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

The most helpful test for diagnosing a delayed hemolytic transfusion reaction is the direct antiglobulin test (DAT). This test detects antibodies or complement proteins bound to the surface of red blood cells. In the case of a delayed hemolytic reaction, the patient's immune system has typically developed new antibodies against transfused red blood cells, which can happen after previous sensitization.

A positive DAT indicates that the patient's immune system is currently reacting against the transfused blood cells, providing strong evidence for hemolysis potentially due to transfusion. This is particularly important because delayed hemolytic reactions can occur days to weeks after the transfusion, making the DAT a key tool for diagnosis during that time frame.

In contrast, other tests have different applications. For example, urine hemosiderin can indicate past hemolysis and is not specific to the acute scenario presented. Plasma hemoglobin measures the amount of free hemoglobin in the plasma, but it may not always be elevated in delayed hemolytic reactions, as red blood cells are often more slowly destroyed. An antibody screen may help identify antibodies, but it may not pinpoint those specifically involved in a delayed hemolytic reaction. Therefore, among the options given, the direct antiglobulin test stands out as