But because this disease can masquerade as other diseases, symptoms can be misleading. Rapid and inexpensive blood tests can detect heterophil antibodies in about 80 percent of persons with a current or recent infection. These antibodies can appear in sufficient strength to give a positive diagnosis as early as the fourth day and generally by the 21st day of illness. Heterophil antibodies can persist for months, however, so their appearance does not prove current infection. Furthermore, the level of heterophil antibodies in the blood does not correlate with the severity of symptoms.
Spot tests are generally accurate, but they can give false positive or false negative results.
Sometimes, appearance of heterophil antibodies is delayed, and a repeat test may be necessary to establish a diagnosis. Moreover, young children, older adults, and individuals with EBV infections that do not resemble classic mononucleosis are less likely to develop heterophil antibodies. Appearance of the antibodies specific for EBV proteins correlates with the stages of infection. The profile is highly accurate, but it is expensive. All physicians have access to laboratories that can perform these tests if they are necessary.