Infectious Mononucleosis - Diagnosis

Diagnosis

The most commonly used diagnostic criterion is the presence of 50% lymphocytes with at least 10% atypical lymphocytes (large, irregular nuclei), while the person also has fever, pharyngitis and adenopathy. Furthermore, it should be confirmed by a serological test. The atypical lymphocytes resembled monocytes when they were first discovered, thus the term "mononucleosis" was coined. Diagnostic tests are used to confirm infectious mononucleosis, but the disease should be suspected from symptoms prior to the results from hematology. These criteria are specific; however, they are not particularly sensitive and are more useful for research than for clinical use. Only half the patients presenting with the symptoms held by mononucleosis and a positive heterophile antibody test (monospot test) meet the entire criteria. One key procedure is to differentiate between infectious mononucleosis and mononucleosis-like symptoms.

A few studies on infectious mononucleosis have been conducted in a primary care environment, the best of which studied 700 patients, of which 15 were found to have mononucleosis upon a heterophile antibody test. More useful in a diagnostic sense are the signs and symptoms themselves. The presence of splenomegaly, and posterior cervical, axillary and inguinal adenopathies are the most useful to suspect a diagnosis of infectious mononucleosis. On the other hand, the absence of cervical adenopathy and fatigue are the most useful to dismiss the idea of infectious mononucleosis as the correct diagnosis. The insensitivity of the physical examination in detecting splenomegaly means it should not be used as evidence against infectious mononucleosis.

In the past, the most common test for diagnosing infectious mononucleosis was the heterophile antibody test, which involves testing heterophile antibodies by agglutination of guinea pig, sheep and horse red blood cells. As with the aforementioned criteria, this test is specific but not particularly sensitive (with a false-negative rate of as high as 25% in the first week, 5–10% in the second and 5% in the third). About 90% of patients have heterophile antibodies by week 3, disappearing in under a year. The antibodies involved in the test do not interact with the Epstein–Barr virus or any of its antigens. More recently, more sensitive tests have been developed, such as the immunoglobulin G (IgG) and immunoglobulin M (IgM) tests. IgG, when positive, reflects a past infection, whereas IgM reflects a current infection. When negative, these tests are more accurate in ruling out infectious mononucleosis. However, when positive, they feature similar sensitivities to the heterophile antibody test. Therefore, these tests are useful for diagnosing infectious mononucleosis in people with highly suggestive symptoms and a negative heterophile antibody test. Another test searches for the Epstein–Barr nuclear antigen, while it is not normally recognizable until several weeks into the disease, and is useful for distinguishing between a recent-onset of infectious mononucleosis and symptoms caused by a previous infection. Elevated hepatic transaminase levels is highly suggestive of infectious mononucleosis, occurring in up to 50% of patients.

A fibrin ring granuloma may be present.