Use of PCR cycle threshold values for toxins A and B quantification in Clostridioides difficile infections

Increasingly, tcdA/tcdB qPCR is being used to diagnose Clostridioides difficile infections (CDI). Under the hypothesis that toxin genes' quantification through Ct values could potentially improve clinical accuracy, this study aimed to assess the linearity of the C. difficile assay on the QIAstat-Dx^® Gastrointestinal Panel 2 (GI2 Panel) and to correlate bacterial to toxins load. Four analytical and thirty-five clinical toxigenic C. difficile isolates were quantified using three validated standard curve qPCR assays targeting adK, tcdA and tcdB genes. Of these, twelve were then tested to characterize the linearity of the C. difficile assay on the QIAstat-Dx^® GI2 Panel. Statistical analysis of the Ct values of adK, tcdA and tcdB obtained from standard curves presented an excellent linear fit (slopes range of 1.008-1.010 ± 0.001). A dynamic range of 1,000-1,000,000 copies/mL with R² ≥ 0.97 was established for the QIAstat-Dx^® GI2 Panel's C. difficile assay. The correlation among tcdA/tcdB and adK genes allows extrapolation to pathogen concentration. The QIAstat-Dx^® GI2 Panel's C. difficile assay demonstrated a wide linear range, allowing the accurate quantification of gene-encoding toxins A and B. This, in turn, presents a tool that could be key in establishing the relevance of toxin concentration and, potentially, a Ct cutoff at the time of CDI.