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Dry Rectal Swabs Prove Effective Alternative to Stool Samples for C. diff Diagnosis

MAY 14, 2018 | RACHEL LUTZ
Clostridium difficile (C. diff) is among one of the top 18-drug-resistant threats to the United States according to the Centers for Disease Control and Prevention, responsible for around 250,000 infections on an annual basis and 14,000 deaths.

When it comes to diagnosis, microbiological testing of stool samples is often used. However, a new study suggests that for simple PCR-based detection of C. diff, dry rectal swabs were an effective substitute for the use of stool samples.

“With this study, we proved that rectal swabs for the diagnosis of C. diff infection by PCR can replace the actually used stool samples,” study author Nathalie Jazmati, MD, University Hospital of Cologne, told our sister publication MD Magazine. “That will be more convenient for both patients and health care workers. Nevertheless, this was only a small study and our results have to be confirmed in a bigger clinical trial.”

In an effort to examine methods other than the analysis of stool specimens for C. diff confirmation, a research team from Germany examined the way rectal swabs with liquid transport medium and nylon flocked dry swabs performed for the detection of C. diff; they also evaluated the impact of storage temperature on the swabs.

For their study, the researchers collected 60 clinical stool samples that tested positive for C. diff by PCR and used them to simulate rectal swabs. Then, researchers dipped both wet and dry swabs into the stool and tested by PCR 3 times.

The first test took place immediately after the simulation “swab,” then, after 1 month and 3 months storage at -80°C. When the researchers tested the frozen samples, they first thawed them at room temperature for 15 minutes and the liquid swabs were vortexed for 30 seconds.

Testing all of the dry swabs 100% successfully detected C. diff, an equal rate of the stool sample testing; this proved true for all 3 phases of testing, and the researchers learned that no significant differences were found on the samples after they were frozen and thawed.

The detection rate for the other 30 liquid swabs was lower, at 83.2% accuracy. However, the researchers determined the temperature and the freezing and thawing of these samples did not have any significant impact.

The authors added that their results fall in line with other studies that tested PCR from rectal swabs in the detection of C. diff. The idea of using rectal swabs instead of stool samples isn’t new—it dates back to 1987.

Liquid swabs are currently cleared by the US Food and Drug Administration (FDA) for transport and the culture of gastrointestinal pathogens, the study authors continued, but it is not FDA approved for use with any molecular gastrointestinal assays.

In the future, dry swabs would “be appropriate and can probably speed up and facilitate the diagnosis of C. diff infection,” the researchers wrote, but warned, “nevertheless, using single step PCR-based detection of C. diff may lead to overdiagnosis of C. diff infection due to the high sensitivity but lower specificity of PCR.”

That marks a heightened importance for the careful clinical evaluation of the patient: Are they an asymptomatic carrier? Is there another reason for the patient’s diarrhea? Do they truly have a C. diff infection? All important questions to continue to ask.

While liquid swabs cannot substitute for the two-step laboratory diagnosis of C. diff, the researchers believe that their study shows the dry swab is a suitable alternative to stool sample testing.

A previous version of this article was posted on MDMagazine.com.
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