title: A simple, rapid enzyme-linked immunosorbent assay for evaluating immunoglobin G antibody avidity in toxoplasmosis.
abstract: This article describes an enzyme-linked immunosorbent assay (ELISA) for evaluating the avidity of Toxoplasma-specific immunoglobin (Ig) G. The test was standardized with the Falcon assay screening test (F.A.S.T) ELISA system using 6 M urea in phosphate-buffered saline for dissociating low-avidity antibodies after the antigen-antibody interaction. The avidity indices for 25 serum samples from patients with a recent Toxoplasma infection ranged from 10 to 40% (mean index = 25.3%), whereas for sera from 25 subjects with preexisting Toxoplasma immunity, the indices ranged from 58 to 94% (mean index = 73.1%). In sequential serum samples obtained over a period of up to 15 months from three patients exhibiting a persistent IgM antibody response to T. gondii, the avidity indices gradually increased during the course of infection. Avidity indices compatible with a recent infection were found in serum samples taken during the first 3 months after the onset of toxoplasmic lymphadenopathy, whereas indices compatible with the presence of a long-term infection were found in serum samples taken 7 or more months after the onset of toxoplasmic lymphadenopathy. The simplicity and short assay time (less than 1 h) make the test a potentially useful tool in assessing the avidity of Toxoplasma-specific IgG.
authors.affiliation: Department of Clinical Pathology, Faculty of Medical Sciences, State University of Campinas, São Paulo, Brazil.
authors.forename: C L
authors.initials: CL
authors.lastname: Rossi
mesh_terms: D000818:Animals; D000913:Antibodies, Protozoan / Q000276:immunology; D000915:Antibody Affinity; D002908:Chronic Disease; D004797:Enzyme-Linked Immunosorbent Assay / Q000379:methods*; D006801:Humans; D007074:Immunoglobulin G / Q000276:immunology*; D011237:Predictive Value of Tests; D011933:Reagent Kits, Diagnostic; D012680:Sensitivity and Specificity; D014122:Toxoplasma / Q000276:immunology*; D014123:Toxoplasmosis / Q000175:diagnosis* / Q000276:immunology / Q000469:parasitology
chemical_list: D000913:Antibodies, Protozoan; D007074:Immunoglobulin G; D011933:Reagent Kits, Diagnostic