The direct antiglobulin test using polyspecific antihuman globulin reagent is not valuable in which instance?

• A. Autoimmune hemolytic anemia evaluation
• B. Diagnosis of Donath-Landsteiner syndrome
• C. Transfusion reaction assessment
• D. Detection of hemolytic disease of the newborn

Answer: (B)

The direct antiglobulin test with polyspecific antihuman globulin detects antibodies or complement that are already bound to red cells in the patient, signaling immune-mediated hemolysis. It is useful when evaluating autoimmune hemolytic anemia, confirming immune involvement in transfusion reactions, and detecting maternal antibodies coating fetal red cells in hemolytic disease of the newborn.

Donath-Landsteiner syndrome, however, is caused by a biphasic autoantibody that binds red cells at cold temperatures and then fixes complement upon warming. This in vivo coating is not a steady state phenomenon detectable reliably by a standard DAT, and the diagnosis hinges on the Donath-Landsteiner test that demonstrates this temperature-dependent hemolysis. Therefore, the DAT is not valuable for diagnosing Donath-Landsteiner syndrome.