In a delayed hemolytic transfusion reaction, which presentation is the least common?

In a delayed hemolytic transfusion reaction, hemoglobinemia is considered the least common presentation. This type of reaction typically occurs days to weeks after a blood transfusion and is primarily characterized by the gradual destruction of transfused red blood cells due to the recipient's immune system recognizing foreign antigens that it has been sensitized to.

The hallmark findings in delayed hemolytic reactions often include a declining hemoglobin level, which reflects the destruction of red blood cells and the inability to sufficiently compensate for this loss. A positive direct antiglobulin test (DAT) is also commonly seen as it indicates the presence of antibodies coating the red blood cells, confirming an immune-mediated process.

Additionally, a positive antibody screen can occur, especially if the patient has developed new antibodies in response to the transfused red blood cells. In contrast, hemoglobinemia, which refers to the presence of free hemoglobin in the plasma usually seen in acute hemolytic reactions due to intravascular hemolysis, is not a typical finding in delayed reactions. As such, the absence of significant hemoglobinemia in this scenario contributes to its classification as the least common presentation in delayed hemolytic transfusion reactions.