Mononucleosis

### Make the Diagnosis: Mononucleosis

#### Prior Probability of Mononucleosis

Mononucleosis is caused by the Ebstein-Barr virus (EBV) and is most common in children and young adults, but especially between ages 16 and 20. Among those aged 16 to 20 years, the community-based annual incidence rate of mononucleosis is 0.7% to 2.7%.1,2,3 About 1 of 13 adolescents presenting with a sore throat will have mononucleosis (prior probability, 7%).1

#### Population in Whom Mononucleosis Should Be Considered

The diagnosis should be considered especially in young adults with sore throat, fever, lymphadenopathy, malaise, and fatigue. Fatigue that is especially pronounced and is excessive for the patient's usual activities can be present. Mononucleosis presents as an acute or chronic illness depending on the phase of disease (chronic symptoms may last months). The differential diagnosis includes other viral illnesses (cytomegalovirus, toxoplasmosis, adenovirus, human immunodeficiency virus), but also acute leukemia.

#### Assessing the Likelihood of Mononucleosis

While symptoms describe the clinical picture when mononucleosis is considered as a diagnosis, no studied individual symptom has a positive or negative likelihood ratio (LR) that is diagnostically useful because the CI for all individual symptoms studied has included 1.0. Most patients will have sore throat and/or fatigue (sensitivity range, 0.81–0.83). Physical examination findings combined with simple laboratory parameters dominate the clinical diagnosis. Palatal petechiae has been reported to occur infrequently (5.5%) among patients evaluated for mononucleosis in 1 study, but when present are associated with an LR+ of 5.3 (CI, 2.1–13).1 The presence of posterior cervical adenopathy or splenomegaly is the most useful physical examination finding (see Table 101-1). The absence of any adenopathy is the most useful finding for identifying patients least likely to have mononucleosis.

Table 101-1.Useful Findings for Assessing the Likelihood of Mononucelosis

Mononucleosis is named for the characteristic mononuclear leukocytosis associated with EBV infections. The presence of atypical lymphocytes ≥ 10% has an LR of 11 (95% CI, 2.7–25), while the presence of atypical lymphocytes at lower percentages of the count reduces the likelihood of mononucleosis (see Table 101-1). The greater the percentage of atypical lymphocytes, the more ...

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