Although most cases of infectious mononucleosis can be diagnosed on the basis of clinical findings, blood count and morphology, and a positive test for heterophil antibody, as many as 20% may be heterophil-negative, at least at presentation (heterophil may become positive when repeated in a few days). In some of these cases, a test for Epstein-Barr virus antibodies may be useful.

The most controversial use of EBV serology is in chronic fatigue syndrome, a complaint predominantly but not exclusively of young to middle-aged women, characterized by long persistent debilitating fatigue and a panoply of usually mild somatic complaints. The high levels of EBV antibodies in the general population, their long persistence, and the poor correlation of antibody titers with symptoms combine to make EBV serology useless in diagnosing, following, or ruling out chronic fatigue syndrome. See table.