
- Nausea and vomiting
- Low blood pressure
What does the test result mean?
If the DAT is positive, then 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 clinical severity, especially if the RBCs have already been destroyed. The DAT detects the presence of the antibody, but it does not tell the doctor 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 haemolytic anaemia.
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 haemolytic disease of the foetus and neonate 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 (Rh Immune Globulin) 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 doctor.