title: Application of fluoroimmunoassay to the identification of low avidity specific IgG against pathogenic human viruses and Toxoplasma gondii.
abstract: BACKGROUND
  Serological diagnosis of primary viral infections is usually made by detection of specific IgM. In some cases, false positive results (mainly due to crossreactions between closely related viruses) can be obtained. Moreover, some primary infections occur without specific IgM response. Thus, alternative serological approaches are required for diagnosis. Detection of low avidity, specific IgG has been applied as a useful serological marker for diagnosing infections caused by several viruses and Toxoplasma gondii.
  
  
  OBJECTIVE
  The standardization and application of specific IgG avidity assays using a semiautomated solid phase immunoassay (fluoroimmunoassay (FIA)) on the basis of the urea elution principle, for the characterization of low avidity specific IgG against rubella virus, herpes simplex virus (HSV), varicella-zoster virus (VZV), cytomegalovirus (CMV) and T. gondii.
  
  
  METHODS
  The method consists of two simultaneous determinations, one as recommended by the manufacturer and the other including a washing step with 8 M urea after the antigen-antibody reaction. A reduction in titer higher than, or equal to, 50% was considered indicative for presence of low avidity specific IgG.
  
  
  RESULTS
  When applied to the diagnosis of infections, this method showed sensitivity ranging from 81% to 100%, and absolute specificity. The detection of low avidity specific IgG allowed the differentiation between primary and recurrent infections caused by VZV. Furthermore, it helped in the identification of CMV as the etiological agent of congenital infection in the absence of specific IgM response, as well as in the elucidation of crossreactivity between antigenically related viruses, i.e., VZV and HSV, and Epstein-Barr virus and CMV.
  
  
  CONCLUSIONS
  FIA can be used for the characterization of the avidity of specific IgG antibody as a diagnostic test in clinical laboratories.
authors.affiliation: Departamento de Diagnóstico, Centro Nacional de Microbiología, Virología e Inmunología Sanitarias, Instituto de Salud Carlos III, Majadahonda, 28220 Madrid, Spain.
authors.forename: F
authors.initials: F
authors.lastname: de Ory
authors.forename: I
authors.initials: I
authors.lastname: Casas
authors.forename: C J
authors.initials: CJ
authors.lastname: Domingo
authors.forename: J
authors.initials: J
authors.lastname: Echevarría