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The United States leads the world in COVID-19 deaths but lags behind many countries — both large and small — in testing capacity. That could soon change.

At the end of August, the US Food and Drug Administration (FDA) granted emergency-use approval to a new credit-card-sized testing device for the coronavirus that costs US$5, gives results in 15 minutes and doesn’t require a laboratory or a machine for processing. The United States is spending $760 million on 150 million of these tests from health-care company Abbott Laboratories, headquartered in Abbott Park, Illinois, which plans to ramp up production to 50 million per month in October.

The tests detect specific proteins — known as antigens — on the surface of the virus, and can identify people who are at the peak of infection, when virus levels in the body are likely to be high. Proponents argue that this could be a game changer. Antigen tests could help to keep the pandemic at bay, because they can be rolled out in vast numbers and can spot those who are at greatest risk of spreading the disease. These tests are also a key element in the testing strategies of other countries, such as India and Italy.

Antigen assays are much faster and cheaper than the gold-standard tests that detect viral RNA using a technique called the polymerase chain reaction (PCR). But antigen tests aren’t as sensitive as the PCR versions, which can pick up minuscule amounts of the SARS-CoV-2 virus that causes COVID-19.

This difference raises some concerns among specialists, who worry that covid-19 antigen tests will miss infectious people and result in outbreaks in countries that have largely controlled coronavirus transmission. Others view the lower sensitivity as an attribute, because some people who receive positive PCR test results are infected, but are no longer able to spread the virus to others. So antigen tests could shift the focus to identifying the most infectious people.

At present, antigen tests are administered by trained professionals, but some companies are developing versions that are simple enough to be used at home — similar to pregnancy tests.

“Making the tests faster, cheaper, easier is definitely the goal — and I think the antigen test is the way to get there,” says Martin Burke, a chemist at the University of Illinois at Urbana-Champaign, who is co-developing rapid tests, including antigen-based assays. “This is by no means the perfect solution, it’s just the fastest thing we could get going now,” he says.

What tests are there and how do they work?

Tests for COVID-19 fall into two categories: diagnostic tests such as PCR and antigen assays, which detect parts of the SARS-CoV-2 virus, and covid-19 antibody tests that sense molecules that people produce when they have been infected by the virus. Antibodies can take several days to develop after an infection and often stay in the blood for weeks after recovery, so covid-19 neutralising antibody tests have limited use in diagnosis (see ‘Catching COVID-19’).