What is the most common reason for mixed-field agglutination?

• A. Dimorphic population of transfused red blood cells
• B. Autoantibody with high titer
• C. Cold agglutinins
• D. Non-specific agglutination

Answer: (A)

Mixed-field agglutination happens when two different populations of red blood cells are present in the same sample, so you see some cells that clump while others do not in the same reaction. The most common cause is a dimorphic population of transfused red blood cells—donor cells with a different antigen profile mix with the recipient’s cells, producing a visible two-population pattern. Autoantibodies with high titer tend to give broad or panagglutination rather than two distinct populations. Cold agglutinins react at low temperatures and can give variable rouleaux or agglutination, but not a true two-population pattern. Non-specific agglutination doesn’t create a true dual cell population either.