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March 2009
Vol. 19 No. 3
Patient Care and Science
EIN: Better Testing for C. difficile

The Emerging Infections Network (EIN) is a forum for infectious diseases consultants and public health officials to report information on clinical phenomena and epidemiological issues with public health significance. Any diagnostic or therapeutic recommendations and all opinions presented are those of the individual contributor. They do not necessarily represent the views of EIN, IDSA (EIN's sponsor), or the Centers for Disease Control and Prevention, which funds the EIN. The reader assumes all risks in using this information.

Some currently available tests for Clostridium difficile have relatively low sensitivity, and EIN members have been discussing how to get an accurate diagnosis quickly.

Some proposed protocols to repeat negative tests. But an EIN member from Georgia said, “Routine, repeat testing using the currently available EIA toxin A and/or B assays is likely not the best answer.” Repeating a negative test increases the likelihood of false positives. Waiting a few days between tests might improve their diagnostic power for patients who continue to have diarrhea. But, the member noted, “this defeats the purpose of earlier diagnosis and delays in treatment can spell disaster in some patients.”

The member added, “Accuracy in the diagnosis would likely improve if laboratories would routinely reject formed stool for testing and if testing was routinely ordered only for patients with three or more unformed stools in a 24 hour period.”

Another member from California recommended homogenizing stool samples. “Bacteria, and presumably toxins as well, are distributed irregularly throughout the fecal mass (although that is probably not so in the case of liquid stools)…We use a Waring blender inside an anaerobic chamber—not practical for a clinical lab, but maybe a Stomacher or simply mixing it up well by hand (with a stick or something, not the hand).”

“Why not use a two-step procedure?” another member suggested. “[Use] one of the immunoassay tests for C. difficile glutamate dehydrogenase antigen probes. They have a good negative predictive value and, if negative, one test is sufficient.”

In a separate thread, an EIN member asked if others had experience with the new PCR assays for Clostridium difficile infection.

One respondent noted that one commercially available PCR assay was more than 95 percent sensitive and specific compared to “gold-standard” toxigenic culture assay according to a poster presented at the Society for Healthcare Epidemiology of America’s annual meeting this March. The conventional ELISA assay for toxin A/B was only 50 percent sensitive in this manufacturer-sponsored study.

“Of 118 PCR-positive patients, 51 were identified only by PCR from their first study sample; only four of these had previously been diagnosed with C. difficile infection by ELISA,” the respondent said. “The increased cost of the assay must be weighed against the potential clinical benefit of rapidly and accurately diagnosing an additional 30 to 40 percent of patients with C. difficile infection.”

Another respondent from California added, “The speed (less than two hours) and sensitivity of the test are impressive, and would be helpful for initial diagnosis depending on index of suspicion. But it will also detect carriers. So the question may be whether it is too sensitive.” The respondent also noted that the PCR assay detects only toxin B genes and would miss rare but pathogenic toxin B-defective mutants, as well as non-toxigenic strains.

In a third thread on C. diff, an EIN member asked, “Is anyone familiar with a commercial lab that performs environmental C. diff cultures in a reliable and affordable manner?”

There were no responses to this question, highlighting the urgent need for more research regarding the role of the environment and testing for C. difficile.


E-mail the Emerging Infections Network.

The Emerging Infections Network (EIN) is a provider-based sentinel network designed to help the public-health community detect trends in emerging infectious diseases.

A joint project of IDSA and the Pediatric Infectious Diseases Society (PIDS) and supported by funding from the Centers for Disease Control and Prevention, EIN tracks emerging infectious diseases and keeps the public-health community up to date with issues that are currently affecting or may soon affect members’ clinical practices.The Network provides a great opportunity for members to share knowledge quickly across large geographical distances. Both IDSA and PIDS members are eligible to join. The EIN listserve allows members to discuss new disease trends and difficult cases. Click here for more information or to join EIN.
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