A mononucleosis spot test (monospot test) is a screening test for mononucleosis. The test detects a type of antibody (heterophil antibody) that forms during certain infections, especially during infection with the Epstein-Barr virus.

For this test, a sample of blood is placed on a microscope slide and mixed with other substances. If heterophil antibodies are present, the blood clumps. This result usually points to a mono infection.

The test can usually find a mono infection in about 2 to 9 weeks after the person has been exposed to the virus. The antibodies are proteins made by the immune system to fight the infection. After they have formed, antibodies usually remain in a person’s blood for life.