Serological diagnosis of<i>Toxoplasma gondii</i>: analysis of false-positive IgG results and implications
Loïc Simon, Judith Fillaux, Aurélie Guigon, Rose‐Anne Lavergne, Odile Villard, Isabelle Villena, Pierre Marty, Christelle Pomarès, Toxoplasma p35 Study Group
Abstract
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 recom Line 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 recom Line ® . 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.