Coombs Test (Direct and Indirect)
Definition
A test that detects antibodies that can attach to red blood cells and cause their early destruction. It has two forms:
| What it looks for | When it is ordered | |
|---|---|---|
| Direct Coombs (DAT) | Antibodies attached to the surface of red blood cells | In cases of anemia or jaundice |
| Indirect Coombs (IAT) | Antibodies circulating in the blood | Usually for pre-transfusion compatibility |
Purpose
The direct test shows whether red blood cells are being destroyed early by the immune system. The indirect test assesses the risk of an adverse reaction to blood that would be given.
Results
If the direct test is positive, there are antibodies against red blood cells. Possible causes: autoimmune hemolytic anemia, chronic lymphocytic leukemia, hemolytic disease of the newborn, infections such as mononucleosis or syphilis, systemic lupus erythematosus, a reaction resulting from an incompatible blood transfusion.
If the indirect test is positive, there are antibodies against red blood cells the body sees as foreign; this may suggest hemolytic disease of the newborn or blood incompatibility.
Normal Values
A negative result is normal, there is no cell clumping and no antibodies against red blood cells are present.