A DAT may 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.
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 hemolytic anemia.
If a DAT is negative, then antibodies are most likely not attached to RBCs and the signs and symptoms should be investigated for some other cause.
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 chronically positive.
This article was last reviewed on September 10, 2012. | This article was last modified on July 21, 2013.
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