Association of tcdB PCR Cycle Threshold with Toxin Antigen Positivity and Clinical Severity in Clostridioides difficile Infection

Abstract

Various methods are used to diagnose and evaluate the severity of a Clostridioides difficile infection (CDI). Among them, the correlation between the tcdB gene PCR cycle threshold (Ct) values and toxin antigen detection is a key area of research. This study examined the relationship between stool tcdB Ct values and toxin antigen results, and their clinical utility in evaluating the CDI severity. A retrospective review was conducted on 300 hospitalized patients suspected of antibiotic-associated diarrhea who tested positive for C. difficile tcdB by PCR (GeneXpert, Cepheid) from Jan 2023 to May 2025. The patients were divided into toxin antigen-positive and -negative groups using C.DIFF QUIK CHEK COMPLETE® (TECHLAB, Inc.). The Ct values in the two groups were compared, and their associations with three severity scoring systems (Zar score, ATLAS score, IDSA/ SHEA guidelines) were analyzed. The median Ct value was significantly higher in the antigen- negative group (29.6) than in the antigen-positive group (25.4) (P=0.0001). The optimal Ct cutoff for predicting antigen positivity was 27.2 (AUC=0.779). Toxin positivity correlated significantly with the ATLAS scores (P=0.041), IDSA/SHEA scores (P=0.012), and CDAD scores (P=0.333). The Ct values showed no significant association with CDI severity. Toxin antigen may be clinically useful in assessing the CDI severity.