The current standard for COVID-19 screening is the polymerase chain reaction (PCR) test, in which genetic material is copied millions of times so that viral RNA can be identified. Trained technicians are required to process results, which take several days to get, making it difficult to scale up to current testing demand. Some estimates suggest the U.S. needs at least 20 million tests daily to safely reopen the economy, but labs across the country are currently running just 150,000 per day.
Antigen testing is receiving attention as an alternative to PCR. Protein fragments, usually from the coronavirus’s surface spikes, are large enough to study without having to replicate. Antigen tests have the potential to diagnosis infections in a matter of minutes, but are difficult to produce. More information about the biology and structure of the virus is required in order to identify the correct viral proteins. Still, antigen tests could provide a solution to some of the limits of PCR, such as time and labor requirements. An antigen test’s immediate results make it suitable for charting progression of the infection over time.
“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,” said Deborah Birx, MD, head of the White House Coronavirus Task Force. “But there might be with the antigen test.”
Developing a working antigen test for COVID-19 will be difficult because of the nature of respiratory illness. Antigen tests work well for bacterial diseases but are less accurate for respiratory viruses like the coronavirus, where the amount of the virus found in the testing area (such as the nasal cavity) varies individually.
In a PCR test, the viral genetic material is amplified millions of times to increase the amount of the infection marker, making it easier to identify the presence of the coronavirus. Since there is no amplification of viral proteins in an antigen test, if the protein is not present in the sample it cannot be detected, making the antigen test less sensitive. Even with these limitations, a combination of antigen and PCR tests may bring the U.S. closer to meeting its testing needs to re-open the economy.