What is antigen testing? How a new test could help track the coronavirus outbreak

Cities and states across the United States are looking to expand coronavirus testing as part of efforts to emerge from lockdowns.
Image: Coronavirus Pandemic Causes Climate Of Anxiety And Changing Routines In America
Workers perform drive-up COVID-19 testing at Mend Urgent Care at the Westfield Culver City mall in Culver City, Calif., on Friday, April 24, 2020.Kevin Winter / Getty Images

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By Denise Chow

Cities and states across the country are looking to expand coronavirus testing as part of efforts to emerge from lockdowns, but two major barriers remain: how to screen huge parts of the population and how to do it quickly.

Antigen testing, which can offer faster results with less lab work, is the newest idea beginning to take hold.

"We have to have a breakthrough innovation in testing," Dr. Deborah Birx, the White House coronavirus task force coordinator, said in an interview Sunday on NBC News' "Meet the Press." "We have to be able to detect antigens, rather than constantly trying to detect the actual live virus or the viral particles itself, and really move into antigen testing."

But antigen tests are not a single fix for the widespread testing shortages. No coronavirus antigen tests have been approved by the Food and Drug Administration, and existing antigen tests for other viruses have their faults. But experts say the tools could become part of the broader solution.

Antigen tests are designed to detect viral proteins — which, in the case of the coronavirus, would come from the spikes that coat the outside surface of the virus — that trigger an immune response in the body.

For now, the main way to screen for COVID-19, the disease caused by the coronavirus, is through what's known as polymerase chain reaction, or PCR, testing. The test lookS for the coronavirus' genetic material in a sample, but because the viral RNA is so tiny, it needs to be "amplified," or copied millions of times in a lab, to get a result.

"They're much more complicated to do, but PCR tests are the most sensitive," said Dr. Pedro Piedra, a professor of molecular virology and microbiology at the Baylor College of Medicine in Houston.

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Although it is accurate, PCR testing is both resource and time intensive.

"Many of the current tests require multiple steps, manual pipetting of reaction components, and very accurate pipetting skills," Dr. Marie-Louise Landry, director of the Clinical Virology Laboratory at the Yale School of Medicine in New Haven, Connecticut, said in an email. "There are large instruments that can automate much of the PCR process and have a higher 'throughput,' but these may still only do 300-400 samples in an eight-hour shift."

That makes it difficult to significantly ramp up PCR testing throughout the country.

"There will never be the ability on a [PCR] test to do 300 million tests a day or to test everybody before they go to work or to school, but there might be with the antigen test," Birx said during a coronavirus task force briefing April 17.

Both antigen and PCR tests require nasal swabs, but antigen testing is simpler and could be conducted by primary care physicians. Patients would also get results in less than an hour.

Antigen testing exists for the flu, in which doctors can administer "rapid influenza diagnostic tests" that deliver results in 15 to 30 minutes. It is hoped that any breakthroughs in antigen testing for the coronavirus would have similar outcomes.

"You don't need specialized personnel to run the test, it's cost-effective and you get rapid results — that's what you're looking for here," said Dr. Aron Lukacher, chair of the department of microbiology and immunology at Penn State College of Medicine in Hershey, Pennsylvania.

But antigen testing is not as sensitive as PCR testing, which means there is a greater chance that antigen tests could deliver false negative results. In other words, a patient could be infected but an antigen test is just not sensitive enough to detect it.

For some antigen tests, the rate of false negatives can be 5 percent to 10 percent, according to Lukacher, which makes them less reliable. PCR testing, although not perfect, is thought to be more finely tuned, because amplifying the virus' genetic material also means smaller amounts of the virus can be detected.

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Antigen tests can also be challenging to develop because scientists need to be intimately familiar with the virus' structure and biology to know what immune response will be triggered by these foreign substances.

"PCR tests have the advantage that you can test for many pathogens, not just one virus," Piedra said. "Antigen tests have to be built for specific viruses."

Health officials are hopeful that significant strides can be made in designing new diagnostic tools. Earlier this month, the federal government's Biomedical Advanced Research and Development Authority awarded more than $710,000 to a Pennsylvania-based company called OraSure Technologies to develop a rapid coronavirus test using saliva samples. Other researchers at E25Bio, a Massachusetts-based biotech company, are working on a coronavirus antigen test that could deliver results in 15 minutes.

Still, Lukacher said, rapid antigen tests are unlikely to be a silver bullet for cities looking to restart their economies and emerge from lockdowns. Rather, any breakthroughs in antigen testing will likely have to be scaled up in tandem with PCR testing for health officials to determine how much the virus has spread in a community.

"The bottom line is we would want both — rapid screening and the ability to test by PCR," Lukacher said. "All of these things have to be done in conjunction with one another."