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

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.