The direct antiglobulin test (DAT) is used primarily to help determine if the cause of hemolytic anemia, 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 hemolytic anemias, which may be 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, and infections such as mycoplasma pneumonia and 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 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 there is a baby-mother Rh incompatibility. Formerly, this was the most common cause of hemolytic disease of the newborn, but this condition is now rare due to preventative treatments given to the mother during and after each pregnancy. The most common cause of hemolytic disease of the newborn is an ABO incompatibility between mother and baby, especially with mothers that are blood group O.

A DAT may also be used to investigate a suspected transfusion reaction. If you are being transfused and have a fever or other significant symptoms suggesting a potential for a hemolytic transfusion reaction, a DAT is done to determine if you have made an antibody to the transfused RBCs. If the antibody is found coating the RBCs, then the RBCs may be destroyed (hemolyzed) or be removed from your circulation faster than normal.

The DAT may be ordered when you are experiencing symptoms suggesting hemolytic anemia, such as:

• Fatigue
• Dark urine
• Back pain
• Jaundice
• Paleness
• An enlarged spleen

This test may be ordered when a newborn exhibits signs of hemolytic disease of the newborn, including:

• Pale appearance
• Jaundice
• Enlarged liver or spleen
• Swelling of the entire body
• Difficulty breathing

A DAT may be ordered when there are signs and symptoms of a blood transfusion reaction, such as:

• Fever, chills
• Rash
• Back pain
• Bloody urine
• Feeling faint or dizzy

 

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. The DAT detects the presence of the antibody, but it does not tell the doctor the cause or exact type of antibody. A patient’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 Rh 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 not attached to RBCs and the signs and symptoms may be due to 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 will often be chronically positive.