There is a high degree of variability in the serologic response to Epstein-Barr virus (
EBV) infection, especially in viral capsid
antigen (VCA)-
IgM antibodies. Therefore, additional tests are needed to confirm primary
infection. We evaluated the value of
IgG avidity determination in diagnosis of
EBV infection in immunocompetent and immunocompromised patients. A total of 236 serum samples from immunocompetent patients with symptoms suggestive of
EBV infection were tested for the presence of VCA-
IgM/
IgG antibodies and
IgG avidity. Using
IgG avidity, acute primary
infection was confirmed in 56.7% of the immunocompetent patients with positive and in 1.8% of patients with negative VCA-
IgM. Recent primary
infection was documented in 8.9% of the
IgM positive and 3.5% of the
IgM negative patients. In patients with indeterminate serology (equivocal
IgM), 6.7% were classified by avidity index (AI) as acute primary
infection, 10.0% as post-acute and 83.3% as past
infection cases. Concerning the 32 immunocompromised patients, recent primary
infection was documented in 3 of the 14
IgM positive patients. High AI was detected in 11 of these patients, indicating an
IgM response due to reactivation. Determination of
IgG avidity in combination with classical serologic markers seems to be a reliable method to confirm primary
infection both in immunocompetent and immunocompromised patients. It may be especially useful to differentiate cases of primary
infection in patients with undetectable VCA-
IgM antibodies or indeterminate routine EBV serology.