[2] The test is specific for heterophile antibodies produced by the human immune system in response to EBV infection.

[5] Rarely, however, a false positive heterophile antibody test may result from systemic lupus erythematosus, toxoplasmosis, rubella, lymphoma and leukemia.

This lack of sensitivity is especially the case in young children, many of whom will not produce detectable amounts of the heterophile antibody and will thus have a false negative test result.

[8][11] Manual versions of the test rely on the agglutination of horse erythrocytes by heterophile antibodies in patient serum.

[citation needed] A 20% suspension of horse red cells is used in an isotonic 3–8% sodium citrate formulation.

Ten microliters of the horse red cell suspension are then added and mixed with each drop of adsorbed serum.