Rapid antigen testing kits have flooded into supermarkets, online shops and pharmacies around the world as countries race to keep up with the Omicron variant’s swift pace of transmission, marking a shift to the self-administered tests from P.C.R. testing, which has long been considered the gold standard for detecting the coronavirus.
But there is no definitive international guideline for the use of antigen tests for Covid-19, and a global patchwork of policies has emerged as each country weighs the advantages and risks of the alternative testing method.
Britain has used at-home rapid antigen tests to fight outbreaks since 2020, even before Omicron, and makes them freely available. France began letting supermarkets sell them late last month. The Biden administration recently said that it planned to make 500 million tests available free and that Americans could request that tests be sent to their homes.
Singapore has allowed people to leave isolation if they get a negative antigen test result after 72 hours. Israel is asking people to swab their throats when using rapid antigen tests, not just their nostrils, to increase the chances of detecting the virus even if it goes against the manufacturer’s instructions.
“There are hugely variable approaches into where, when and how antigen tests are used across different countries,” said Deborah Williamson, a professor of public health at the University of Melbourne in Australia.
The lack of consistency in how antigen tests are deployed raises a question of how the world should monitor the severity of the pandemic, she said, when some countries are identifying every single case and others are prioritizing severe ones.
In addition, while promising quicker results, antigen tests are significantly less reliable in detecting infection than the P.C.R. tests, studies have shown.
The self-swab in an at-home antigen testing kit, which is meant to reach the inside of the nostril, is less than 30 percent capable of detecting a positive coronavirus case compared with the nasopharyngeal swab used in a P.C.R. test, which reaches the wall at the end of the nasal cavity, said Dr. Kiho Hong, a professor of laboratory medicine at Yonsei University in South Korea.
Accordingly, health experts at the Infectious Diseases Society of America said that P.C.R. testing was the method of choice for diagnosing Covid-19 infection. But they added that antigen tests could help identify cases when P.C.R. tests were not readily available.
A shortage of P.C.R. tests in countries like the United States and Australia has also left some people waiting for hours in line and several days to get the result. That prompted Australian officials to order people without Covid symptoms to seek a P.C.R. test only if they had received a positive antigen test result.
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The demand for antigen tests has become so overwhelming in Australia that the government last week began prohibiting price gouging on the tests and said it would limit the number that people can buy. And the state of New South Wales ordered residents to report their positive rapid antigen test results starting on Jan. 19. Those who don’t comply will be fined 1,000 Australian dollars (about $720).
“Omicron has been a game changer,” Professor Williamson said, adding that it “has really catalyzed the shift to large-scale rapid antigen testing.”
There is a pitfall to that shift, she added: Governments may lose their ability to track new variants and the scale of the virus’s spread, because the shift to self-testing won’t capture all of the results.
In addition, Professor Williamson said, with the Omicron variant, it is no longer realistic in many countries to keep track of every single case.