A child may have a mild sore throat or tonsillitis or have no symptoms at all, and the illness often goes unrecognized by the parent or teacher. As mononucleosis symptoms appear, the body reacts to the virus in certain distinctive ways that can be detected through laboratory tests. Blood tests that measure lymphocytes and antibodies aid in the diagnosis of mononucleosis. These antibodies indicate that an EBV infection is present in the body, but they are not directed against the virus itself and do not serve a protective function. Because other types of infections and immunologic reactions also induce heterophil antibodies, their presence suggests, but sometimes does not indicate specifically, an EBV infection.
Symptoms play an important role in the diagnosis of mononucleosis.
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.