Do COVID-19 rapid tests work on Omicron?

By Jennifer Ferreira

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    TORONTO (CTV Network) — With Canadians across the country still faced with limited access to polymerase chain reaction (PCR) testing, rapid antigen tests remain a quick and easy way to tell whether or not someone has COVID-19.

But a recent study performed in the United States revealed that these rapid tests may be less sensitive to newer variants of the virus, and therefore less effective at detecting their presence. This may leave some wondering just how accurate rapid antigen tests actually are, especially as Canadians navigate a sixth wave driven by the Omicron variant.

“The tests do work, they are still detecting COVID,” Dr. Christopher Labos, a Montreal-based cardiologist and epidemiologist, told CTVNews.ca in a phone interview on May 18. “We just have to understand their limitations.

“What we’re seeing with the Omicron variant and other variants is that false negatives are an issue.”

The possibility of receiving a negative result while being infected with COVID-19 is the biggest limitation of using these tests, Labos said. The explanation for a false negative result is unlikely to stem from issues with identifying the latest COVID-19 variant of concern due to new mutations, said Dr. Lisa Barrett, an infectious disease specialist and professor at Dalhousie University in Halifax. Instead, it might have more to do with how the test is being performed, and where a person is in their course of infection.

“It’s not a test detection problem,” Barrett told CTVNews.ca on May 18 in a phone interview. “It may well be that the virus itself hangs out in different parts of your respiratory tract at different times in the infection.”

At the start of an infection, the Omicron variant tends to be most present in the throat, Barrett said. As the infection progresses, it begins to concentrate in the nasal area and later on, can even move down further into the respiratory tract, which might then produce a negative result, she said. This is why experts, including Barrett, suggest swabbing more than just the nose when using a rapid antigen test, despite manufacturer instructions.

“We suggest using a swab that reaches the back of the throat, swab both sides – you should get a little bit of a gag – and then do both sides of the nose,” she said. “That’s your best sample to get the best sensitivity, even for early and low infection.”

Particularly for those displaying symptoms of COVID-19, a single negative test may not completely rule out infection, especially since Omicron continues to circulate at high levels in different parts of the country, Barrett said. But it may indicate that the chances of passing the virus on to others at that point are low. Ultimately, the goal of rapid tests is not necessarily to rule out infection, but determine the likelihood of spreading COVID-19 to other people, said Barrett.

“Positive tests are really important and still seem to be connected with higher infectiousness to other people, which is the way we’re trying to use the tool,” she said. “The tool was never meant to rule out infection for someone so they can go and do something that might be risky if they’ve got symptoms – it’s to rule in infection.”

WHEN SHOULD YOU TAKE A RAPID TEST?

If someone is showing symptoms of COVID-19 but receives a negative result using a rapid test, they should test themselves again at least once or twice more, Labos said, waiting about 24 to 48 hours between each test.

“The point is to test repeatedly,” Labos said. “There are quite a large number of people who tested negative on their first test, only to test positive on their second or third test.”

Earlier this year, Ontario’s COVID-19 Science Advisory Table published data indicating that the sensitivity of rapid antigen tests towards the Omicron variant varied depending on how far along someone was in their infection. Results showed that test sensitivity was just under 20 per cent when taken two days after an initial positive PCR test. By day three, sensitivity shot up to over 80 per cent.

As a result, Labos advises people to wait a few days after the onset of symptoms before taking a rapid antigen test.

“If your symptoms started today, you want to wait [for results from] a repeat test on day three, that’s the one that’s going to give you the biggest bang for your buck in terms of diagnostic accuracy.”

For those who are not symptomatic, but want to take a rapid test before getting together with friends and family, for example, Barrett recommends performing the test immediately before meeting with others.

“If you’re going to test in that situation, without symptoms, do it right before you walk into a gathering … and [swab] your throat and nose,” she said. “Don’t do it 12 hours before because [the virus] can change in terms of its level.”

WHAT EXACTLY DO RAPID TESTS MEASURE?

Rapid antigen tests are typically designed to target the N-gene product, or the nucleocapsid protein, of SARS-CoV-2, Barrett explained. This is different from detecting the virus’ genetic material or antibody levels, which typically rise when someone has either been infected with the virus, or received a COVID-19 vaccine, Labos said.

“You will only have that protein if you are actively infected with COVID-19, the vaccine won’t impact that,” he said. “Even if you got vaccinated, you will still test negative if you do not actively have COVID.”

This is different from how PCR tests are performed, as they detect parts of the S-gene of SARS-CoV-2, otherwise known as the spike protein. PCR tests are considered more accurate in detecting COVID-19 than rapid antigen tests, Labos said, but that doesn’t mean it’s necessary to take one in order to confirm results from a rapid antigen test, especially if someone is symptomatic.

“If it looks like a duck and quacks like a duck, it’s probably a duck,” he said.

Given the limited eligibility to PCR testing in different parts of the country, rapid tests continue to play a useful role in detecting COVID-19, particularly because they are self-administered, can conveniently be performed at home and produce results within about 20 minutes, Barrett said.

With the recommendation that people should be taking multiple COVID-19 tests to determine if their symptoms are caused by COVID-19, Labos said it’s helpful to reserve one rapid test kit for each person living at home. Each kit typically comes with five tests.

“If you think about the practical realities of having to do multiple tests if you get sick … you should make sure that you have enough rapid test kits to make sure that every person in your house can do two, three or four rapid tests if the need arises.”

WHAT SHOULD YOU DO IF YOU TEST POSITIVE?

Finally, if someone were to test positive using a rapid antigen test, the first step should be to self-isolate, Barrett said. Provinces have varying requirements for self-isolation after exposure to COVID-19. In Ontario, for example, residents are expected to quarantine for at least five or 10 days, depending on their vaccination status, and must continue isolating if symptoms haven’t improved or if they continue to have a fever. In Nova Scotia, however, the requirement is to self-isolate for seven days from the onset of symptoms, regardless of vaccination status.

In order to reduce the chances of infecting others, Barrett recommends that those who are sick stay home for at least seven days and wait until their symptoms have resolved before ending their self-isolation period. It’s also a good idea to take a rapid test towards the end of the isolation period to determine whether or not a person is still infectious.

“That’s most useful to demonstrate that you’re still positive and may want to stay home a day or two longer, particularly if you work with vulnerable people … or you have people in your immediate circle that are vulnerable.”

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Sonja Puzic