A patient received 2 units of red blood cells and had a delayed transfusion reaction, yet initial antibody screening showed no agglutination until sensitized cells were added. What explains the original results?

In this scenario, the patient's initial antibody screen did not show any agglutination until sensitized cells were introduced, indicating the potential presence of antibodies that were not detectable under standard testing conditions. The choice that indicates that the patient's serum was omitted during the original testing is particularly relevant.

When performing an antibody screen, the patient's plasma or serum is mixed with a standardized panel of red blood cells that express different antigens. If the serum is omitted, there would be no antibodies present to react with the antigens on the test cells, resulting in a negative agglutination reaction. Once sensitized red blood cells are introduced, which carry the specific antigens against which the patient’s antibodies are directed, a reaction can occur, demonstrating that the patient indeed has antibodies that were simply not detected in the absence of their serum.

Other choices suggest issues with the testing process or technique that could interfere with results, such as overwashing red cells or prolonging centrifugation, but these factors would affect the reaction ability rather than result in a complete absence of agglutination if the patient's serum were present. Hence, the most likely explanation for the original negative results is the omission of the patient's serum, which prevented any antibody-antigen interactions in the screening