Accuracy Of Rapid Tests: April 2022
Article below is courtesy of CBC.ca
Medical experts say Canadians should keep a supply of rapid antigen tests handy as we head into a summer with almost no public health restrictions in much of the country. But experts add a negative result doesn't necessarily mean someone's in the clear.
Canada is already seeing a sixth wave of COVID-19 in the weeks after mask mandates and other measures lifted across the country.
But while cases are on the rise, public PCR testing availability hasn't ramped back up after it became overwhelmed during the Omicron-driven wave that sent case counts soaring in January and February.
"I think using the rapid test prudently at home is what most Canadians will have to do," said Dr. Prabhat Jha, a global epidemiologist at St. Michael's Hospital in Toronto.
When should I use a rapid test?
Jha recommends people use a rapid antigen test if they start showing COVID-19 symptoms, or if they had a high-risk exposure to an unvaccinated or symptomatic person with COVID-19.
Even then, he says, not all situations would necessitate taking a test.
Instead, Jha suggests considering: "Was the person that I had contact with vaccinated? Was the person actively symptomatic? Was it an indoor, closed space where I might have gotten a good load of their virus if they were hacking away?"
A positive result can also help an infected person get a better idea of the risk to their family members and others around them, especially as mask mandates lift and other respiratory viruses begin to circulate more, said Dr. Susy Hota, medical director of infection prevention and control at University Health Network in Toronto.
What does a negative result mean?
Medical experts continue to warn that a negative result on a rapid test doesn't necessarily mean you don't have COVID-19. New Swiss research, which is yet to be peer-reviewed, suggests some rapid tests have "significantly lower sensitivity" to Omicron than to the Delta variant.
Similarly, research by Ontario's COVID-19 Science Advisory Table earlier this year found rapid tests are less sensitive for Omicron than Delta variant in nasal samples, especially in the first one-to-two days after infection.
Doctors now recommend isolating immediately after symptoms or exposure, then waiting a day or two before using a rapid test, to get the most accurate result possible from an increasing viral load.
"By that time, the rapid tests are less likely to give you a false negative result," Jha says.
But it can still happen, says Hota, who recommends taking another rapid test 24 hours later, keeping in mind that a second negative test "doesn't necessarily rule it out."
However, she adds, a positive result should always be accepted as a "true positive."
Should I use a rapid test before an event?
As spring and summer social calendars fill, both Jha and Hota said it's important to remember a negative rapid test result is never a guarantee — and that other safety measures, such as keeping events outdoors, are still important.
"A single rapid test just kind of tells you with poor sensitivity what your status is at that point that you take the test," Hota said.
"If you have the virus, in theory, it's possible that you're less infectious to others at that time. It might change again in the two hours that you're at that place … but it's just not something you should hang your hat on."
Jha points to a recent Washington gala, the Gridiron Dinner, as a case in point: more than 10 per cent of the 630 guests at the mask-less indoor event — including cabinet secretaries, members of Congress and White House advisors — have since tested positive.
However, Jha says, rapid tests can be useful for identifying when you've recovered enough to return to work and resume socializing, "typically five days after you've had the first positive test or the symptoms started."
"If it's turned negative by then, you're pretty much in the clear to go and meet others."
What's the best technique for taking a test?
The technique you use in administering a rapid test to yourself or someone else also matters a great deal. And experts say a quick swish around each nostril is no longer sufficient, despite what the instructions in the box might say.
For a more accurate result, Hota recommends swabbing the bottom inside of both cheeks, then your throat, tonsils or the back of your tongue — "depending on what you can tolerate" — then swabbing both nostrils. The swab should go about 2 centimetres into each nostril, for several circles, she said.
How many test kits should I keep at home?
Jha suggests making sure you have at least two tests per household member. "If you're a typical family of four, you should have maybe 10 on hand."
But the ease or difficulty of getting your hands on a free rapid test depends on where in Canada you live, with provinces and territories distributing them through different channels.
In British Columbia, for instance, rapid tests are available free of charge at pharmacies, but those stores say they're having a hard time convincing people to take them.
Raj Rakholiya, manager of Wilson Pharmacy in Port Coquitlam, B.C., says uptake is increasing as cases rise, but it's still below expectations: he's currently sitting on a stockpile of about 550 test kits.
"Most of the people are saying that they already got their three shots, now they are going to book for their boosters, so they don't need it. Some people are saying they already got COVID so they're less likely to catch the virus again, so they aren't getting [tests]."
Infectious disease specialists say that's the wrong strategy: thousands of Canadians have caught COVID-19 more than once, and reinfections are becoming more common, as the more transmissible Omicron subvariant BA.2 spreads in Canada.
"Although the risks are small, you can still get reinfected even if you had COVID before," Jha said.
"I think having rapid tests at home is a sensible strategy which is considered kind of the new normal."
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