What indicates a positive reaction in the heterophil antibody test?

A positive reaction in the heterophil antibody test is indicated by agglutination, which is the clumping together of particles, in this case, red blood cells or other antigens in the test. This test is primarily used for the diagnosis of infections such as infectious mononucleosis, where specific heterophil antibodies are produced in response to the infection.

When the test sample is mixed with a reagent containing antigens, the presence of heterophil antibodies in the serum will cause the antibodies to bind to the antigens, leading to visible clumping or agglutination. This is a direct indicator that the immune response has recognized and reacted to these specific antigens.

Other options, such as the absence of agglutination, color change in the solution, or rapid sedimentation of cells, do not signify the presence of heterophil antibodies or a positive test result in this context. Instead, they may represent either a negative test result or unrelated laboratory observations. Hence, agglutination is the definitive sign of a positive reaction in a heterophil antibody test.