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'We just don't know if it's necessary'

Gabriel Borrud | Conor Dillon
March 5, 2021

Despite the discomfort, rectal COVID-19 probes do have some benefits over mouth and nose swabs. But DW spoke to a researcher who says the case still isn't clear for their use as a mass diagnostic tool.

USA New York JFK International Airport | Menschen stehen Schlange
Image: Reuters/S. Stapleton

Anal COVID-19 swabs — why is China doing them?

After independent complaints out of the United States and from Japanese officials, China's CDC has confirmed that at some airports passengers are being tested for COVID-19 through the use of anal swabs. These probes are also being used on local populations. On March 3 a foreign ministry spokesperson told Reuters that the decision to use these "virus prevention and control measures" was based on science.

DW spoke with Wendy Szymczak, who researched the utility of stool PCR for detecting SARS-CoV-2 RNA and implemented diagnostic testing for COVID-19 at the Montefiore Medical Center in New York.

DW: What can a stool PCR test do differently than one of those oral or nasal swabs that most of us are familiar with?

Wendy Szymczak: The main advantage of a stool PCR is that you can detect the SARS-CoV-2 RNA for a longer period of time, post-symptom onset. So, for the traditional nose and throat, or nasopharyngeal PCRs, you can detect virus usually for about two weeks if the patient has mild symptoms. Whereas in the stool, it's been shown that you can usually detect the virus for about four weeks, and for some patients even longer, out to 70 days. The biggest caveat, though, is that not all patients will shed viral RNA in the stool.

So there is some sense in testing this way if you're trying to figure out, over a longer time frame, whether or not someone still has the coronavirus in them.

Pressebild Wendy Szymczak
Dr. Wendy Szymczak is the director of the clinical microbiology lab at Montefiore Medical CenterImage: Montefiore Medical Center

Yes. It makes sense. It just depends on the purpose of why you're doing the test. For a patient where you're trying to make the diagnosis, if they're presenting to a medical center or urgent care center, it can be helpful because you have that extra timeframe to be able to pick up the infection. Now whether it's useful in an asymptomatic patient and for screening purposes, that's a little bit harder to say. And the reason is that we just don't know if those patients are only shedding virus in the stool or if they are able to transmit the infection to others.

Why does the virus stay in the stool longer than in the throat?

I don't think we really know that. We do know that the virus can infect the cells within the gastro-intestinal tract, but we don't really know why you tend to shed [virus] longer in the stool versus the upper respiratory tract.

Anal COVID-19 swabs — why is China doing them?

When it comes to testing for COVID-19, what are the disadvantages of using an anal swab?

It can never be used as a standalone test. It should always be done with that nasopharyngeal PCR. The other disadvantage is that people just don't like it. They don't want to have an anal swab done, especially if they've just traveled and they've just gotten off a plane. It can be an uncomfortable specimen to collect. Doing stool testing is another option. But I think, as you can imagine, it's probably a logistical issue trying to collect those specimens.

How does an anal swab work, exactly? Are people doing this themselves? Do they go into a bathroom stall? Or is there a medical professional with them?

Yeah, I think these are normally done by a medical professional. The swab is put in about an inch, rotated, and pulled back out.

If you were given a choice to test a large group of people, what would be the logic for opting for anal swabs rather than the swabs we're all used to?

You know I don't think you could do the [anal] swabs alone. I think the question is, does it make sense to do both the nasopharyngeal swab and the anal swab? To pick up people that you may miss by the upper respiratory testing? And that's where I think we just don't know. It's a lot of extra work, it's a lot of extra resources, it's uncomfortable for the people that are traveling. And we just don't know if it's necessary. We don't know what it means to be shedding that virus in the stool. Are you really putting others at risk of having a secondary infection?