| Abstract: | ![]()
A comparative study of the Toxoplasma IgGI and IgGII Access (Access I and II, respectively; Beckman Coulter Inc.), AxSYM Toxo IgG (AxSYM; Abbott Diagnostics), Vidas Toxo IgG (Vidas; bioMerieux, Marcy l''Etoile, France), Immulite Toxo IgG (Immulite; Siemens Healthcare Diagnostics Inc.), and Modular Toxo IgG (Modular; Roche Diagnostics, Basel, Switzerland) tests was done with 406 consecutive serum samples. The Toxo II IgG Western blot (LDBio, Lyon, France) was used as a reference technique in the case of intertechnique discordance. Of the 406 serum samples tested, the results for 35 were discordant by the different techniques. Using the 175 serum samples with positive results, we evaluated the standardization of the titrations obtained (in IU/ml); the medians (second quartiles) obtained were 9.1 IU/ml for the AxSYM test, 21 IU/ml for the Access I test, 25.7 IU/ml for the Access II test, 32 IU/ml for the Vidas test, 34.6 IU/ml for the Immulite test, and 248 IU/ml for the Modular test. For all the immunoassays tested, the following relative sensitivity and specificity values were found: 89.7 to 100% for the Access II test, 89.7 to 99.6% for the Immulite test, 90.2 to 99.6% for the AxSYM test, 91.4 to 99.6% for the Vidas test, 94.8 to 99.6% for the Access I test, and 98.3 to 98.7% for the Modular test. Among the 406 serum samples, we did not find any false-positive values by two different tests for the same serum sample. Except for the Modular test, which prioritized sensitivity, it appears that the positive cutoff values suggested by the pharmaceutical companies are very high (either for economical or for safety reasons). This led to imperfect sensitivity, a large number of unnecessary serological follow-ups of pregnant women, and difficulty in determining the serological status of immunosuppressed individuals.Toxoplasmosis, caused by Toxoplasma gondii, is widespread in humans and warm-blooded animals. Although it is usually asymptomatic in immunocompetent humans, toxoplasmosis may cause severe disorders in pregnant women, because of the high risk of transplacental transmission and the occurrence of abortion or multiple congenital lesions in the fetus, and in immunocompromised individuals (5, 9).Life-threatening reactivation of a previous infection is commonly observed in cases of severe immunodeficiency (human immunodeficiency virus-infected patients, organ and hematopoietic stem cell transplant patients). For these patients, the detection of Toxoplasma-specific antibodies showing serological reactivation or primary infection is therefore essential for the appropriate diagnosis and prevention of severe toxoplasmosis (2, 7).The follow-up of patients with obstetric toxoplasmosis mainly depends on the detection of antitoxoplasma-specific immunoglobulin M (IgM) and IgG antibodies (14, 16, 18). The presence of toxoplasma-specific IgM at the time of the first blood test is a cause for concern. The presence of toxoplasma-specific IgG without IgM permits confirmation of the immunization of the patients and thus allows unnecessary and expensive follow-up to be avoided.For both obstetric follow-up and diagnosis in immunocompromised patients, tests for IgG are crucial. Since the 1980s, toxoplasma-specific IgG assays have been standardized by different generations of World Health Organization (WHO) standards (15), and test results have been reported in international units per milliliter (IU/ml). The dye test (DT), first described by Sabin and Feldman 60 years ago, is still the reference method for the serodiagnosis of toxoplasmosis. However, this test uses live Toxoplasma gondii and is now used in only a few laboratories (13). A good alternative, the test Toxo II IgG Western blot (LDBio, Lyon, France) has been proposed to be a confirmatory technique by Franck et al. (6). The results of this test appear to be consistent with those of DT, with a specificity of 100% and a sensitivity of 99.2%. Thus, this immunoblotting technique can be used as a very reliable and easy confirmatory test in laboratories where DT cannot be implemented.Despite the international standardization and the availability of a reference (or confirmatory) test, automated immunoassays frequently show discordance and moderate degrees of correlation (6, 12). A comparison of six random-access immunoassays (that report IgG levels in IU/ml) and the Toxo II IgG Western blot (LDBio) as a confirmatory technique was undertaken to review the analytical performance characteristics and the degree of standardization of the tests. |
