The latest guidance from the federal health agency is for negative COVID-19 antigen test results regardless of the presence or absence of symptoms. The federal agency said recent studies from people likely infected by the omicron variant shows repeat testing increases the chance of an accurate result, reducing the likelihood of spreading the virus.
“At-home COVID-19 antigen tests detect proteins, called antigens, from the SARS-CoV-2, the virus that causes COVID-19,” the FDA said yesterday. “At-home COVID-19 antigen tests are less likely to detect the SARS-CoV-2 virus than molecular tests, such as polymerase chain reaction (PCR) tests. This is especially true early in an infection or in people who do not have COVID-19 symptoms. Currently, all at-home COVID-19 antigen tests are FDA-authorized for repeat, or serial use. This means people should use multiple tests over a certain time period, such as 2-3 days, especially when the people using the tests don’t have COVID-19 symptoms.”
The FDA said its guidance applies both to at-home COVID-19 antigen tests available over the counter as well as point-of-care COVID-19 antigen tests performed at a clinic or doctor’s office. A negative point-of-care antigen result calls for serial testing, which can include at-home tests.
False positives are much less likely than false negatives with antigen testing, which is faster and less expensive than the more reliable PCR testing.
People who have COVID-19 symptoms but test negative should test at least one more time 48 hours after the first negative test. Asymptomatic people who get a negative test should test again 48 hours later, and if that second test is also negative, the FDA recommends a third test in another 48 hours.
How accurate are PCR COVID-19 tests versus at-home antigen tests?
The FDA said molecular COVID-19 tests such as PCR tests detect SARS-CoV-2 virus infections at least 95% of the time, while antigen tests detect infections at least 80% of the time. Both tests are highly accurate when it comes to positive results.
In a study from October 2021 into February 2022, the FDA, National Institutes for Health (NIH) and University of Massachusetts Chan Medical School compared at-home COVID-19 antigen test performance against molecular testing.
Researchers made the results available to manufacturers of all at-home COVID-19 antigen tests. The FDA urged at-home antigen test developers with emergency use authorization (EUA) to engage with the agency on how to use the study results.
More than 7,000 participants in the study tested at home using an antigen test, and those with negative results repeated testing every 48 hours over 14 days. All participants also sent nasal swabs in to clinical labs for molecular testing to compare results against the at-home results.
“Results from this studyExternal Link Disclaimer show that repeat testing over a longer timeframe improves test performance and increases the likelihood that an at-home COVID-19 antigen test will detect an infection,” the FDA said. “These results have further guided the FDA’s thinking that repeat testing after a negative result with an at-home COVID-19 antigen test reduces the risk of a false negative result.”
New COVID-19 guidance from the CDC
FDA issued the new advice as the CDC dropped its recommendation that people exposed to COVID-19 quarantine to reduce the chances they could spread the virus.
The CDC’s new guidance now says people exposed to COVID-19 should wear a “high-quality mask” (the guidance does not use the term “N-95”) for 10 days and get tested on the fifth day. The CDC said people who are sick and suspect COVID-19 should isolate until they test negative.
The CDC also stopped recommending screening testing of asymptomatic people without known exposures in most community settings.
The highly infectious omicron variant is showing increased evasion to immunity from vaccines and past infections, and leading antibody treatments have lost effectiveness against the variant. The FDA has warned that while antigen tests can still detect omicron infections, they may do so less reliably.
Testing remains a key tool in the fight against COVID-19, along with vaccinations, masking and improved ventilation, the CDC said.