What Is an Antigen Test?

Understanding a new type of coronavirus test authorized by the FDA

A Sofia machine, made by Quidel Corporation, which is used to run the company’s 15-minute Covid-19 antigen test.

A new category of coronavirus tests is here. On May 9, the U.S. Food and Drug Administration authorized the first antigen test to diagnose Covid-19, made by Quidel Corporation, a San Diego-based company that develops medical testing. Former FDA commissioner Scott Gottlieb told CBS’s Face the Nation that the technology behind it is “a real game changer.”

So what is an antigen test and how does it work?

Antigens are molecules on the surface of a virus or other germs that cause the body to produce an immune response. They can also come from substances in the environment like chemicals or pollen. A viral antigen test is designed to look for fragments of antigens from a virus, unlike most diagnostic tests, which look for the presence of the virus itself. Quidel’s test looks for antigens found on SARS-CoV-2, the virus that causes Covid-19, using a nasal swab. If you take the test and the results come back positive, it means that you are currently infected with Covid-19.

One of the benefits of antigen tests is that they provide rapid results, which means they could be done in emergency rooms or doctor’s offices to quickly determine whether someone is sick with Covid-19. Quidel’s test takes just 15 minutes to return results.

That’s a lot faster than the conventional method of diagnosing Covid-19, which uses a lab technique known as PCR, or polymerase chain reaction. PCR works by amplifying a tiny amount of genetic material from a patient sample to identify the virus. It’s a highly reliable method and is typically very accurate, but running PCR tests and analyzing the results can take up to a few days.

The downside of antigen tests is that they may not identify everyone who is actively sick with Covid-19. Antigen tests are very specific to the virus they’re meant to detect, but they’re not as sensitive as PCR tests. “This means that positive results from antigen tests are highly accurate, but there is a higher chance of false negatives, so negative results do not rule out infection,” the FDA explains in a statement. The agency says negative results from an antigen test may need to be confirmed with a PCR test.

An advantage of the test is that it can be produced more cheaply than PCR tests, which means they could be scaled up more quickly to potentially screen millions of Americans. Quidel hasn’t yet said when or where the tests will be available, but Gottlieb said around 40,000 machines needed to run the tests are already in doctors offices.

Former staff writer at Medium, where I covered biotech, genetics, and Covid-19 for OneZero, Future Human, Elemental, and the Coronavirus Blog.

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