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Diagnostic Laboratory Immunology

Evaluation of the New Elecsys Toxo IgG Avidity Assay for Toxoplasmosis and New Insights into the Interpretation of Avidity Results

Jean-Benjamin Murat, Coralie L'Ollivier, Hélène Fricker Hidalgo, Jacqueline Franck, Hervé Pelloux, Renaud Piarroux
Jean-Benjamin Murat
aLaboratory of Parasitology and Mycology, Grenoble University Hospital, Grenoble, France
bUMR 5163 CNRS—Joseph Fourier University Grenoble I, Grenoble, France
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Coralie L'Ollivier
cLaboratory of Parasitology and Mycology, La Timone University Hospital, Assistance Publique-Hôpitaux de Marseille (AP-HM), Marseille, France
dUMR MD3, Aix-Marseille University, Marseille, France
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Hélène Fricker Hidalgo
aLaboratory of Parasitology and Mycology, Grenoble University Hospital, Grenoble, France
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Jacqueline Franck
cLaboratory of Parasitology and Mycology, La Timone University Hospital, Assistance Publique-Hôpitaux de Marseille (AP-HM), Marseille, France
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Hervé Pelloux
aLaboratory of Parasitology and Mycology, Grenoble University Hospital, Grenoble, France
bUMR 5163 CNRS—Joseph Fourier University Grenoble I, Grenoble, France
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Renaud Piarroux
cLaboratory of Parasitology and Mycology, La Timone University Hospital, Assistance Publique-Hôpitaux de Marseille (AP-HM), Marseille, France
dUMR MD3, Aix-Marseille University, Marseille, France
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DOI: 10.1128/CVI.00333-12
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  • Fig 1
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    Fig 1

    Elecsys Toxo IgG Avidity assay principle. Samples are diluted in parallel with either the reference diluent (DilUni) or avidity diluent (DilToxoAv) and incubated for 10 min. During this time, IgG antibodies against T. gondii are bound to unlabeled T. gondii-specific recombinant antigen present in the avidity diluent. The remainder of the assay is fully automated and follows the one-step double-antigen sandwich principle. Briefly, the samples are incubated with a biotinylated recombinant T. gondii-specific antigen (surface antigen 1 [SAG1]) and a ruthenium-labeled T. gondii-specific antigen (surface antigen 1) to form a sandwich complex. In the sample preincubated with the avidity diluents, the labeled antigens compete with the unlabeled antigens already bound to IgG antibodies. As high-avidity antibodies bind more strongly to the antigen than low-avidity antibodies, competition with the unlabeled form of the antigen is more efficient; thus, there is less binding of labeled antigens in samples with high-avidity antibodies. Streptavidin-coated microparticles are then added and the complex binds to the solid phase via biotin-streptavidin interactions. The reaction mixture is transferred to a measuring cell and the microparticles are magnetically captured onto the surface of an electrode. Unbound sample is washed away before a chemiluminescent reaction is induced by applying a voltage to the electrode, and chemiluminescence is measured by a photomultiplier. Avidity (in percent) can be calculated for samples with a reference measurement (i.e., an aliquot treated with reference diluent) of greater than 3 IU/ml according to the following equation: 100 − [(concentration of aliquot treated with DilToxoAv/concentration of aliquot treated with DilUni) × 100], where concentrations are in IU/ml.

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    Fig 2

    Correlation of avidity values obtained from 217 serum samples analyzed with the Elecsys and Architect assays (A) and 267 serum samples analyzed with the Elecsys and Vidas assays (B). r, correlation coefficient.

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    Fig 3

    Avidity values according to time since infection determined by the Elecsys (total number of serum samples [n] = 497) (A), Architect (n = 230) (B), and Vidas (n = 273) (C) assays. Each serum sample is represented by a dot. Dotted line, low-avidity and gray-zone cutoff; solid line, gray-zone and high-avidity cutoff. For each category (low avidity, gray zone, high avidity), the number of serum samples from each group is given within boxes. For each time point after infection, the median value is represented by a wide horizontal bar, while the median ± 1 standard deviation is represented by narrow horizontal bars.

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  • Table 1

    Cutoff values recommended by the manufacturer for each of the three avidity assays compared in this studya

    Toxo IgG avidity assayCutoff value or index for the following assay interpretation:
    Low avidityGray zoneHigh avidity
    Elecsys (Roche Diagnostics)<70%70% ≤ value <80%≥80%
    Architect (Abbott Laboratories)<50.0%50.0% ≤ value <60.0%≥60.0%
    Vidas (bioMérieux)<0.2000.200 ≤ index <0.300≥0.300
    • ↵a For all three assays, a high avidity value allows exclusion of a primary infection within the previous 4 months.

  • Table 2

    Comparison of results obtained with Elecsys and Vidas assays on samples collected in Marseille

    Elecsys IgG Avidity assay resultNo. of samples with the following Vidas IgG avidity:
    LowGray zoneHighTotal
    Low139146159
    Gray zone1483052
    High515056
    Total1582386267
  • Table 3

    Comparison of results obtained with Elecsys and Architect assays on samples collected in Grenoble

    Elecsys IgG Avidity assay resultNo. of samples with the following Architect avidity:
    LowGray zoneHighTotal
    Low129135147
    Gray zone134320
    High304750
    Total1451755217
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Evaluation of the New Elecsys Toxo IgG Avidity Assay for Toxoplasmosis and New Insights into the Interpretation of Avidity Results
Jean-Benjamin Murat, Coralie L'Ollivier, Hélène Fricker Hidalgo, Jacqueline Franck, Hervé Pelloux, Renaud Piarroux
Clinical and Vaccine Immunology Oct 2012, 19 (11) 1838-1843; DOI: 10.1128/CVI.00333-12

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Evaluation of the New Elecsys Toxo IgG Avidity Assay for Toxoplasmosis and New Insights into the Interpretation of Avidity Results
Jean-Benjamin Murat, Coralie L'Ollivier, Hélène Fricker Hidalgo, Jacqueline Franck, Hervé Pelloux, Renaud Piarroux
Clinical and Vaccine Immunology Oct 2012, 19 (11) 1838-1843; DOI: 10.1128/CVI.00333-12
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