Clostridioides difficile (CD) is an anaerobic, Gram-positive, spore-forming bacillus that causes C. difficile –associated infections (CDI). CDI is a clinical diagnosis; laboratory confirmation usually involves a combination of methods including culture, detection of other markers such as glutamate dehydrogenase (GDH), PCR screening for toxin genes and toxin detection. Although it is widely accepted that a two-step algorithm is preferable, it remains unclear which approach is optimal to detect toxigenic strains and still be cost effective. For screening, PCR and GDH can be expensive, but may be more rapid, depending on the laboratory workflow. In contrast, culture using selective agar takes approximately 24 hours, but has the advantage of making isolates available for further investigation if required. There are concerns about the highly sensitive PCRs for toxin genes overcalling disease where the genes are detected but are toxin negative. Conversely, there is concern of a reduction in sensitivity of culture-based screening compared to PCR screening, particularly when patients may have already been on treatment for presumed CDI.
This study sought to evaluate whether there was an unacceptably high false negative rate using selective C. difficile culture as the screening method, compared to GDH or toxin gene PCR, combined with toxin testing directly on the sample. During February 2021, fifty unformed or liquid faeces from patients that were negative by selective culture on CDIF Chrom ID™ (BioMerieux, France) were also tested for GDH and toxin using the C.diff Quik Chek Complete® (Techlab®, USA), and by an in-house Taqman PCR with two gene targets in the C. difficile PaLOC. This demonstrated 100% correlation between culture and GDH and 98% between culture and GDH compared to PCR.
Conclusion: These three methods correlate strongly for use as a screening test to exclude CDI in symptomatic patients. The differences in costs, batching and other logistical issues remain the major factor when determining the optimal initial screening method for CDI.