How Should Laboratories Be Testing for C. difficile?
Abstract & Commentary
By Ellen J. Baron, MD, PhD(ABBM), Professor of Pathology and Medicine, Stanford University; Medical School Director, Clinical Microbiology Laboratory, Stanford University Medical Center, is Associate Editor for Infectious Disease Alert.
Dr. Baron reports no financial relationships relevant to this field of study.
On November 11, 2008, the Association for Practitioners in Infection Control and Epidemiology (APIC) announced the results of a one-day prevalence survey conducted between May and August of last year.1 Almost 650 hospitals in 47 states, 12.5% of all the hospitals in the United States, sent in their data. According to the results, more than 12 of every 1,000 in-patients were infected with C. difficile. One major short-coming of the survey was that 94.4% of the positive results were based on an enzyme immunoassay performed by the laboratory. Thus, the point-prevalence survey actually underestimated the depth of the problem by at least 50%!
At a recent SHEA meeting, Gelone reported that laboratories overwhelmingly are using enzyme immunoassays (EIAs) for either toxin B or toxins A and B, and often adding a second test for glutamate dehydrogenase (GDH), an enzyme thought to be uniformly present in all C. difficile strains whether they produced toxin or not.2 Although it had been assumed by most microbiologists that the cell culture cytotoxin neutralization test, technically difficult and slow to generate results (requiring at least overnight incubation for a preliminary result), was the most reliable laboratory assay for CDI diagnosis, only 2% of laboratories in the United States still performed that test. Dale Gerding has long advocated a more rigorous assay, the cytotoxic culture, in which the stool is cultivated onto C. difficile selective agar, today's best formula being cycloserine-cefoxitin-fructose horse blood taurocholate agar, incubated anaerobically. Organisms phenotypically (and by smell) resembling C. difficile are further incubated in an anaerobic broth at least overnight and sometimes for several days, and the broth is then tested for cytotoxin. This test is slow, but it does yield an isolate that can be typed, for example, as the NAP1 hypervirulent strain currently sweeping across the United States.3
Laboratory and Infection Prevention scientists at Johns Hopkins Hospital recently threw the longstanding reliance on both EIAs and cytotoxin neutralization into question. Their in-house cell culture cytotoxin neutralization test was only 67% sensitive compared to cytotoxic culture assay (the true gold standard).4 A PCR for toxin B gene sequences (tcdB gene) was 83.6% sensitive compared with cytotoxic culture, but increased to 90.9% when the previous "standard" cell culture cytotoxin B assay was used as the comparison, thus highlighting that the sensitivity of a test can vary based on the rigor of the comparative standard. Three years ago, the same Hopkins' workers published an influential paper advocating a two-step approach using GDH as a preliminary screening test and following a positive GDH result with a confirmatory test, such as cell culture cytotoxin neutralization.5 In that paper, they reported that a popular toxin A and B EIA test, likely the primary assay used to test for CDI in the APIC prevalence study, was only 36% sensitive against cell \culture cytotoxin neutralization.4 The results were delayed because of the need to perform two tests, with batching of the second confirmatory test. That approach was revisited in their 2009 paper, in which a real-time PCR test for a toxin B gene sequence was superior to other methods, with a sensitivity of 86.3%, the most sensitive of all tests compared with the gold-standard cytotoxic culture.4
Sloan et al from Mayo Clinic also suspected that laboratories were missing important cases using the two-step approach.6 They developed an in-house PCR test for a toxin B genetic sequence and evaluated several EIAs and a GDH test, comparing it to the appropriate cytotoxic culture. At best, the sensitivity was 48% among all the EIA assays tested. Contrary to popular belief, the GDH assay also detected only 32% of the toxin-positive C. difficile recovered by culture. Even their home-brew PCR was only 86% sensitive compared with cytotoxic culture. If the point-prevalence study data are reassessed with the assumption that the EIAs used for 94.4% of the testing performed during the survey are failing to detect as many as 52% of the patients with CDI, the prevalence reported in the study could be only half of the true prevalence.
Commentary
Clearly the use of more accurate and rapid molecular tools in the laboratory will contribute to the ability to rapidly identify patients who need more aggressive treatment and will facilitate infection control interventions with enhanced potential to be effective.
References
- Jarvis WR, et al. National point prevalence of Clostridium difficile in US health care facility inpatients, 2008. Am J Infect Control. 2009;37:263-270.
- Gelone S. Clostridium difficile-associated disease: results of an international web-based surveillance project. Late breaking abstract. 16th Annual Meeting of the Society for Healthcare Epidemiology of America (SHEA). March 18-21, 2006; Chicago, IL.
- Gerding, D. N. 2007. New definitions will help, but cultures are critical for resolving unanswered questions about Clostridium difficile. Infection Control and Hospital Epidemiology. february 2007;28:113-115.
- Paul D. Stamper, et al. Comparison of a commercial real-time PCR assay for tcdB detection to a cell culture cytotoxicity assay and toxigenic culture for direct detection of toxin-producing Clostridium difficile in Clinical Samples. Journal of Clinical Microbiology. 2009;47:373-378.
- Ticehurst JR, et al. Effective Detection of Toxigenic Clostridium difficile by a Two-Step Algorithm Including Tests for Antigen and Cytotoxin. Journal of Clinical Microbiology. 2006;44:1145-1149.
- Sloan, L.M. et al. Comparison of Real-Time PCR for Detection of the tcdC Gene with Four Toxin Immunoassays and Culture in Diagnosis of Clostridium difficile Infection. JCM. 2008;46:1996-2001.
You have reached your article limit for the month. Subscribe now to access this article plus other member-only content. TESTING
- Award-winning Medical Content
- Latest Advances & Development in Medicine
- Unbiased Content