One hour after receiving a platelet apheresis, a patient develops severe hypoxemia and tachycardia. What test would best investigate the cause?

In this scenario, the patient’s sudden hypoxemia and tachycardia after receiving a platelet apheresis transfusion raise concerns about a transfusion reaction, particularly one that may involve antibodies from the donor’s leukocytes, such as in a transfusion-related acute lung injury (TRALI) scenario.

Testing the patient for leukocyte antibodies is insightful because TRALI can occur when antibodies in the donor's blood react with the recipient's leukocytes, causing an inflammatory response in the lungs leading to acute respiratory distress. In TRALI cases, it is often the recipient's leukocytes that are targeted by donor antibodies, and identifying these antibodies can help confirm the diagnosis and understand the etiology of the reaction.

The other testing options have their own relevance, but they do not directly address the likely cause as effectively. For instance, testing for bacterial contamination is crucial in cases where there is a risk of sepsis or febrile reactions but may not specifically explain the patient's respiratory symptoms. Similarly, evaluating the DAT on red cells focuses on hemolytic reactions, and checking for red cell antibodies in the patient's plasma, while important in some contexts, does not target the leukocyte-mediated immune response that could be implicated here. Thus, testing specifically for