Bacteriology
Evaluation of the Roche Elecsys Toxo IgG and IgM electrochemiluminescence immunoassay for the detection of gestational Toxoplasma infection

https://doi.org/10.1016/j.diagmicrobio.2010.07.011Get rights and content

Abstract

Unidentified gestational infection with Toxoplasma gondii may lead to fetal infection with severe complications later in childhood. Because diagnosis of maternal infection solely depends on serology, routine tests with high sensitivity and specificity are required. In this study, the new Roche Elecsys Toxo IgG and IgM immunoassay was compared with Sabin–Feldman dye test and immunosorbent agglutination assay-IgM as reference test. Serum samples were analyzed from 927 pregnant women, including 100 negative, 706 chronic, and 121 acute infections. The combination of both Elecsys IgG and IgM assays demonstrated high sensitivity and specificity of 97.1% and 100.0%, respectively, and a positive and negative predictive value of 100.0% and 81.3%, respectively. The Elecsys assay is a useful tool as a first-line screening method to detect gestational infections. However, if gestational infection is assumed, confirmatory testing by a reference laboratory might be necessary to discriminate between pre- and postconceptional infection to start antiparasitic treatment to avoid mother-to-fetus transmission and severe sequelae.

Introduction

Unrecognized maternal postconceptional infection with the protozoan Toxoplasma gondii may cause a broad spectrum of clinical diseases, varying from subclinical infection to connatal toxoplasmosis with severe symptoms such as retinochoroiditis, intracerebral calcifications, hydrocephalus, and mental retardation (Montoya and Liesenfeld, 2004, Remington et al., 2000). Fetal infection risk increases with gestational age at maternal seroconversion, resembling a risk of 15% at 13 weeks, 44% at 26 weeks, and 71% at 36 weeks (Thiebaut et al., 2007). In contrary, severity of fetal damage after infection declines with gestational age. In the first trimester, infections mostly lead to abortions and may cause severe neurologic lesions within the second trimester. Within third trimester, infected fetuses very often present asymptomatically at birth (Wilson et al., 1980).

In Austria, pregnant women are subject to a nationwide routine serologic prenatal screening program. Initial Toxoplasma serology is carried out within the first trimester, mostly after the first visit at the gynecologist. In seronegative pregnant women, follow-up serology is recommended at 8-week intervals to identify seroconversions. Seropositive sera are further investigated to discriminate acute (postconceptional) from chronic (preconceptional) infection to avoid unnecessary retesting and, more important, to avoid unnecessary therapy. In the majority of laboratories, serologic screening is based on commercially available enzyme immunoassays, and every test system has to fulfill criteria of adequacy including high sensitivity and specificity, cost effectiveness, and easy handling under routine laboratory conditions.

The Sabin–Feldman dye test (DT), still considered as the “gold standard” for the detection of Toxoplasma infection, is expensive and time consuming (Reiter-Owona et al., 1999). Its application is therefore restricted to specialized laboratories as confirmatory test and as standard for validation of new test systems.

In case of acute Toxoplasma infection, amniocentesis for direct fetal diagnosis by polymerase chain reaction (PCR) is performed (Gratzl and Hayde, 1998, Kasper and Sadeghi, 2009, Knerer and Hayde, 1995) and antibiotic treatment according to the PCR result is recommended (Aspock et al., 1994). The transmission rate (Thiebaut et al., 2007) and dimension of sequelae depend on the period between seroconversion and initiation of treatment (Gras et al., 2004). Long serologic testing intervals are favoring fetal infection, whereas early prenatal treatment is able to reduce the risk of cerebral lesions and retinochoroiditis, the main complications of connatal toxoplasmosis (Gras and Wallon, 2005, Kieffer and Wallon, 2008).

The aim of the study was to evaluate the recently introduced Roche Elecsys Toxo IgG and IgM immunoassays on a Cobas 2010 system as a routine screening method for the detection of T. gondii antibodies in the sera of pregnant women. The Elecsys test results were compared with the DT results and the immunosorbent agglutination assay (ISAGA)-IgM for determination of anti–T. gondii-specific IgM.

Section snippets

Serum specimens

A total of 927 blood samples from pregnant women were collected and assessed according to the recommendations of the Austrian Toxoplasmosis Screening Program by the toxoplasmosis laboratory located at the Department of Pediatrics and Adolescent Medicine, Medical University of Vienna. The discrimination of chronic and acute infection status was performed according to the criteria of Lebech et al. (1996) in A) no infection: DT negative; B) postconceptional (acute) infection: seroconversion during

Performance of Roche Elecsys Toxo IgM and IgG assay

Roche Elecsys IgM results were compared to ISAGA-IgM test. Table 1 shows the comparison of 927 serum samples measured with Elecsys IgM with the ISAGA-IgM assay. Six hundred nineteen (66.7%) samples tested with Elecsys assay were nonreactive, 33 indeterminate (3.6%), and 275 (29.7%) reactive. Of the latter sera, 260 of 275 (94.5%) were positive, 12 (4.4%) borderline, and 3 (1.1%) negative in ISAGA-IgM. From the 619 serum samples that were nonreactive in the Elecsys IgM, 498 of 619 (80.5%) were

Discussion

Untreated infection with Toxoplasma gondii during pregnancy can lead to fetal infection with serious consequences (Remington et al., 2000). For screening purposes, economic, effective, and reliable laboratory systems are required to minimize costs for analysis and retesting. A first-line serologic test system has to be both highly sensitive not to miss any women at risk and highly specific to minimize subsequent confirmatory testing.

So far, several different test systems were evaluated (Hayde

Acknowledgments

The authors thank in particular Birgit Panzenböck and the work done by the staff members of the Reference Toxoplasmosis Laboratory.

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