Coombs, Direct

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.

A blood sample is obtained by inserting a needle into a vein in the arm.

No test preparation is needed.