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Chinese researchers say anal swabs are effective in testing for COVID-19. U.S. experts aren’t so sure. Dowell/Getty Images
  • Researchers in China say people with COVID-19 have active gut viral infections.
  • They say using anal swabs can therefore be effective in detecting the disease.
  • However, U.S. experts say nasal and throat swabs are better because COVID-19 is an upper respiratory disease.

Experts in the United States are questioning the use of anal swabs in China to detect COVID-19.

Researchers from the Faculty of Medicine of Chinese University of Hong Kong (CU Medicine) say they’ve found for the first time that people with COVID-19 have active and prolonged gut viral infection.

They say the infections occur even in the absence of gastrointestinal symptoms and even after the respiratory infection has cleared.

That means anal swab testing, which requires inserting a swab up to 2 inches into the rectum and rotating several times, could detect COVID-19 cases that the standard nose and throat swabs would miss, experts say.

Or does it?

While the findings impact China’s clinical management of COVID-19 patients, U.S. experts caution against taking the study — or anal swab testing for the novel coronavirus — seriously.

Anal swabbing and stool screening is not a new method of testing for COVID-19 in China.

Since September 2020, CU Medicine has been providing 2,000 tests a day, including stool screening tests. Free stool sample testing of children and select populations has been occurring since last March.

“It is hoped that this helps identify asymptomatic people carrying the COVID-19 virus as early as possible in order to stop its spread in our community,” CU Medicine officials say in a press release.

These stool tests are focused on high-risk babies and young children, the pediatric population returning to Hong Kong from high-risk areas, and people who have difficulties collecting specimens of sputum, nasal swabs, and throat swabs.

“Stool specimens are more convenient, safe and non-invasive to collect in the [pediatric] population and can give accurate results,” Paul Kay Sheung CHAN, chairman of the Department of Microbiology and associate director of the Centre for Gut Microbiota Research, said in the press release.

“Among the confirmed cases in the territory, we note that there is more than one COVID-19 patient who had stool test positive while tests for respiratory samples were negative,” added Francis Ka Leung Chan, dean of the Faculty of Medicine and director of the Centre for Gut Microbiota Research.

How many more?

Three patients from a sample of 15 showed active viral infection up to 6 days post-respiratory infection clearance at the beginning of the pandemic.

“Some countries, including the U.S. Food and Drug Administration, are currently in contact with us for details and arrangements of stool tests for COVID-19,” according to the press release.

“That report is false,” said Omai Garner, PhD, an associate clinical professor, clinical microbiology section chief, and point of care testing director in the Department of Pathology and Lab Medicine at UCLA Health.

Garner told Healthline that at first, he was surprised to see the report on anal swabbing being used to test for the disease.

“It’s well established that the gold standard or best sample to take is something upper respiratory,” he said.

“This was something that was looked at early on, in March, April, May timeframe, and it was found that the PCR positive signal that is found in the stool does not equate to live virus,” Garner added.

Dr. Gary W. Procop, MS, medical director and co-chair of the Cleveland Clinic’s Enterprise Laboratory Stewardship Committee and director of Molecular Microbiology, Virology, Mycology and Parasitology, is currently responsible for COVID-19 testing at his facility.

He told Healthline that “although the virus may be found in the stool, going to the focus of infection (i.e., the source) is preferred for specimen collection.”

Procop says that “at least one early study supports respiratory specimens as superior to the other end of the alimentary canal.”

“I think we’re far enough into this pandemic, and there is a substantial amount of data out there now saying that that’s the best source if you’re trying to figure out who has acute disease,” Garner added.

“Right away, in the very early days of the pandemic, when we were sending samples to the CDC for testing, they asked for lots of different samples,” said Garner.

“They’d ask for stool, they’d ask for urine, they’d ask for respiratory swabs, they’d ask for blood, because this question was still trying to be determined: What samples should we test?” he said.

“But it became very apparent, like most other respiratory viruses, the most accurate place to find it is in the upper respiratory tract, so this is why I was a little bit surprised and confused by the reports coming out on large scale anal swab testing.”

At the same time, Garner says he doesn’t want to eliminate stool testing.

“We do some stool testing here at UCLA, but it is in a very, very limited capacity… and I cannot think of a screening context in which anal swabbing or stool sample testing would be preferred,” said Garner.

Wide-scale sewage screening at buildings is another story.

“If you end up finding it in the sewage, then it means somebody in the building is potentially infected, so that’s a well-established and reasonable way to do sort of surveillance on large groups of people without having to swab all of them,” Garner said.