Factors affecting the detection of Clostridium difficile in faecal samples

Clostridium difficile infection (CDI) laboratory diagnostic assays have variable performance, but reasons behind this variability are not well defined. In contrast to previous findings, the PCR ribotype of the organism only appears to be a factor in reduced sensitivity for toxin enzyme immunoassays (EIAs), not glutamate dehydrogenase (GDH) EIAs. Growth curve data demonstrated that GDH is produced during the exponential phase of growth, and the sensitivity of the GDH assay in vivo may be related to the amount of protein produced by the organism, as very high levels of GDH were detected during growth of C. difficile in an in vitro gut model. Indeed the levels of GDH, measured in both gut model and patient samples, correlated with organism bioload. In addition, the median faecal levels of GDH in recurrent CDI cases were significantly higher than in patients with a single infection episode. Interestingly, when patients had sequential faecal samples tested, 27% with an initial GDH-positive/toxin-negative result had a subsequent toxin positive sample, after a median of eight days. Further studies, with supplementary assays for gut inflammation, are required, to determine if these are ‘missed’ infections or insignificant sub-clinical levels of toxin. A laboratory test that could predict risk of recurrence would be an important tool to inform choice of appropriate C. difficile treatment and prevention options. Indeed GDH detection may offer such an opportunity; a cohort of patients has been identified who were consistently GDH positive, even after resolution of symptoms, who subsequently developed recurrent CDI following additional antimicrobial therapy. The cycle threshold (CT) value of PCR assays for the detection of toxin gene may also provide additional information, as low CT (<25) was significantly associated with toxin positivity, presence of PCR ribotype 027 and mortality. Low CT was also associated with recurrence but was not a significant finding.