# P-1008. Polymerase Chain Reaction Cycle Threshold as a Predictor for Cell Culture Cytotoxicity Assay Results in Hospitalized Adults With PCR-Positive Clostridioides Difficile Infection

## TL;DR

This study investigates whether PCR cycle threshold values can predict the results of a more accurate but slower test for C. difficile infection.

## Contribution

The study explores the potential of PCR CT values as a predictor for CCCA results in C. difficile diagnosis.

## Key findings

- Lower PCR CT values showed a potential association with CCCA positivity, but no definitive cutoff was identified.
- 55.7% of PCR-positive specimens were confirmed positive by the gold standard cell culture cytotoxicity assay.
- NPV and PPV trends suggest CT values may reflect bacterial burden, but results are limited by sample size.

## Abstract

Diagnosing Clostridioides difficile infection remains challenging due to high sensitivity but limited specificity of polymerase chain reaction (PCR) assay, which often detect colonization rather than true infection. Alternative testing with improved specificity like the cell culture cytotoxicity assay (CCCA), the gold standard for toxin confirmation, remains time intensive. Emerging evidence suggests PCR cycle threshold (CT) values may predict toxin presence. This study explores the relationship between CT values and CCCA results to improve diagnostic accuracy and guide treatment decisions.

This retrospective cohort study analyzed adults with both C. difficile PCR and CCCA test results available at multiple Texas Health Resources hospitals between April 2023 and December 2024. Patients receiving C. difficile treatment prior to specimen collection or with incomplete data were excluded. Demographic, laboratory, and clinical data were collected from electronic health records. PCR CT values and CCCA results were assessed using ROC curve analysis to determine diagnostic performance. Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) were analyzed at different CT thresholds.

A total of 456 samples from 417 unique patients were included; the median age was 68 years, and 33.5% were male. Most patients were classified as community-acquired (82.5%). The median PCR CT value was 27.8. ROC curve analysis yielded an AUC of 0.288, indicating poor predictive performance. As CT values increased, sensitivity and NPV increased, while specificity and PPV decreased. No definitive CT cutoff was identified. Overall, 55.7% of PCR-positive specimens were confirmed positive by CCCA.

Our preliminary findings suggest a potential relationship between lower CT values and CCCA positivity, but no definitive CT cutoff was identified. The trends observed in NPV and PPV of CT values may reflect differences in bacterial burden, but interpretation is currently limited by small sample size. While CT values may provide contextual value in diagnostic interpretation, they remain investigational and should not guide clinical decisions alone.

All Authors: No reported disclosures