In my opinion, the direct antiglobulin test is the most important concept that a transfusion medicine physician or technologist must understand in interpreting complex serology patterns.
Like all other testing, the DAT must not be interpreted alone but rather in the context of other laboratory and clinical results. Still, it is very important to understand the significance of the DAT in hemolysis.
The mere presence of immunoglobulin on the RBC surface does not necessarily mean severe hemolysis. The DAT strength increases with increased immunoglobulin coating of the RBCs but does not necessarily indicate how quickly the RBCs will be cleared. That depends on the class and subclass of the antibody, whether and if so, how avidly it fixes complement, etc.
One trick question I give in my lessons to staff is, “What is the clinical significance of a negative DAT?”
In my career, I have seen severe hemolysis with either negative or weak DAT, the latter especially if there is weak C3 staining. The DAT can be negative because there is no significant antibody OR there is a highly destructive antibody causing massive hemolysis, leaving only the antigen-negative cells (and in that case, there is still the possibility of innocent bystander hemolysis).
I show them the following case of an ABO-incompatible acute hemolytic transfusion reaction:
In this case, a group A patient received a group B RBC unit intended for a patient with a similar name who was group B and was in a bed next to him.
Notice the patient’s complete loss of the reaction to group B cells in the reverse type and the supernatant hemolysis. The DAT was negative. The transfused B cells were not even present in the post-transfusion gel.
Here is the urine specimen from that case showing gross hematuria:
So, in severe, life-threatening hemolysis, even antibody-mediated, you may have a negative DAT. DAT negativity may also be seen in non-immune hemolysis. I will discuss causes of DAT-negative hemolytic anemia is a future post.
In summary when doing a hemolysis investigation, a negative DAT does not mean everything is all right. Everything must be interpreted in the context of the clinical and other laboratory findings.