Jan. 11, 2022 — Many Americans are familiar with the rapid antigen tests for COVID-19 that involve swabbing the nose. But some new evidence suggests a saliva sample could boost the tests’ accuracy.
Experts agree on one thing: If you’re going to test both the nose and throat, swab the throat first.
In terms of an official stance, the FDA says to follow the test instructions. In other words, stick to the nose for now.
“We don’t have enough data suggesting throat swabs are an accurate or appropriate method for at-home tests,” the agency wrote Jan. 7 on Twitter.
One leading infectious disease doctor might disagree.
Robert Wachter, MD, shared a story on Twitter about how his symptomatic son tested negative at first using a rapid nasal swab test. After a search for more tests, sampling both his throat and nose yielded a positive result.
“I’ve been tweeting about Covid for nearly 2 years. But this week it became personal when my 28-year-old younger son got it. With his permission, I’ll describe his experience & how I approached his situation, given the realities of life and the rapidly changing evidence,” he tweeted on Jan. 8.
Wachter is chair of internal medicine at the University of California San Francisco and a contributor to Medscape Medical News, WebMD’s sister site for medical professionals.
Preprints Support Saliva Sampling
Two recent preprint studies support the saliva swabbing strategy. A caveat with preprints is that the research is not yet peer-reviewed.
One study by Gert Marais, MD, and colleagues says using saliva is better than nasal swabbing for detecting coronavirus infections.
The researchers compared the accuracy of saliva and nasal sampling on 382 people with symptoms who also had PCR tests done.
The saliva rapid test findings agreed 100% of the time with a standard laboratory-based PCR test for presence of the Omicron variant, Marais and colleagues at the University of Cape Town and the National Health Laboratory Service in South Africa reported.
In contrast, the nasal rapid test results agreed 86% of the time.
The results, the authors say, may mean we need to rethink the typical diagnostic testing done for suspected cases of COVID-19.
Saliva Appears More Accurate
Saliva sampling and throat swabbing are not necessarily the same thing, says Christina Wojewoda, MD, chair of the College of American Pathologists Microbiology Committee.
For example, in one of the new studies, participants were given these instructions:
“Participants should not have had any food, drink, tobacco or gum in the 30 minutes preceding saliva swab collection. Participants were initially instructed to cough 3-5 times, covering their mouths with the inner elbow. They were then asked to swab on the inside of both cheeks, above and below the tongue, on the gums and hard palate. A minimum swabbing duration of 30 seconds was required.”
This technique makes more sense than swabbing the throat directly, she said. “To get a well-collected throat swab on yourself … would be difficult. I would gag and spit.”
More research is needed “to nail down” the best way to collect saliva, says Wojewoda, who also is director of the Clinical Microbiology Laboratory at the University of Vermont Medical Center.
“We know from rapid strep testing studies that accurate sample collection from the back of the throat is not easy,” says Michael Blaivas, MD, chief medical officer at Anavasi Diagnostics.
This means clinical expertise can make a big difference in sample collection. A throat swab done by a doctor, nurse, or other health care professional in general will always be more accurate than one done by someone less familiar with proper technique, he said.
Rapid Tests May Need to Be Rethought
Anavasi Diagnostics has a rapid molecular test based on lower nose swabbing that can return results in 30 minutes.
“This Marais study looked at saliva PCR and mid-[nostril] PCR. This is very different from saliva antigen tests,” Blaivas said.
“However, if the study findings and conclusions are validated by others and it is true that Omicron causes higher viral shedding in the mouth or saliva than the nose, companies will need to consider redesigning their tests,” he said.
Another Caveat on Nasal Rapid Testing?
Researchers published a different preprint study comparing the performance of saliva-based PCR tests with that of nasal rapid antigen tests in 30 people who tested positive as part of a screening program.
Blythe Adamson, PhD, and colleagues reported that most people with Omicron tested positive by PCR days before antigen tests yielded the same result.
“This study sounds reasonable to me,” Wojewoda says. “We know that PCR will always be more sensitive than the antigen test.”
“So I think this really limits the use of antigen tests to ‘Oh, wow, I feel crummy today. Let me get an antigen test, and if positive, I’m considering myself positive. If it’s negative, I still could be positive, and I’ll need to get a PCR test.’”
In contrast, for coronavirus patients without symptoms, it looked like antigen tests “performed pretty poorly,” Wojewoda says.
The Adamson study also confirms a widespread belief that “rapid antigen tests are often not sensitive enough to diagnose an infection when it is early or symptoms are mild and little virus is shed,” Blaivas said.
“My clinical experience in the emergency department regularly showed, early in the pandemic, that antigen tests were not reliable for our clinical use,” Blaivas said. “We accidentally admitted multiple patients to non-COVID wards when they were actually COVID-positive … because the hospital ran out of in-house PCR tests during a repeat wave of infections.”
Blaivas recommended consulting a regularly updated FDA website that lists antigen tests that fail to recognize the Omicron variant reliably.
Best of Both Worlds?
Some experts support swabbing both the throat and nose, “and there is literature support for this being more accurate than just throat swabs alone,” Blaivas said.
But, he says, “some tests will fail to perform if faced with a sample from either the nose or throat, if not geared for and tested on it.”