Diagnosis of Infectious Mononucleosis

The diagnosis of infectious mononucleosis implies primary EBV infection.

A presumptive diagnosis may be made by the presence of typical clinical symptoms with atypical lymphocytosis in the peripheral blood. The diagnosis is usually confirmed by serologic testing, either for heterophile antibody or specific EBV antibodies.

Culture of EBV is tedious and requires 4–6 wk. The culture method is the transformation assay, which is performed by co-cultivating oropharyngeal or genital secretions, peripheral blood (10–30 mL), or tumor with human umbilical cord lymphocytes. The cultures are observed for 6 wk for signs of cell transformation: proliferation and rapid growth, mitotic figures, large vacuoles, granular morphology, and cell aggregation. EBV immortalizes the umbilical cord cells, resulting in cell lines that can be maintained in perpetuity that harbor EBV isolated from the patient.

In >90% of cases there is leukocytosis of 10,000–20,000 cells/mm3, of which at least ? are lymphocytes; atypical lymphocytes usually account for 20–40% of the total number. The atypical cells are mature T lymphocytes that have been antigenically activated. Compared with regular lymphocytes microscopically, atypical lymphocytes are larger overall, with larger, eccentrically placed indented and folded nuclei with a lower nuclear-to-cytoplasm ratio. Although atypical lymphocytosis may be seen with many of the infections usually causing lymphocytosis, the highest degree of atypical lymphocytes is classically seen with EBV infection.

Heterophile antibodies agglutinate cells from species different from those in the source serum. The transient heterophile antibodies seen in infectious mononucleosis, also known as Paul-Bunnell antibodies, are IgM antibodies detected by the Paul-Bunnell-Davidsohn test for sheep red cell agglutination. The heterophile antibodies of infectious mononucleosis agglutinate sheep or, for greater sensitivity, horse red cells but not guinea pig kidney cells.

Results of the sheep red cell agglutination test are often positive for several months after infectious mononucleosis; those of the horse red cell agglutination test may be positive for as long as 2 yr. The most widely used method is the qualitative rapid slide test using horse erythrocytes. It detects heterophile antibody in 90% of cases of EBV-associated infectious mononucleosis in older children and adults but in only up to 50% of cases in children <4 yr of age because they typically develop a lower titer.

EBV-specific antibody testing is useful to confirm acute EBV infection, especially in heterophile-negative cases, or to confirm past infection and determine susceptibility to future infection. Several distinct EBV antigen systems have been characterized for diagnostic purposes.

Differential Diagnosis.
Infectious mononucleosis-like illnesses may be caused by primary infection with cytomegalovirus, T. gondii, adenovirus, viral hepatitis, HIV, or possibly rubella virus. Cytomegalovirus infection is a particularly common cause in adults.

Streptococcal pharyngitis may cause sore throat and cervical lymphadenopathy indistinguishable from that of infectious mononucleosis but is not associated with hepatosplenomegaly.

Failure of a patient with streptococcal pharyngitis to improve within 48–72 hr should evoke suspicion of infectious mononucleosis.

The most serious problem in the diagnosis of acute illness arises in the occasional patient with extremely high or low white blood cell counts, moderate thrombocytopenia, and even hemolytic anemia. In these patients, bone marrow examination and hematologic consultation are warranted to exclude the possibility of leukemia.