From the Guidelines
To test stool for Clostridium difficile, order a C. difficile toxin PCR test, which is the preferred first-line diagnostic test. This molecular test detects the genes responsible for toxin production and provides results within hours with high sensitivity and specificity 1. When collecting the specimen, a walnut-sized portion of loose stool should be placed in a clean container without preservatives and transported to the lab promptly. Only diarrheal stools should be tested, as C. difficile can colonize the gut without causing disease in formed stools.
The following are key points to consider when testing for C. difficile:
- Testing for cure is not recommended as patients may remain positive for weeks after clinical resolution.
- Alternative testing methods include enzyme immunoassay (EIA) for toxins A and B, which is less sensitive but may be used in two-step algorithms, and glutamate dehydrogenase (GDH) antigen testing, which is sensitive for detecting the presence of C. difficile but cannot distinguish between toxigenic and non-toxigenic strains.
- The PCR test is preferred because it directly identifies the pathogenic potential of the organism by detecting the genes responsible for toxin production.
- The use of NAAT-only testing is a recommended practice for detection of the C. difficile toxin gene, with a high strength of evidence 1.
- Other testing algorithms, such as GDH-positive EIA followed by NAAT, and GDH-positive/toxin-negative EIA followed by NAAT, are also recommended as American Society for Microbiology (ASM) best practices for the detection of the C. difficile toxin gene or organism 1.
From the Research
Testing Options for Clostridium Difficile
- There are several options available for testing stool for Clostridium difficile, including toxin testing using an enzyme immunoassay method 2, cell culture cytotoxicity neutralization assays (CCNAs) 2, and nucleic acid amplification tests (NAATs) 2, 3, 4, 5, 6.
- Toxin testing using an enzyme immunoassay method has sensitivities ranging from 60 to 70% and specificities of 98% 2.
- NAATs have been shown to be superior to toxin EIAs, CCNA, and 2-step algorithms, but not to toxigenic culture 2, 3, 4, 5.
- The cobas Cdiff test is a NAAT that has been evaluated for the detection of toxigenic Clostridium difficile in stool samples, with a sensitivity of 92.9% and a specificity of 98.7% compared to the combined direct and enriched toxigenic culture method 4.
- The DNA amplification assay has been shown to be superior to the traditional toxin A/B EIA for the diagnosis of CDI, with a sensitivity and positive predictive value of 100% 5.
Recommended Testing Approach
- The use of a sensitive assay, such as a NAAT, is recommended for testing stool for Clostridium difficile 2, 3, 4, 5, 6.
- Repeat testing is not cost-effective when using a sensitive assay 2.
- The quantitative results of the NAAT can be used as a predictor of toxin presence, but additional toxin testing is still needed due to the suboptimal correlation between the two tests 6.