Photo by Jernej Furman via Flickr.
Rapid antigen tests are now the standard way for people to determine if they have COVID-19, but studies show they are less sensitive than laboratory tests and can result in a false negative for infection, creating public confusion about about how to use rapid tests.
In July 2022, a Cochrane supported review of 155 antigen test studies showed that on average, rapid tests correctly identified COVID-19 in 73% of people with symptoms and 55% without symptoms.
The tests were most accurate (on average correctly identified COVID-19 infection in 82% of people) when used within the first week that COVID-19 symptoms occurred, likely because that is when people have the most virus in their nasal passages. For people without symptoms, the tests were most accurate (on average, correctly identified COVID-19 infection in 64% of people) in someone who had close contact with a person with confirmed COVID-19.
So how should people interpret this data? An antigen test will correctly identify if someone is infectious but may not correctly identify someone infected, says Mara G. Aspinall, a professor of practice in biomedical diagnostics at Arizona State University and managing director of Health Catalysts Group, a life sciences consulting firm. She is also co-author of a weekly newsletter called: “Sensitive and Specific: The Testing Newsletter.”
“When used correctly, antigen tests are highly effective,” Aspinall said. “What is most important to understand is that they are a measure of infectiousness, not a measure of infection. It’s really important to understand this difference.”
The time between infection (meaning the presence of a pathogen in a body) and infectiousness (meaning being contagious to others) is the reason antigen test manufacturers specify that people with and without symptoms should take two tests in a row within 24 to 48 hours to confirm a case of COVID-19. It can take several days before enough virus replicates in the nose to be detected, or the person becomes contagious, which is why someone might test negative with an antigen test and positive with a laboratory-confirmed PCR test. (To understand the differences in COVID-19 tests, see my guide to understanding COVID testing.)
On Aug. 11, the Food and Drug Administration advised people who have been exposed to COVID-19, but don’t have symptoms, to take three tests in a row over two to three days. Taking a third test, when a person doesn’t have symptoms, increased the accuracy of the test to correctly identifying COVID-19 infection in 79% of people, from 62%.
Also on Aug. 11, the CDC updated its COVID-19 quarantine guidance aimed at reducing spread of the diseases. This too makes use of antigen tests. The agency advised that once a person tests positive for COVID-19, they should isolate for five days and then wear a mask for another five days, when in public, or take two antigen tests in a row. If they are both negative, then a person can unmask.
As of mid-August, there were 17 over-the-counter antigen tests for sale under FDA emergency use authorization and three over-the-counter PCR tests. Aspinall provided a comparison of tests and their accuracy.
At-home PCR tests are much more accurate than antigen tests — at least 90% or better in correctly detecting COVID-19 — but are much more expensive (in the range of $75 to $314 a test) than antigen tests. The federal government provides eight antigen tests for free to people with a mailing address, and health insurers reimburse beneficiaries for eight antigen tests per person a month.
Story ideas to explore
- Are people still using antigen tests before social gatherings?
- Are employers still using them?
- Should parents be using them during this school season?
- Though the federal government is giving away tests and health insurers are reimbursing for them, accessing tests remains an issue, especially for the uninsured. What are communities doing about it?
- Will consumers have access to more at-home tests in the future? What about for testing for the flu?
- What are the regulatory challenges of allowing consumers to do more at home testing for diseases?
Experts to contact
- Mara Aspinall, professor of practice in biomedical diagnostics at Arizona State University and managing director of Health Catalysts Group; aspinall@ASU.edu
- Bradley Connor, M.D., clinical professor of medicine at Weill Cornell Medicine; bac2004@med.cornell.edu
- Jacqueline Dinnes, Ph.D., senior researcher, Institute of Applied Health Research at the University of Birmingham; dinnes@bham.ac.uk
- Celine Gounder, M.D., clinical associate professor of medicine and infectious diseases at New York University’s Grossman School of Medicine and editor-at-large at Kaiser Health News; @celinegounder
- Nate Hafer, Ph.D., assistant professor of molecular medicine at the UMass Chan School of Medicine; hafer@umassmed.edu
- Michael Mina, M.D., Ph.D., chief science officer for eMed; @michaelmina_lab
- Apurv V. Sonia, M.D., Ph.D., assistant professor of medicine at the UMass Chan School of Medicine; apurv.soni@umassmed.edu
- Bruce J. Tromberg, Ph.D., director of the National Institute of Biomedical Imaging and Engineering; bruce.tromberg@nih.gov