Foreign visitors have been upset by China’s anal swab tests for COVID-19, prompting complaints of inconvenience and even psychological trauma, and stoking debate over their necessity.

A few cities, such as the capital, Beijing, Shanghai and the port city of Qingdao require the checks, in addition to nasal or throat swab tests, for some international arrivals, state media say.

What are anal swabs?

The Chinese Center for Disease Control says the test is performed with a sterile cotton swab, which looks like a very long ear bud, that is inserted 3 cm to 5 cm (1.2 inches to 2 inches) into the anus before being gently rotated out.

Why anal swabs?

Such tests can ensure infections are spotted, since coronavirus traces can be detectable in the anus for longer than in the respiratory tract, some Chinese doctors told state media.

But a positive result does not necessarily mean the person tested can spread the virus, as inactive traces unable to replicate or infect others can also show positive, Jin Dongyan, a virology professor at the University of Hong Kong, told Reuters.

Foreigners only?

Early this week, Tokyo complained about the tests on some Japanese travellers to China, saying they had caused “great psychological pain”.

South Korean visitors can now submit stool samples instead of “Chinese authorities taking them directly”, Choi Young-Sam, a spokesman of the South Korean foreign ministry, said on Tuesday.

Last month, U.S. media outlet Vice cited a State Department official as saying U.S. diplomats had been subjected to the tests. A Chinese foreign ministry spokesman said that to his knowledge, such tests had not been required of U.S. diplomats in China.

In a statement to Reuters on Thursday, the ministry said the virus prevention and control measures China was taking were based on science.

Anal swab tests are not exclusive to foreign visitors. During China’s last major round of infections in January, some cities performed them on unspecified segments of the local population.

Only in China?

Travellers flying into Shanghai must undertake a full battery of tests including anal swabs if more than five people on their airplane test positive for the virus, state media reported, citing one of the local CDC staff.

Travellers from regions where the virus is rampant or those who test positive on arrival also need to undergo such tests, according to the staffer.

While anal swabs are not compulsory for all international arrivals in China, one staffer of Beijing’s Daxing district epidemic control department told state-backed Global Times that international visitors to Beijing were subject to such testing.

“If people are not familiar with the procedure for taking an anal swab test, our employees will help explain how it will be done,” she said.

Anal swab tests for the virus are not unique to China.

Galicia, in northwest Spain, has performed them on some hospitalised patients, a few newborns and those with psychiatric illnesses for whom it was impossible to administer nasal swabs, its health department told Reuters.

What does the WHO say?

The World Health Organization (WHO) recommends testing respiratory tract specimens, where possible, to diagnose respiratory diseases because they give the best samples, spokesman Christian Lindmeier said in an email to Reuters.

“Faecal samples may offer an alternative testing material, especially in patients with gastrointestinal symptoms,” he said, but they are “less likely than respiratory samples to be positive in the first week of symptoms.”