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Epstein Barr Virus (EBV), also classified as Human Herpes Virus 4, infects the vast majority of adults worldwide and establishes both non-productive (latent) and productive (lytic) infection. Classical EBV diagnosis includes quantitative determination of viral DNA and serological analysis, based on the determination of IgG and IgM responses against the viral capsid antigen (VCA) and the IgG response against the EBV nuclear antigen-1 (EBNA-1). EBV-serology can be misleading in some cases, such as acute infections with low or undetectable VCA IgM, convalescent patients with persistent or reactivated VCA IgM and negative anti- EBNA-1 IgG, due to a loss of this marker during immunosuppression. In all these cases avidity determination of IgG is helpful to prevent false diagnosis. Avidity represents the stability of the antigen-antibody interaction. Its value increases during the infection, so high avidity never associates with a primary acute infection. We studied the importance of avidity determination of p18 (VCA)-IgG to achieve unequivocal interpretation of serological results. The amount of IgG and IgM is determined by Chemiluminescent Immune Assay (CLIA), a rapid and highly sensitive method suitable for automation. The intensity of luminous signal produced by antibody-antigen recognition is expressed as Relative Light Unit (RLU). p-18 IgG is determined using a recombinant p18 antigen expressed in E. coli. Avidity index is determined in CLIA by the ratio between denaturated and not denaturated IgG specific antibodies expressed in RLU. These results demonstrate that avidity determination represents an important additional marker particularly in cases with aberrant serological profile.
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