American Board of Internal Medicine (ABIM) Certification Practice Exam

When do delayed hemolytic transfusion reactions typically occur after a transfusion?

1. Within 24 hours

2. 1 to 3 days

3. 2 to 10 days

4. More than 10 days

The correct answer is: 2 to 10 days

Delayed hemolytic transfusion reactions commonly occur between 2 to 10 days after a transfusion. This timing is significant because it is usually associated with the anamnestic immune response, where previously sensitized patients exhibit a secondary immune response to blood group antigens that they have been exposed to in a prior transfusion or pregnancy. This response may lead to hemolysis, which can be less overt than the acute reactions occurring within the first 24 hours. During this window, the patient's immune system recognizes the antigens present on the transfused red blood cells, triggering the production of antibodies. Because this reaction is not immediate, it often goes unnoticed at first and may only be detected during follow-up blood tests, such as post-transfusion hemolysis lab work. Understanding this timeframe is crucial for clinicians when monitoring patients after transfusions, as symptoms can be subtle and can easily be missed if the time frame isn't kept in mind.