The diagnosis of mononucleosis in the office laboratory.

Infectious mononucleosis has been known for a long time to be a common infection in young adults. It also infects children. In particular, children under the age of 2 years may not express the illness clinically. Diagnostic criteria vary but in young adults usually include the symptoms of fever, exudative tonsillitis, and cervical lymphadenopathy. Usually, there is a relative and absolute lymphocytosis, with 10 to 20 per cent or greater atypical lymphocytes. Rapid slide tests are accurate and economical and support the diagnosis when positive. False-positive results are known to occur with several other important disease processes; therefore, the clinical presentation and laboratory results must be interpreted in their appropriate context. Quality control is essential even for the rapid slide tests, and experienced examiners are required to review blood smears. The morphology of atypical lymphocytes varies greatly. In addition, morphology interpretation can be hampered by problems in preparation of slides or the holding of blood samples awaiting slide preparation. EBV-specific serodiagnosis has significantly enhanced our ability to study further and differentiate heterophil-negative syndromes that share clinical features with infectious mononucleosis. Acute, past, chronic, and no EBV infection can now be differentiated. Further diagnostic tests for other etiologic agents are necessary when all EBV tests are negative and the clinical situation requires appropriate diagnosis.