By Rose Hoban
In the coming days, people seeking a rapid test to see if they’re positive for COVID-19 will be able to put information into a federal website and ask for up to four rapid tests. Ten to 12 days later, they should arrive in the mail.
And over the past weekend, federal rules recently put in place mean that insurance companies now have to reimburse plan members for up to eight over the counter rapid COVID-19 tests per month.
All this comes as the Biden administration is finally putting its weight behind scaling up the number of rapid tests being manufactured and released to the public – about a billion rapid tests should be online by the first week of February. That should make it easier for people to get their hands on the tests, which have flown off of pharmacy shelves since late December.
This late-to-the-dance emphasis on rapid tests is in stark contrast to what’s happened in other countries. Former NC Health News intern Mona Dougani, who’s studying in Spain, said tests are everywhere there.
“You literally go to a pharmacy, you go to a pharm technician and you ask for a take-home test, a COVID antigen test,” she said. “Five to eight euros per rapid test,15 minutes super quick and easy to do at home.”
But in the U.S. rapid test availability has lagged. And with the flood of Omicron cases, labs that run the PCR tests that have been used all through the pandemic have been completely swamped.
On top of that, the guidance on testing has been confusing and changing in recent weeks. We talked to several testing experts to ask some basic questions and get advice on the way to think about testing going forward.
First, the basics
Two years ago, few members of the general public had ever heard of a PCR test, much less had one. But now, it’s hard to find someone who hasn’t had one – or multiple – PCR tests.
PCR tests, short for polymerase chain reaction, are tests used to detect genetic material of the virus.
“PCR testing detects everything,” said Laura Murray, the area medical director for Greensboro-based Cone Health. “Everything that used to be COVID, or is COVID, or is infectious, isn’t infectious anymore, is debris from prior infections.”
She described PCR as “extremely, exquisitely sensitive.”
“It’s not always useful when we’re trying to decide if somebody is infectious, is a risk to others,” Murray said.
Melissa Miller, a professor in the pathology department at the school of medicine at the UNC Chapel Hill also runs the microbiology lab services for UNC Health.
She explained that PCRs can pick up the presence of an infection that’s really new, maybe only a day or two old, before a person shows symptoms.
In contrast, she said, a rapid test that looks for antigens “is actually looking for a protein that lives on the outside of the virus, so you can see it, and you’re able to detect it in these tests.”
“Antigen tests help us know, with a high degree of likelihood, who is a contagious threat right now,” Murray said. “So, they’re quite useful right now.”
PCRs became the gold standard for COVID testing early in the pandemic largely because of the fact that people carrying the original variant of the virus could take a week or more to develop symptoms, yet could still be infectious. Early detection was key. PCR also was preferred for a long time, because many physicians were leery of antigen tests, based on past experience, both women said.
Miller said that from the perspective of someone who runs a lab, she was one of those mistrustful of antigen tests for respiratory diseases, based on how poorly they performed in the past.
Check out this online guide comparing rapid tests and their relative ease of use.
“We’ve been burned by the RSV rapid antigen test. We’ve been burned by the influenza rapid antigen test,” she said. “We discontinued our influenza rapid antigen testing in 2009.”
She said those influenza rapid tests were only accurate a small fraction of the time.
“Those were not a test worth doing,” Miller said.
“With that lens, though, the COVID antigen tests have been designed, have been engineered to be quite a bit more sensitive,” Murray explained. “The range of variability from test to test to test is less and so the confidence in these tests is much, much better.”
A recent study, conducted by a group of researchers in San Francisco earlier this month, compared readings on both PCR tests and antigen tests in the same patients. They found that about the point where PCR testing could tell that COVID was infectious, antigen tests were also positive, with a correlation that topped 95 percent. The paper has still to be peer-reviewed, but others have found similar results.
OK, so I can get a test, what now?
Much of the advice from federal health officials has been complicated and has evolved over time as science provided more information. It hasn’t been clear how long to stay home, versus how long to avoid others, versus how long to just mask up. Even the language has been confusing: many people have had difficulty distinguishing between “isolation,” which translates to staying home, and “quarantine” which translates to avoiding others while wearing a tight-fitting mask.
Murray said that at the end of December, the Centers for Disease Control and Prevention came out with its clearest guidance yet. Even still, it’s complicated.
“For me, the important thing is — if you spend substantial time with somebody face to face without good masking, and they turn up positive, it’s a good idea to get a test in a few days even if you feel fine,” she wrote in a follow-up email.
Why antigen tests now?
With labs that perform PCR testing overwhelmed with demand, the time it takes to get results back has stretched into days instead of hours or the minutes required for an antigen test. But since Omicron replicates quickly, it becomes infectious sooner and creates symptoms sooner.
“We think it’s a faster incubation period for Omicron than it was for the other variants. We thought [earlier variants] were more in the five to seven day period, and this is maybe more in two to three day period,” said Tom Denny, an immunologist and the chief operating officer at Duke’s Human Vaccine Institute.
Around five days after exposure, the antigen test can start to detect an active COVID infection.
“The good news is when you’re using an antigen assay early on, if you don’t get a positive test, and you just happen to be exposed and infected, you are most likely not infectious or able to transmit to someone else,” Denny said.
“Where the antigen test is very useful is at day five,” he said.
For a person having symptoms, who still has a negative antigen test, it’s useful to do another rapid test a day or two later to see if they’ve become antigen-positive, and infectious.
It took a long time – some say too long – for U.S. manufacturers to create antigen tests that were sensitive and specific enough for COVID. And it took U.S. regulators at the Food and Drug Administration a long time – some say too long – to give authorization for those antigen tests.
So, for now, Denny said the U.S. is under-resourced. This means that sometimes, someone who’s been exposed to a person with COVID may be flying blind.
When it comes to schools, the recommendations are changing too. In Europe, schools have remained open and many practice a “test to stay” strategy, i.e. if a child has been exposed, they wear a mask and have serial antigen tests, if the test is negative, the child can stay in school. Positive tests send kids home.
In late December, the CDC endorsed these types of strategies for keeping students in school, and this month, the North Carolina Department of Health and Human Services followed suit. Some school districts are starting to use this method of continuing attendance in classrooms.
Nose or throat?
There’s been some evidence that Omicron reproduces more actively in the throat than in the nose, although the science is still not clear.
Nonetheless, some people have been using their rapid test swabs on their throats instead of the nose. Murray says that’s a bad idea.
“The technology that we have embraced as a nation and the antigen tests available right now, by and large, are nasal swab tests,” she said. Murray explained that the tests are engineered to collect samples from nasal mucus, not from the cells and mucus of the throat, which are different.
“You should not be YouTubing instructions on how to convert the nasal kits, the antigen kits to saliva,” she said. “I just don’t think there’s a lot of science for that.”
But she said that the swab needs to go pretty far into the nose, not the “brain biopsy” that people complained about at the start of the pandemic, but a good ways in.
“The important message that I’m giving, at least this week, about throat swabs, is that people should not be using them off label in rapid antigen tests,” Miller said. “It’s a very different environment, it’s actually more challenging to collect a good throat swab.”
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