Detection of immunoglobulin M antibodies specific for Toxoplasma gondii with increased selectivity for recently acquired infections

Abstract

Toxoplasma gondii infections can cause serious complications in pregnant women, leading to miscarriage, stillbirth, and birth defects. Definitive diagnosis of T. gondii acute infection is therefore critical for the clinical management of a mother and her fetus. Positive immunoglobulin M (IgM) results are not sufficient as evidence of recent infection, as these antibodies are often present for many months. Further, IgG avidity and differential agglutination tests, two tests used by reference laboratories to distinguish between recent and past infections, are not always in agreement, and both methods yield a significant number of indeterminate results. We report the development of a new toxoplasma IgM immunoassay that is performed by using a bead-based immunoassay on an automated analyzer (BioPlex 2200). Initial validation included 204 samples from pregnant women and 198 samples from asymptomatic healthy adults. An overall specificity of 99% was observed. Further, 100% sensitivity for acute infections was observed for 10 well-characterized seroconversion panels. We then examined 50 samples from pregnant women, all of which were IgM positive by ELISA, which had been fully evaluated in a reference laboratory. Of the 50 samples, 34 (68%) tested positive in the BioPlex 2200 toxoplasma IgM assay, of which 32 of 34 (94%) exhibited an acute or equivocal pattern by differential agglutination. Of the 16 negative samples, 15 (94%) showed high-IgG-avidity antibodies. Collectively, these results suggest that this new toxoplasma assay shows a preferential response to IgM antibodies produced by recent infections, reducing the number of positive results for pregnant women that will require extensive additional clinical evaluation.

FIG. 1.

Correlation of test results obtained by the BioPlex 2200 toxoplasma IgM assay and the VIDAS toxoplasma IgG avidity assay for 50 pregnant women. The solid vertical lines indicate VIDAS threshold levels for low, equivocal, and high avidity, with test results of ≤0.2 representing low avidity, results from 0.2 to 0.3 representing equivocal avidity, and results of >0.3 representing high avidity. The solid horizontal line indicates the threshold for discrimination of IgM-negative from IgM-positive samples by the BioPlex 2200 assay. AI values that were ≥1.0 are scored as positive.