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Rectal Swabs an Optional Diagnostic Tool for Clostridium difficile

It can’t replace two-step laboratory testing, but it is a suitable alternative as a diagnostic tool.

Nathalie Jazmati, MD

In simple Clostridium difficile (C. difficile) diagnostic tests, dry rectal swabs were an effective substitute for the use of stool samples, according to new findings.

C. difficile infection confirmation is almost always done through the analysis of stool specimens, but instead, a research team from Germany wanted to examine other methods. Researchers examined the way rectal swabs with liquid transport medium and nylon flocked dry swabs performed in the detection of C. difficile infection, and evaluated the impact of storage temperature on the swabs.

The investigators collected 60 clinical stool samples that tested positive for C. difficile 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. difficile, an equal rate of the stool sample testing. This was 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 temperature and the freezing and thawing of these samples did not have any significant impact.

“With this study we proved that rectal swabs for the diagnosis of C. difficile infection by PCR can replace the actually used stool samples,” study author Nathalie Jazmati, MD, University Hospital of Cologne, told 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 the paper, study authors added that their results fall in line with other studies that tested PCR from rectal swabs in the detection of C. difficile. They also said 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. difficile infection,” the researchers wrote, but warned, “nevertheless, using single step PCR-based detection of C. difficile may lead to over-diagnosis of C. difficile infection due to the high sensitivity but lower specificity of PCR.”

That marks a heightened importance for 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. difficile infection? All important questions to continue to ask.

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

The full paper, titled “Evaluation of the use of rectal swabs for the laboratory diagnosis of Clostridium difficile infection,” was published in the Journal of Clinical Microbiology.