Detection of IgA and low-avidity IgG antibodies for the diagnosis of recent active toxoplasmosis

Abstract

Objective: To determine the clinical value of testing IgA and the avidity of IgG (by two commercial systems) for the detection of recent active toxoplasmosis (RAT), and t o study the IgG avidity during the course of infection.Methods: The IgA was tested by a capture ELISA (Pasteur, France) and the avidity of IgG was determined by two modified commercial indirect ELSA methods (Sorin, Italy; Behringwerke, Germany) in 12 patients who were not immunosuppressed (group I) and 57 healthy subjects with a past infection by Toxoplasrna gondii (group 11). Results:IgA was present in 75% of patients from group I and 21% of subjects from group II. The reliabilityfor diagnosis of RAT was: sensitivity 75%, specificity 84%, positive predictive value 52.9% and negative predictive value 93.3%. In group I, 91.7% of patients had more than 50% low-avidity IgG, by both methods; in group 11, 21% of subjects had lowavidity IgG at levels from 40% to 50%, by both methods. The diagnostic reliability of the two methods for the detection of low-avidity IgG in the first samples of RAT was similar when a breakpoint of 50% was used, with values of: sensitivity 91.7%, specificity loo%, positive predictive value 100% and negative predictive value 98%. Conclusions:The study of IgA is not on its own adequate for diagnosis of RAT. However, testing the avidity of IgG is more reliable for the diagnosis of RAT, in studies of one serum sample or sequential samples.

Diagnosis and Management of Toxoplasmosis

“…For these patients, the authors have recommend a conservative approach-we suggest that the patient be managed similarly to those patients for whom serology results suggest an infection acquired during gestation. More recently, however, testing for T gondii-specific IgG avidity for certain parasite antigens [17,[23][24][25][26][27][28][29] has been reported to be useful for confirmatory testing in patients who have positive IgG and IgM titers or equivocal TSP results [17]. High-avidity IgG antibodies develop at least 12 to 16 weeks (depending on the test kit used) after acquisition of infection.…”

“…A number of tests for avidity of toxoplasma IgG antibodies have been introduced to help differentiate between recently acquired and distant infection [17,[23][24][25][26][27][28][29]. This method is based on the observation that during acute T gondii infection, IgG antibodies bind antigen weakly (ie, have low avidity), whereas chronically infected patients have more strongly binding (high avidity) antibodies [23].…”

Identification of Potential Serodiagnostic and Subunit Vaccine Antigens by Antibody Profiling of Toxoplasmosis Cases in Turkey

“…High avidity IgG in the first trimester indicates the exposure probably occurred before pregnancy and the fetus is at no risk for congenital toxoplasmosis. In contrast, low avidity IgG may help in the diagnosis of acute infection, although the result should be interpreted with caution as some individuals have low affinity IgG that persists for several months after infection (32)(33)(34). In all cases, it is recommended that a diagnosis of recent or acute infection is re-tested by an experienced toxoplasmosis reference laboratory using a panel of serologic and molecular tests, including the complement fixation test.…”

Clinical and diagnostic relevance of the Toxoplasma IgG avidity test in the serological surveillance of pregnant women in Austria

“…Finally, an IgG avidity test was established by Hedman et al (1989) for purposes of diagnosing Toxoplasma gondii infections. Since the beginning of the 1990s, Toxoplasma IgG avidity has been evaluated due to its applicability in the diagnosis of ocular (Vinhal et al 1994;Garweg et al 1998) and cerebral toxoplasmosis , of toxoplasmosis in immunocompetent (Joynson et al 1990;Gutierrez et al 1997;Rossi 1998) or immunocompromised patients (Radkowski et al 1995), and, in particular, of Toxoplasma infections during pregnancy (Koskiniemi et al 1992;Lappalainen et al 1993;Holliman et al 1996;Jenum et al 1997;Liesenfeld et al 1997;Asburn et al 1998;Cozon et al 1998;Pelloux et al 1998).…”