Toxin B PCR Amplification Cycle Threshold Adds Little to Clinical Variables for Predicting Outcomes in Clostridium difficile Infection: a Retrospective Cohort Study

The objective of the present study was to evaluate the value of the PCR cycle threshold (C T ) for predicting the recurrence/severity of infection compared to that of toxin detection plus clinical variables. First episodes of Clostridium difficile infection (CDI) diagnosed during 2015 at our institution were included. Samples were tested for glutamate dehydrogenase (GDH) and toxin A/B by use of a single enzyme immunoassay (EIA). The Xpert C. difficile PCR assay was performed on GDH-positive samples. Medical data were reviewed by investigators blinded to diagnostic results for comparison of patients with and without recurrence or a poor outcome (severe/severe-complicated CDI episodes and all-cause death). We generated two sets of predictive models by incorporating the presence of a positive toxin EIA (EIA-including model) or the optimal PCR C-T cutoff value (PCR-including model) into the clinical variables. Among 227 episodes of CDI included in the study, the rates of recurrence and poor outcome were 15.8% and 30.8%, respectively. The mean PCR C-T was lower for episodes with recurrence (24.00 +/- 3.28 versus 26.02 +/- 4.54; P = 0.002) or a poor outcome (24.9 +/- 4.24 versus 26.05 +/- 4.47; P = 0.07). The optimal cutoff value for recurrence was 25.65 (sensitivity, 77.8% [95% confidence interval {CI}, 60.9 to 89.9]; and specificity, 46.6% [95% CI, 39.4 to 53.9]). The area under the receiver operator characteristics curve (auROC) for the PCR-including model was similar to that for the EIA-including model (0.785 versus 0.775, respectively). The optimal PCR C-T value for poor outcome was 27.55 (sensitivity, 78.6% [95% CI, 67.1 to 87.5]; and specificity, 35.7% [95% CI, 28.2 to 43.7]). The auROC of the PCR-including model was again similar to that of the EIA-including model (0.804 versus 0.801). Despite the inverse correlation between PCR C-T and the risk of CDI recurrence/severity, this determination does not meaningfully increase the predictive value of clinical variables plus toxin EIA.