Use of Toxoplasma gondi Specific IgG Avidity Assay for Diagnosis of Acquired Toxoplasmosis in Pregnant Women

Abstract

Fetal infection by Toxoplasma gondii develops when non-immune mother becomes infected during pregnancy. Accurate dating of infection is mandatory before intervention to prevent dangerous complications. T. gondii IgM persists in patient's serum months after infection, providing no evidence of recent infection, beside its lack of specificity. T.gondii IgG avidity assay is expected to be a tool to distinguish recent and past infection. High avidity excludes recent infection in the preceding 16-20 weeks, however, low avidity doesn't help to differentiate between recent and old infection. Sera from 2070 asymptomatic Saudi Arabian pregnant women at different gestational stages were screened for T.gondii IgG by Indirect hemagglutination test. Enzyme–linked fluorescent assay (ELFA) was used for detection of T.gondii IgM, quantitative assay of IgG and IgG-avidity index (AI). Out of 401 IHA-positive sera, 151 (37.7%) samples were positive for T.gondii IgM, among which AI was low in 17 samples (11.3%), intermediate in 1 (0.6%), and high in 133 samples (88.1%). The possibility of recent infection in the IgM-positive sera is excluded since they contained high IgG AI (95% CI 83-93%). IgM-negative sera showed either high (in 97.8% of sera) or intermediate AI. This finding minimized the value of a positive IgM finding as single indicator for acute recent infection. Actually, it highlighted the value of IgG avidity assay to exclude recent infection. No significant correlation between level of IgG, and presence of either low or high AI (r = 0.096, p = 0.054). Likewise, no correlation was found between IgG level and presence of IgM in serum (r =0.092, p =0.046). To conclude, data suggested that pregnant women should be initially screened for IgG, to determine the patient's immune status; followed by IgM assay of IgG-positive sera. Negative serum IgM essentially excludes recent infection. Regardless the serum IgG level,AI assay should follow to exclude recent infection in the past 16-20 weeks. Assays should rely on accurate techniques (e.g., ELFA). This will reduce the cost of repeated assays in the majority of pregnant women during 1st trimester, avoid unnecessary examinations, antibiotic therapy, and anxiety burden.