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what to know about rapid antigen tests

the rapid covid test is the hottest, most sought-after product this holiday season, and experts say they should be used as just another layer of protection against the virus.

by maija kappler

dec 18 2021

4 minute read

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if you have symptoms, book a pcr test. but if you feel healthy and want some extra reassurance, try a rapid test. (getty)

the spread of omnicron comes at a most inconvenient time. with many people having already made plans for holiday gatherings and travel to see loved ones, the experts are saying that rapid tests is one way to help slow transmission of this new and highly transmissible variant.

“we should flood canada with rapid tests,” dr. isaac bogoch, an infectious disease specialist at university health network told cbc news . “there should be no rapid test sitting on a shelf, collecting dust in a warehouse. they should be in the hands of canadians.”

two ways to test for covid

rapid tests come on the heels of the more familiar polymerase chain reaction (pcr) test , where a healthcare provider swabs a patient and then gives that sample to a lab technician, who isolates the genetic material and tests it in a thermal cycler to detect whether or not the sars-cov-2 virus is present. results can take days.

the rapid antigen test, which is most accurate when a person has covid symptoms — less so if they are asymptomatic or in the early stages of infection — deliver results in about 15 to 20 minutes, are easy to use, and are supposed to be east to access — although people are lining up for hours to snag the free tests provided by the ontario government, for example.

but as reassuring as it may sound to be able to confirm you are covid-free before meeting up with others, rapid tests are not the be-all-end-all solution, says devon greyson, an assistant professor of public health at the university of british columbia. rather, we should think of them as just “another layer of protection” in the fight against the virus.

“when we talk about safety in medicine or public health, we often talk about this swiss cheese model,” greyson says. “nothing is 100 per cent effective, everything has some holes. so you want to have multiple slices of swiss cheese up against each other to try to reduce the risk that your holes will line up.”

canada’s chief public health officer dr. theresa tam said something similar at a news conference earlier this month: “we need to use everything we have available to add those layers of protection, of which a rapid test is one.”

so what should you know about rapid tests?

when should i take a rapid test?

rapid tests work best when a person has symptoms of the virus, dr. fatima kakkar, a pediatric infectious disease specialist and doctor at the centre hospitalier universitaire sainte-justine in montreal, told cbc . if you don’t have symptoms or in the early stages of infection the positivity rate drops to between 35 to 50 per cent, she says, since there may not be a lot of the viral protein in your system for the test to detect.

still, taking a rapid test before you see other people can add an extra layer of security. according to dr. ashley lipps , an infectious diseases physician at the ohio state university wexner medical center, a negative result “strongly suggests you are negative at that particular time.”

how do rapid tests work?

most come in a small kit with a nasal swab, solution, test strip and test tube. generally, you’d add the solution to the test tube, then swab your nostril, put the swab into the tube, stir it, and then drop several drops onto the test strip.

many tests are similar to pregnancy tests in that you’re looking for visual signs in two places: a control area, and the test area. when the control line — identified by the letter ‘c’ — becomes visible, that indicates that the test is working. if test line — identified by the letter ‘t’ — becomes visible, that means the test is positive for covid.

a covid- 19 rapid test device at a pop-up testing site on the dalhousie university campus in halifax. (andrew vaughan / the canadian press)

if you’re a visual learner, you can watch webmd’s chief medical officer dr. john whyte demonstrate the process of taking a rapid test.

of course, there are a variety of rapid tests, so make sure to read the directions carefully on any test you’re taking.

how rapid is a rapid test?

generally, results should show up in about 15 minutes, health canada reports .

how accurate is a rapid test?

that’s the tricky part. accuracy ranges between 50 and 95 per cent, depending on a number of factors. one major factor is viral load, or how much of the virus is present in the body. rapid tests are most likely to be accurate when the viral load is the highest, the cdc explains — which tends to happen several days after the infection occurs.

someone who just contracted covid, but is asymptomatic, could potentially get a negative result on a rapid test because their viral load is still relatively low. that’s why public health officials encourage healthy people to take rapid tests every few days — inaccurate negative results can occur often.

“the amount of, or the degree of, contagiousness can change quite a bit over even a couple of days,” dr. raj bhardwaj, a family doctor in calgary, told cbc news .

where can i get a rapid test?

it depends where you live. people in saskatchewan, new brunswick, and nova scotia can get tests from their provincial governments. people in quebec and alberta can soon get them free at pharmacies, and people in ontario can, oddly, get them free at some government-run liquor stores.

canadians can also buy rapid tests — health canada regularly updates its list of approved brands.

when is a pcr test better than a rapid test?

if you have covid symptoms, or were recently exposed to someone who tested positive, the only way to know definitively if you contracted the virus is have a pcr test.

maija kappler is a reporter and editor at healthing.

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what to know about rapid antigen tests