How Omicron is changing the way rapid tests should be used

Science moves quickly in the arms race against the ever-evolving COVID-19 virus. Now, a growing number of medical experts and health authorities say the way at-home COVID tests are used needs to be adjusted to better detect Omicron and its subvariant.

This involves collecting samples from the inside of the mouth in addition to the nose, as well as waiting a day or two after the onset of symptoms to start testing.

Ontario is now in the midst of a sixth wave of COVID-19, fueled by an even more contagious subvariant of Omicron.

At the same time, public health measures meant to blunt virus spread, such as the mask mandate, have largely ended. Estimates indicate there could be more COVID infections in Ontario now than there were at the peak of the Omicron wave in January.

So, how do you know if you have COVID? How does Ontario know who has COVID?

Ontario can only estimate the number of people infected with COVID because the use of polymerase chain reaction (PCR) tests, the gold-standard COVID tests, was restricted by the province in December to high-risk individuals and health-care workers. The arrival of Omicron saw skyrocket infections, overburdening Ontario’s testing infrastructure. Testing rates plummeted, and Ontario now relies on monitoring wastewater to gauge the presence of the virus.

In December, Ontario started giving out free rapid antigen tests, which are less accurate than PCR tests but can generate results at home, rather than in a lab. The distribution of free rapid tests, originally slated to end this week, will now continue until “at least” July 31.

While package instructions on most rapid tests, and guidance from Health Canada, maintain that test takers should only collect samples from their nostrils, both the government of Ontario and its COVID-19 Science Advisory Table (prior to its amalgamation with Public Health Ontario on Monday ) have said people should now be swabbing the inside of their cheeks and throat, too.

Doing so increases the sensitivity of the test, according to Dr. Kevin Schwartz, an infectious disease physician at St. Joseph’s Health Center in Toronto.

“The virus is in different places in the body at different stages of the illness,” he said. “What happens early on in the illness course, especially with the (Omicron variants), is people are having higher concentrations of the virus in their saliva and the throat. Then, over the next few days, it becomes more concentrated in the nose.”

Schwartz said the “dynamics of the virus,” and when people are most infectious, is shifting, because of vaccinations and widespread partial immunity. As a result, COVID has become more detectable one to two days after the onset of symptoms, now considered the ideal time to use a rapid test.

Dr. Susy Hota, medical director of infection prevention and control at University Health Network in Toronto, agreed both that the cheeks and throat should be swabbed in addition to the nose and that rapid testing will be less accurate right after illness begins.

Hota said vaccinated people are now understood to have an earlier immune response to COVID infection, prompting earlier symptoms, which arise before a rapid test is able to detect the virus.

“It might take up to four days (after symptoms begin) for your rapid test to be positive,” she said, adding that a PCR test would likely catch an infection sooner.

“Probably the most important message is, if you start to get symptoms, don’t go anywhere. Self-isolate immediately. Don’t believe a negative rapid test right away.”

Further testing advice, taken from Ontario Health’s latest rapid test sample collection guide, is that test-takers avoid eating, drinking, smoking, vaping and chewing gum for at least 30 minutes before collecting samples. As well, people should blow their noses before swabbing their nostrils.

The guide goes on to recommend testers swab both inner cheeks and their lower gums — or, alternatively, the back of their throats and tonsils — before using the same swab to collect a sample from their nose as usual.

In a February brief, the science table wrote that rapid tests are less sensitive for Omicron than they were for the Delta variant in nasal samples — especially in the first two days of infection. This can be offset by taking combined oral-nasal samples to “more reliably detect infectious cases of the Omicron variant.”

The science table said people can collect oral-nasal samples by “initially swabbing both cheeks, followed by the back of the tongue or throat, and then both nostrils.”

Dr. Peter Jüni, scientific director of the science table, confirmed to the Star Wednesday that the science table still recommends combined oral-nasal swabbing, as well as other testing advice detailed in the February brief.

Asked about any potential drawbacks to taking oral-nasal samples, Schwartz said none have been observed thus far, despite the fact rapid tests were not designed to process saliva.

“It’s not the Health Canada-approved way to use the tests, but it looks like it’s helpful, especially early on,” he said. “The antigen tests are very good at detecting high amounts of virus, so the more virus you can get on the swab, the more likely it is to be positive.”

This is particularly important given rapid tests’ penchant for producing false negatives. In December, a study out of Switzerland, which has not yet been peer-reviewed, suggested rapid tests could only be about 50 per cent accurate on average when detecting early onset cases of Omicron.

However, when rapid tests show positive results, they’re usually right, experts say. A peer-reviewed study conducted in January by researchers from the University of Toronto found that only 0.05 per cent of rapid test results they surveyed were false positives.

“It’s important not to assume you don’t have COVID if you get a negative (rapid) test, if you have symptoms,” Schwartz said. He added that people who are at higher risk of severe illness, including the immunocompromised, unvaccinated or those of advanced age, can still book the more accurate PCR COVID test at a local assessment centre.

Ben Cohen is a Toronto-based staff reporter for the Star. Follow him on Twitter: @bcohenn

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