A 23-year-old patient is seen for a same-day visit for a few days of fever, fatigue, sore throat, and body aches. Suspicion for infectious mononucleosis (IM) leads the provider to check a heterophile antibody test (commonly known as the Monospot test), which is negative. Additional EBV testing reveals negative early antigen IgG, positive viral capsid antigen IgM, positive viral capsid antigen IgG, and positive nuclear antigen IgG. How would you interpret these results?
Primary EBV infection of B-lymphocytes induces antibodies against viral antigens as well as unrelated antigens found on sheep and horse RBCs, known as heterophile antibodies (which is how the Monospot test works). A reactive Monospot test in the presence of a compatible syndrome is considered diagnostic of EBV infection.
- False-negative heterophile antibody tests are common early in the course of illness (25% in the first week, 5-10% in the second week, and 5% in the third week) and in children under 4 years of age.
Testing for EBV-specific antibodies is warranted in patients with suspected IM who have a negative heterophile test.
Viral Capsid Antigen (VCA): IgM and IgG antibodies directed against VCA are usually present at the onset of illness because of EBV’s long incubation period, which is typically 4-8 weeks. IgM anti-VCA levels wane approximately 3 months later, and thus are a reliable marker of acute infection. IgG anti-VCA antibodies persist for life.
- Although the presence of IgM anti-VCA antibodies is highly suggestive of acute EBV infection, other herpesviruses (e.g., CMV) can induce IgM anti-VCA antibodies to cell lines that express EBV antigens.
Epstein-Barr Nuclear Antigen (EBNA): IgG antibodies to EBNA (a protein expressed only when EBV begins to establish latency) appear 6-12 weeks after the onset of symptoms and persist throughout life; their presence early in the course of illness effectively excludes acute EBV infection.
- While the presence of IgM anti-VCA antibodies suggests acute EBV infection, the diagnosis is almost certain in the presence of IgM anti-VCA antibodies and the absence of IgG EBNA antibodies.
Early Antigen (EA): IgG antibodies to EA often appear early in the course of illness, then disappear after recovery. Their absence does not exclude acute illness because the antibodies are not expressed in a significant number of patients.
Red Book pearls:
- Because IgG anti-VCA antibodies occur in high titer early in infection and persist for life at modest levels, testing of acute and convalescent serum specimens for VCA IgG alone is not useful for establishing the presence of active infection. In contrast, testing for the presence of IgM anti-VCA antibody and the absence of (or very low) titers of EBNA antibodies is useful for identifying active and recent infections.
- Because serum antibody against EBNA is not present until several weeks to months after the onset of infection and rises with convalescence, a very elevated anti-EBNA antibody concentration typically excludes active primary infection. Testing for antibodies against EA is not usually required to assess EBV-associated mononucleosis.
The patient’s heterophile antibody test is negative, thus it makes sense to perform additional EBV testing given the clinical suspicion for IM. The presence of IgM and IgG anti-VCA antibodies suggest a recent infection, but the presence of EBNA antibody early in the course of the patient’s current illness effectively excludes acute EBV infection.