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In recent years, rapid at-home COVID testing, which can identify infection with a simple nasal swab, has been a mainstay of efforts to diagnose infection with the SARS-CoV-2 virus and control its spread. to prevent it.
But what if these tests are often wrong?
New research conducted at Caltech suggests that in many cases, rapid tests using a nasal swab yield false negatives, suggesting a person is infection-free even though other areas of their airways are teeming with the virus. The findings, from the lab of Rustem Ismagilov, Ethel Wilson Bowles and Robert Bowles Professor of Chemistry and Chemical Engineering, appear in a paper published in the journal Spectrum microbiology on June 15.
Researchers in Ismagilov’s lab tracked viral loads in three places in the human body during a COVID-19 infection: the nose, throat and mouth. Because the nose, throat and mouth are so closely connected, one might expect similar virus levels in those locations. That turns out not to be the case.
“Overall, we saw that in most people, the virus shows up first in their throat and saliva, and then, sometimes days later, in their nose,” said Alexander Viloria Winnett, a biology graduate student and co-author of the study. “Actually, each type of sample from a single person follows its own distinct rise and fall in viral load, so it makes a big difference what type of sample is used for testing.”
At the start of the pandemic, the gold standard for testing was the deep nasal swab (PCR test) administered by a medical professional, which is highly sensitive and accurate, but uncomfortable and slower to give results for many people. As the pandemic progressed, people increasingly relied on rapid home antigen tests, which can be performed without the help of a medical professional and provide results in as little as 15 minutes.
However, in their study, the Caltech researchers found that most people showed a delay of several days between when the virus first appeared in the throat or saliva and when it appeared in the nose. Importantly, 15 of the 17 study participants had high and presumably infectious virus levels at least one day prior to a positive antigen test.
“In one subject, virus levels in throat swabs were extremely high and presumably contagious for nearly two weeks, while viral loads from nasal swabs were undetectable or remained so low that they would have been detected only by a highly sensitive PCR test,” says Natasha Shelby . , the administrator of the Caltech COVID-19 study and a co-author of the paper. “This person never tested positive on her daily rapid antigen tests.”
Shelby says antigen tests have two major limitations: “First, they only test for viruses in the nose, even though the SARS-CoV-2 virus is known to be in the mouth, and in fact countless studies show it pops up often days for nose to mouth Secondly these rapid tests have low sensitivity meaning it takes a lot of virus to indicate a positive result it is believed these tests will be positive if people are contagious , but we now know from countless studies that this is not always true.”
That means home tests can indicate a person is negative for the virus even though they are actually infected — and contagious even at this point.
“Rapid tests have some advantages. You can get results in about 20 minutes, they’re usually less expensive, and they can be more portable than other testing methods,” says Viloria Winnett. “But because they are only approved for nasal swabs and have low sensitivity, the proper detection of infection by those tests is often delayed by several days.”
Viloria Winnett says he hopes the Caltech COVID-19 study findings will spur the development of home testing kits that sample both the nose and throat. Such tests are common in other parts of the world, such as the United Kingdom, but are not authorized for use in the United States.
“This has implications,” says Viloria Winnett. “People are still getting infected with COVID. We’re still seeing waves and we’re still seeing the development of new variants. I don’t want to be alarming, but we still need to be aware of and respond to the pandemic. And hopefully this can update our initial response to other upper respiratory viruses that may emerge so that testing strategies can be more effective.”
Exactly why such different viral pathways are seen in these parts of the body remains an open question that should be the subject of future research.
“Why is there a virus in one type of sample and not another? It could be the route of initial infection or the susceptibility of different anatomical sites to the virus. This is what I’ll try to find out in follow-up studies,” says Viloria Winnett.
More information:
Alexander Viloria Winnett et al, Daily SARS-CoV-2 Nasal Antigen Tests Miss Infected and Presumably Infectious People Due to Differences in Viral Loads Between Specimen Types, Spectrum microbiology (2023). DOI: 10.1128/spectrum.01295-23
Quote: Study: Rapid home COVID tests may miss many infections (2023, June 15) Retrieved June 15, 2023 from https://medicalxpress.com/news/2023-06-at-home-rapid-covid-infections. html
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