Serological diagnosis of Toxoplasma gondii: analysis of false-positive IgG results and implications

Primary infection by Toxoplasma gondii in pregnant women can result in serious outcomes for the foetus. A false-positive IgG result during pregnancy can lead to a misdiagnosis of past infection and to stopping preventive measures. We collected 189 sera with positive Architect Toxo IgG assay (Abbott Laboratories) and negative IgG results with at least two other serological tests, in order to find an explanation for the suspected false-positive IgG results. We used the recomLine Toxoplasma IgG immunoblot (Mikrogen Diagnostik) to search for specific antigenic reactivities of the sera, and the LDBio Toxo II IgG immunoblot (LDBio Diagnostics) as a confirmatory test. The bands GRA8 and/or GRA7 were positive for 148 samples (78.3%). GRA8 was the most frequent band, appearing in 133 patterns (70.4%), whereas GRA7 was present for 49 samples (25.9%). Of the 81 samples tested with LDBio , 23 (28.4%) turned out to be positive. Of the 58 negative LDBio tests (71.6%) (real false-positive Architect IgG), 23 samples (39.6%) did not show either a GRA8 or p30 band by recomLine . Their false positivity with Architect remains unexplained since Abbott uses these two recombinant antigens for their assay. The Architect IgG false positivity for T. gondii seems to be due to reactivity against GRA8 for the majority of the sera and GRA7 to a lesser extent. The hypothesis of past contact with parasites genetically close to T. gondii such as Hammondia hammondi or Neospora caninum seems promising and should be assessed further.