A patient transfused 20 days ago is suspected of having a delayed hemolytic transfusion reaction. Which test is most helpful for diagnosis?

In the context of a suspected delayed hemolytic transfusion reaction, the most helpful test for diagnosis is the direct antiglobulin test (DAT). This test detects the presence of antibodies or complement proteins bound to the surface of red blood cells.

In delayed hemolytic reactions, which typically occur days to weeks after transfusion, the immunologic response may result in the production of antibodies against transfused red blood cell antigens that were not previously encountered by the patient’s immune system. The DAT helps identify whether there has been an immune-mediated destruction of red blood cells, indicating that the body has recognized the transfused cells as foreign.

While urine hemosiderin and plasma hemoglobin can provide information about hemolysis, they are not specific to the diagnosis of a delayed transfusion reaction. The antibody screen is useful for detecting the presence of antibodies in the patient's serum but may not provide conclusive information specific to the delayed reaction itself. The direct antiglobulin test, however, is specifically designed to assess immune-mediated hemolysis, making it the most suitable choice in this situation.