College of American Pathologists



Laboratory Detection of Clostridium difficile Infection

Posted August 30, 2013

Yaolin Zhou, MD

Clostridium difficile infection (CDI) is caused by toxigenic strains of the organism that produces toxins A and B, with clinical disease requiring both the presence of significant diarrhea (> 3 stools per day) and laboratory detection of the toxin.1 Rapid identification and treatment of infected patients is crucial, given the organism’s highly transmissible nature through hardy spores and potential for causing pseudomembranous colitis.

Virulence of C. difficile is conferred by two exotoxins, toxin A and toxin B, encoded by their respective genes tcdA and tcdB. In addition, a hypervirulent strain of toxigenic C. difficile was identified in 2005.2 This strain is characterized by the production of a binary toxin, encoded by the genes ctdA and ctdB.2 While the exact role of the binary toxin in the disease caused by C. difficile remains unclear, epidemic strains (BI/NAP1/027) carrying this toxin show hypervirulence and behave synergistically with tcdA and tcdB, resulting in severe colitis.2

The traditional “gold standard” for the detection of CDI is the toxigenic culture, necessary for epidemiological studies but rarely used clinically due to its lack of standardization and four to seven-day turnaround-time (TAT). In contrast, enzyme immunoassays (EIAs), which detect toxin A and/or B are still widely used, despite their low sensitivities (30%–70%). Because of EIAs’ known high false-positive rates, clinicians often order them on patients repeatedly, with sequentially lower positive predictive values.3,4 The Society for Healthcare Epidemiology of America and the Infectious Diseases Society of America (SHEA-IDSA) discourages the use of EIAs.5 An alternative method is a two-step algorithm: first detect GDH, a common antigen; then detect toxin with either EIA or cytotoxin neutralization assay.4

Since 2009, the US Food and Drug Administration (FDA) has approved several molecular-based tests with fast TAT and high sensitivities and specificities. These nucleic acid amplification assays include the PCR-based BD GeneOhm Cdiff assay (Becton Dickinson), proGastro Cd (Gen-Probe/Prodesse), and Xpert C. difficile (Cepheid). The Meridian illlumigene C. difficile assay, which uses loop-mediated isothermal amplification to detect tcdA, is also FDA-approved. Currently, tests for the binary toxin are for research use only.

Laboratory considerations in assay selection include test accuracy, equipment and reagent costs, and technician time; but infection control and its impact on patient care are critical. Patients suspected of CDI must be placed on isolation precautions appropriate for those who are truly positive. Unfortunately, even false-positive test results will lead to patient isolation and treatment with oral vancomycin or metronidazole. Conversely, patients with false negative test results are not treated, potentially prolonging hospitalization with risk of transmission to hospital staff and other patients. Therefore, the indirect cost-savings of timely diagnoses and decreased transmission can offset assay costs.

A concerted effort by both pathologists and clinicians is needed to implement new tests and hospital policies successfully. Guidelines developed through collaboration can include the discouragement of “test for cure,” which is not endorsed by SHEA-IDSA,5 as well as a mandatory waiting period. Though newer molecular tests might appear cost-prohibitive, tests that deliver results quickly and accurately may result in long-term cost savings and improved medical care.


  1. Petersen LR, Mehta MS, Patel PA, et al. Infection control of Clostridium difficile. Is this concept an illusion? Semin Infect Control. 2001;1:172–183. doi:10.1309/AJCPTP5XKRSNXVIL(2011) American Journal of Clinical Pathology, 136,372–280.
  2. McDonald LC, Killgore GE, Thompson A, et al. An epidemic, toxin gene-variant strain of Clostridium difficile. N Engl J Med. 2005; 353(23):2433–41.
  3. Petersen LR, Robicsek A. Does my patient have Clostridium difficile infection? Ann Intern Med. 2009;151(3):176–179.
  4. Ticehurst JR, Aird DZ, Dam LM, Borek AP, Hargrove JT, Carroll KC. Effective detection of toxigenic Clostridium difficile by a two-step algorithm including tests for antigen and cytotoxin. J Clin Microbiol. 2006; 44(3):1145–1149.
  5. Cohen SH, Gerding DN, Johnson S, et al; Society for Healthcare Epidemiology of America; Infectious Diseases Society of America. Clinical practice guidelines for Clostridium difficile infection in adults: 2010 update by society for healthcare epidemiology of America (SHEA) and the infectious diseases society of America (DSA). Infect Control Hosp Epidemiol. 2010;31(5):431–455. doi:10.1086/651706.

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NewsPath® Editor: Kyle L. Eskue, MD, FCAP
This newsletter is produced in cooperation with the College of American Pathologists Member and Public Communications Committee and the NewsPath Editorial Board and may be reproduced in whole or in part as a service to the medical community. Copyright © 2013 by the College of American Pathologists.
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