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

Background: 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. Results: 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. Conclusions: 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.