China is requiring some travelers arriving from overseas to receive an invasive anal swab test as part of its coronavirus containment measures, a move that has outraged and shocked several foreign governments.

Japanese officials said on Monday that they had formally asked China to exempt Japanese citizens from the test, adding that some who had received it complained of “psychological distress.” And the United States State Department last moth said it had registered a protest with the Chinese government after some of its diplomats were forced to undergo anal swabs, though Chinese officials denied that.

It is not clear how many such swabs have been administered or who is subject to them. Chinese state media has acknowledged that some arrivals to cities including Beijing and Shanghai are required to take the tests, though the reports said the requirements might vary depending on whether the travelers were deemed to be high-risk.

Chinese experts have suggested that traces of the virus may survive longer in the anus than in the respiratory tract and that samples of the former may prevent false negatives. China has imposed some of the strictest containment measures in the world, including barring most foreign arrivals, and has largely suppressed the epidemic.

Lu Hongzhou, an infectious disease specialist at Fudan University in Shanghai, told the state-controlled Global Times tabloid that nasal or throat swabs could cause “uncomfortable reactions,” leading to subpar samples. He acknowledged that fecal samples could replace anal swabs, to prevent similar discomfort.

But other experts — including in China — have questioned the need for anal samples. The Global Times quoted another expert, Yang Zhanqiu, as saying that nasal and throat swabs are still the most effective because the virus is contracted through the respiratory tract.

Benjamin Cowling, a public health professor at the University of Hong Kong, said in an interview that even if someone did test positive on an anal swab but not a respiratory one, he or she would likely not be very contagious.

“The value of detecting people with the virus is to stop transmission,” Professor Cowling said. “If someone has got an infection but they’re not contagious to anyone else, we didn’t need to detect that person.”

A spokesman for the Chinese Foreign Ministry said this week that the government would make “science-based adjustments” to its containment policies.

Professor Cowling said he did not know what the scientific rationale was behind the existing policies. “I presume there’s some evidence leading to this decision, but I haven’t seen that evidence,” he said.