Diagnosing infectious mononucleosis.

Infectious mononucleosis can be diagnosed with certainty only when suggestive clinical findings are corroborated by relative and absolute lymphocytosis, lymphocyte atypia of more than 20 percent and a positive serologic test. A serologic test is not absolutely necessary when atypical lymphocytes exceed 40 percent since this is specific to infectious mononucleosis. The diagnosis is difficult when clinical findings are scanty and the blood picture is unhelpful.