What Tests for COVID-19 Antibodies Can and Can’t Tell Us About the Disease2020-05-15
- Only one antigen test has been permitted — a rapid test kit created by the Quidel Corporation in San Diego that can show results within 15 minutes.
- The FDA hopes to evaluate and authorize more antigen tests in the coming weeks.
- Being able to immediately know whether someone’s been exposed to the new coronavirus could help reopen the economy and allow people to get back to work.
Antigen tests are the latest tool that’s been given
The tests, which function similarly to rapid flu or strep tests, are designed to quickly detect tiny pieces of protein from the novel coronavirus that causes COVID-19. The test is done by swabbing a person’s nasal cavity.
So far, only one antigen test has been permitted — a rapid test kit created by the Quidel Corporation in San Diego that can show results within 15 minutes.
The FDA hopes to evaluate and authorize more antigen tests in the coming weeks.
The main advantage of these tests is their speed, according to the FDA. Being able to immediately know whether or not someone’s been exposed to the virus could, in theory, help reopen the economy and allow people to get back to work safely.
Antigen tests are also cheaper than other diagnostic tests available, and could easily be scaled up and distributed widely.
But there are some drawbacks to consider when administering these types of tests, health experts say.
Antigen tests aren’t as accurate as other diagnostic tests
The current diagnostic tests available are polymerase chain reaction (PCR) tests.
By swabbing a person’s naval cavity, PCR tests can detect the virus’s genetic material and diagnose if a person is actively sick with COVID-19.
PCR tests are thought to be very accurate because of how sensitive they are to picking up genetic material. They’re able to detect small amounts of genetic material, but it takes time to process and interpret the results in a lab.
Antigen tests, on the other hand, swab for antigens, or protein fragments on the surface of a virus that triggers an immune response.
“This test looks for parts of the virus in tissue as a way of detecting infection,” said Dr. James Zehnder, director of clinical pathology at Stanford Medicine.
They’re lightning fast, providing results within minutes. But — there’s a lot of room for error.
“They’re faster but less sensitive [than PCR tests]. They miss many cases; very much limiting their value,” said Dr. Sheldon Campbell, a Yale Medicine pathologist and professor of laboratory medicine at the medical school.
If a person tests positive on an antigen test, there’s a high chance they do in fact have COVID-19.
That said, the FDA states that antigen tests miss a lot of active infections, and lead to a lot of false negatives. (Quidel’s test allegedly only catches 85 percent of positive cases.)
“It’s very concerning,” said Campbell. “They need 1,000 or more times as many virus particles to detect as the RNA tests do.”
False negatives from an antigen test should be confirmed with a PCR test, the FDA advised.
Experts don’t want a lot of people walking around thinking they don’t have COVID-19, when in actuality they do.
How should we use antigen tests?
Zehnder said that PCR tests remain “the most sensitive and specific test to determine if someone is infected.”
Antigen tests are limited, considering the whole purpose of diagnostic testing is to accurately detect cases and prevent transmission.
If your test results are crucial — i.e. someone needs to know if they’re able to go to work or return to a long-term care facility — it’s probably best to take a PCR diagnostic test.
If your results are less urgent — perhaps you just want confirmation whether or not you have COVID-19 and will continue quarantining regardless — then an antigen test alone may be useful, Campbell noted.
The tests aren’t as accurate as health experts would like them to be, but they’re another resource that can better help us manage the ongoing pandemic.
The bottom line
Antigen tests are the latest tool that’s been given emergency use authorization by the FDA in our fight against the new coronavirus.
The tests, which function similarly to rapid flu or strep tests, are designed to rapidly detect tiny pieces of protein from the virus by swabbing a person’s naval cavity.
Though they provide rapid results and are cheap to produce, the tests are known to produce many false negatives and the results often need to be validated via a polymerase chain reaction test, or PCR test.
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