Article Figures & Data
Figures
- 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.
- 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.
- 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.
Tables
- Table 1
Cutoff values recommended by the manufacturer for each of the three avidity assays compared in this studya
Toxo IgG avidity assay Cutoff value or index for the following assay interpretation: Low avidity Gray zone High avidity Elecsys (Roche Diagnostics) <70% 70% ≤ value <80% ≥80% Architect (Abbott Laboratories) <50.0% 50.0% ≤ value <60.0% ≥60.0% Vidas (bioMérieux) <0.200 0.200 ≤ 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 result No. of samples with the following Vidas IgG avidity: Low Gray zone High Total Low 139 14 6 159 Gray zone 14 8 30 52 High 5 1 50 56 Total 158 23 86 267 - Table 3
Comparison of results obtained with Elecsys and Architect assays on samples collected in Grenoble
Elecsys IgG Avidity assay result No. of samples with the following Architect avidity: Low Gray zone High Total Low 129 13 5 147 Gray zone 13 4 3 20 High 3 0 47 50 Total 145 17 55 217
