 The heterophile antibody test, or monospot test, works by agglutination of red blood cells from guinea pig, sheep and horse.This test is specific but not particularly sensitive (with a false-negative rate of as high as 25% in the first week, 5–10% in the second, and 5% in the third).About 90% of diagnosed people have heterophile antibodies by week 3, disappearing in under a year.