This quick screening test detects a type of antibody (heterophil antibody) that forms during certain infections. A sample of blood is placed on a slide and mixed with other substances. If heterophil antibodies are present, the blood clumps (agglutinates). This result usually indicates a mono infection. Monospot testing can usually detect antibodies 2 to 9 weeks after a person is infected. It typically is not used to diagnose mono that started more than 6 months earlier.