To add to Americans' growing confusion over how to navigate the Covid-19 pandemic, some medical experts are disagreeing on how to best use at-home rapid antigen tests. Early evidence suggests throat swabbing your throat in addition to your nose may increase the chances that a test can detect the highly contagious Omicron variant sooner, but the Food and Drug Administration (FDA) says the at-home kits should only be used as directed.
Saliva swabs are frequently used to diagnose other respiratory infections like influenza and rhinovirus, and recent research suggests it could also be useful with Covid-19. While the Omicron variant replicates 70 times faster than the Delta variant in human airways, infection in the lungs appears to be less severe, which means an Omicron infection can remain undetected—especially by at-home rapid antigen tests—for longer.
A recent pre-print study of 30 participants in New York and California found that rapid tests, including the Abbott BinaxNOW and Quidel QuickVue tests, weren’t detecting Covid-19 during an individual’s first few days of infection. It took an average of three days for people to test positive via a rapid antigen test after their first positive PCR result. Researchers also found that viral loads peaked in saliva one to two days before they peaked in tests taken from nasal swabs, suggesting that throat swabs may detect Covid-19 better than nasal swabs alone, Matthew Herper reports for STAT.
While the at-home tests in the recent study didn’t detect the virus in the early days after infection, each test showed a positive result eventually. At-home tests may not work well to spot infections early, but false positives are rare.
Another pre-print study from South Africa found that saliva swabs may be better than nasal swabs at detecting Omicron as the variant. In the study, researchers found that PCR tests picked up a higher share of Omicron infections when saliva samples, taken by rubbing a swab inside the mouth, were used instead of nasal samples, per Emily Anthes and Christina Jewett for the New York Times.
Because those infected with Omicron appear to get sick faster, “this means that there is a chance the virus isn’t yet growing in the nose when you first test,” tweeted Michael Mina, a former assistant professor of epidemiology at the Harvard T. H. Chan School of Public Health who now works as the chief science officer of eMed. The “virus may start further down. Throat swab + nasal may improve chances a swab picks up (the) virus,” he wrote.
In addition to the possibility that Omicron spreads aggressively in the throat and mouth than in the nose, there is also evidence rapid tests may be less sensitive to detecting the highly contagious variant, the FDA said last week. The agency noted that early results suggest that antigen tests “do detect the Omicron variant but may have reduced sensitivity.” That means it's possible the tests could miss an infection, known as a "false negative,” especially in the early days of infection, Dartunorro Clark reports for NBC.
Despite increasing evidence that saliva samples could help detect Omicron infections, the FDA advises against using self-collected throat swabs for Covid-19 tests, noting that Americans should only use the tests as instructed. Eating or drinking before swabbing the throat, for example, could cause the virus to not appear on the test. As the FDA learns more about the Omicron variant, the agency may revise its guidance—but for now, they say Americans should keep the nasal swabs out of their mouth if they want reliable results.
“The FDA has noted safety concerns regarding self-collection of throat swabs, as they are more complicated than nasal swabs—and if used incorrectly, can cause harm to the patient,” says Jim McKinney, a spokesperson for the agency, to Today.
Rochelle Walensky, director of the U.S. Centers for Disease Control and Prevention, said those who test negative with an at-home rapid antigen test may want a PCR test for added insurance.
“We do know that the most sensitive test that you can do is a PCR test, so if you have symptoms and you have a negative antigen test, then we do ask you to go and get a PCR to make sure that those symptoms are not attributable to Covid,” she said on ABC’s Good Morning America.