In serologic testing, which finding is considered a positive reaction besides agglutination?

• A. Hemolysis
• B. Color change
• C. Precipitation only
• D. No reaction

Answer: (A)

In serologic testing, a positive result isn’t limited to visible clumping of cells. Hemolysis can occur when an antibody binds to red cells and fixes complement, triggering destruction of the cells. The released hemoglobin or the appearance of hemolyzed cells signals a positive reaction just like agglutination does, indicating a true antigen–antibody interaction.

Color change of the mixture is not a reliable or standard positive endpoint for these tests, so it isn’t used to declare a positive reaction. Precipitation can indicate a positive result in certain assay types, but in routine red cell serology, the key positive readouts are agglutination or hemolysis, while no reaction remains negative.