Direct Antiglobulin Test (DAT)
The third and generally last screening test which should be utilized in the transfusion reaction workup is the Direct Antiglobulin Test, more commonly referred to as the "Direct Coomb's Test" or simply the "DAT".
In order to understand why we would do a DAT to rule out an acute hemolytic reaction, we must first address the mechanics of the DAT and differentiate it from the Indirect Antiglobulin Test (IAT). The DAT and the IAT both detect the presence of red cells coated with antibodies and/or complement; the difference is that the DAT looks for red cells that were coated in the body, while the IAT detects the same coating done in the laboratory.
Acute Hemolytic Transfusion Reactions will usually give a positive DAT due to coating of the transfused red cells with the patient's antibody. Like the visible hemolysis check, though, a positive DAT must be evaluated with caution, because it may not always mean acute hemolysis. I recommend checking whether the pretransfusion DAT was positive as well before you go any further, because some sources suggest that as many as 15-25% of hospitalized patients have a positive DAT which is completely unexplainable. In addition, a wide array of things unrelated to acute hemolysis can cause a positive DAT, including drugs, disease processes, and passively acquired antibodies.
Likewise, a negative DAT does not always exclude acute hemolysis. In a situation of massive acute hemolysis, as may be seen in an ABO-incompatible transfusion, there may be no incompatible cells left to be coated by the AHG reagent, so the DAT will be negative. (The hemolysis check and the clerical check will likely show problems, however).
So, if all three of the screening tests for acute hemolysis are perfect, it is extremely unlikely that acute hemolysis has occurred. If, however, one or more are suspicious, or if clinical suspicion is high, you may move on to doing some confirmatory tests.
Back to Transfusion Reaction Workup
Back to Transfusion Reactions Index