SAGE IVD recommended fully listing the Clostridioides difficile combined glutamate dehydrogenase antigen and toxins A and B test in the EDL 5:
• as a disease-specific IVD for use in health care facilities with clinical laboratories (Section II.b. Clostridioides difficile infection);
• using a RDT as assay format;
• using stool as specimen type (diarrhoeal stool being the ideal specimen);
• to aid in the diagnosis of infection caused by Clostridioides difficile. SAGE IVD also noted that other tests for glutamate dehydrogenase antigen and toxins A and B are available as separate tests, if used, theyshould be done sequentially in alignment with the algorithm for C. difficile management.

The test proposed combines detection of glutamate dehydrogenase antigen to determine the presence of C. difficile and toxins A and B to confirm the presence of toxigenic C. difficile. It was submitted by WHO HQ/AMR Division/SPC Department/SEL Unit. The importance of a rapid point-of-care test for C. difficile infection was stressed as it has a high disease burden globally and high mortality. Furthermore, misdiagnosis leads to inappropriate antibiotic use and the risk of antimicrobial resistance. Thus, two-step testing (glutamate dehydrogenase as a sensitive test, followed by toxin immunoassay as a specific test) is recommended. However, limited data were provided on combined testing, so more evidence of the tests used together would be useful. It was emphasized that the tests are suitable for controlled environments (hospitals or laboratories), not community settings. The data are mostly based on one study in 2013 as the others cited were flawed. In addition, there was a lack of clinical utility data and reliance on evidence from one manufacturer. Some SAGE members had cost concerns. The data are from high-income countries and the cost is about US$ 35 per test which could be a barrier in LMICs, although one member wondered about the cost given as in his experience it was substantially lower. Furthermore, comparisons were drawn with similar diagnostic tools that have become affordable over time with volume-driven cost reductions. Additionally, inclusion in the EDL could allow negotiation for lower costs. It was also pointed out that if there is no feasible alternative, cost should not be a barrier; for example, most populations in sub-Saharan Africa live in rural areas where PCR and culture are not available. And it was up to countries themselves to decide whether to include the test or not. The consensus leaned towards full listing of the test in the EDL while advocating for pricing and accessibility strategies and collection of additional data on cost–effectiveness.

Rapid diagnosis of Clostridoides difficile infection (CDI) allows for timely administration of antibiotic therapy and prevention of transmission in healthcare settings. The reference standard for CDI is toxigenic culture, a labor-intensive and slow method that can test for both presence of C. difficile and toxin production. Several alternative tests are available for CDI, including enzyme immunoassays and nucleic acid amplification tests. The current EDL application focuses on rapid immunoassays that detect both glutamate dehydrogenase (GDH; indicating bacterial presence) and toxins A and B (indicating current infection) in combination. While several studies exist that have evaluated GDH and toxins A/B as separate tests, few studies have evaluated these in combination. The evidence about the diagnostic accuracy of this combined test rests heavily on one large study by Planche and colleagues (2013), as other studies have serious limitations. Using culture as reference standard, Planche and colleagues showed that sensitivity and specificity estimates of the GDH + toxins A/B combination were 57.0% (95% CI 53.9 to 60.0) and 99.4% (95% CI 99.3 to 99.6), respectively. The study also shows a much higher estimate of sensitivity (81.8%) when cell cytotoxin assay is used as the reference standard. We can illustrate the consequence of this test in a hypothetical cohort of 1000 people. If we consider culture to be the gold standard and we assume a prevalence of approximately 8.3% (from the Planche 2013 study sample), meaning 83 people would have CDI, 47 people would be correctly identified as having CDI and 36 people will be missed. Among 917 people without CDI, 911 people would be correctly ruled out while 6 people would be incorrectly diagnosed as having CDI. However, this study used a combination of two separate commercial tests for GDH (C. DIFF CHEK-60 [TECHLAB]) and toxins A/B (C. DIFF TOX A/B II [TECHLAB]) rather than a single combined assay, and the study did not report how a positive combined test result was defined. Diagnostic accuracy studies on other commercially available rapid immunoassays are lacking. No studies about clinical utility were submitted for review. The 2017 updated clinical practice guidelines for CDI in adults and children (Infectious Disease Society of America and Society for Healthcare Epidemiology of America) state: “Use a NAAT alone or a multistep algorithm for testing (ie, GDH plus toxin; GDH plus toxin, arbitrated by NAAT; or NAAT plus toxin) rather than a toxin test alone when there are preagreed institutional criteria for patient stool submission…” for diagnosis of CDI in stool specimens from patients likely to have CDI based on clinical symptoms (weak recommendation, low quality of evidence). The 2017 guidelines also state: “Use a stool toxin test as part of a multistep algorithm (ie, GDH plus toxin; GDH plus toxin, arbitrated by NAAT; or NAAT plus toxin) rather than a NAAT alone for all specimens received in the clinical laboratory when there are no preagreed institutional criteria for patient stool submission…” for detecting patients at increased risk for clinical significant CDI in commonly submitted stool specimens. (weak recommendation, low quality of evidence). In summary, although combined assays for the detection of CDI are recommended, the supporting evidence predominantly relies on data from two commercial tests from a single manufacturer. As the performance of rapid immunoassays can vary depending on the specific assay and the choice of reference standard, further evaluations are warranted.

The selection and use of essential in vitro diagnostics: report of the fifth meeting of the WHO Strategic Advisory Group of Experts on In Vitro Diagnostics, 2024. World Health Organization. (To be published)