Q:

What serologic tests (other than heterophile antibodies) can be used to diagnose infectious mononucleosis?

A:

Infections due to the herpes virus, Epstein-Barr virus (EBV), are very prevalent in most populations. Primary infection in young children is often asymptomatic or accompanied by only non-specific minor illness. In persons infected in adolescence, primary infection is often manifested as infectious mononucleosis (IM). In this case, the disease is usually self-limiting, characterized by fever, sore throat, myalgias, lymphadenopathy, and hepatosplenomegaly. As with other herpes viruses, EBV causes persistent, latent infection that can be reactivated. It has been estimated that Epstein-Barr virus will infect more than 90 percent of the worldwide human population during their lifetime.

Diagnosis of acute disease due to this agent depends mostly on testing for the immune response to EBV by:

• Heterophile agglutination assay:
  • Infectious mononucleosis slide test

• Enzyme linked immunosorbent assays:
  • Epstein-Barr Virus Antibody to Viral Capsid Antigen (VCA), IgM
  • Epstein-Barr Virus Antibody to Viral Capsid Antigen, IgG
  • Epstein-Barr Virus Antibody to Nuclear Antigen (EBNA), IgG
  • Epstein-Barr Virus Antibody to Early D Antigen (EA-D), IgG

The immune response during acute infection, however, may be significantly delayed, especially in children and immunocompromised hosts. Thus, extensive workups may be undertaken in the markedly febrile patient with enlarged nodes, who remains seronegative. Testing for EBV DNA may be useful in these rare settings.

Specific EBV serology can be used to differentiate heterophile-negative infections from IM-like illnesses caused by other agents (e.g., CMV, toxoplasmosis, etc.). Antibodies to EBV viral capsid antigen (VCA) arise early in the course of EBV infection and can be demonstrated in a majority of cases. IgM antibodies to VCA are particularly sensitive and specific for IM, as VCA-IgM antibodies are rarely seen in the general population. Most individuals possess nearly peak titers of VCA-IgG antibodies by the time clinical symptoms of acute infection are apparent. Once present, IgG antibodies to VCA will persist for life. The absence of VCA-IgG antibodies in a patient's serum indicates susceptibility to infection by EBV.

Epstein-Barr nuclear antigen (EBNA) antibody responses may be used in conjunction with VCA antibody results to aid the diagnosis of primary EBV infections. A gradual increase in EBNA antibody titer occurs during convalescence and near-peak titers are maintained for life. Thus, the appearance of EBNA antibodies in a patient who was previously VCA positive and EBNA negative is strong evidence of recent infection. However, neither the height of the antibody titer to EBNA nor the time required for its development correlates with the severity or duration of illness in IM.

In a study reported by Henle, et al, 49 of 62 IM patients had antibody titers to Epstein-Barr early antigens (EA-R and EA-D) in the range of 1:20–1:320 in the same time frame as VCA-IgM antibodies. Persistently elevated antibody titers to EA and EBNA together may suggest reactivation of or persistently active EBV infection.