COVID-19 rapid tests aren’t always accurate. But they’re still useful, experts say
Rachel Gilmore globalnews.ca
Ontario announced its plans to start a rapid antigen testing blitz on Wednesday, as the Omicron variant continues its swift spread across Canada.
The province follows others, like New Brunswick, Nova Scotia and Saskatchewan, which are already handing out rapid tests. Alberta, Quebec and British Columbia have also all vowed to follow suit.
But reports have suggested the accuracy of these tests could be anywhere between 50 and 95 per cent, depending on what kind you buy and how you use them. That means even if you take one of these tests, you can’t always trust that it’s giving you the right result.
“If someone tests positive with a rapid antigen test, it makes sense to go and get PCR testing as confirmation,” said Dr. Matthew Miller, assistant dean at McMaster University’s department of biochemistry and biomedical sciences.
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This reality has prompted some — including, on Tuesday, a Liberal MP — to say that because rapid tests are “not as efficacious” as other kinds of tests, it means they are “unreliable” in the fight against Omicron.
But public health experts say it’s a bit more complicated than that — and that, when used properly while adhering to public health measures, rapid antigen tests can be an incredibly useful and effective tool in anyone’s COVID-19 toolkits.
Here’s what you need to know.
What is a rapid antigen test?
Rapid antigen tests can be bought online, at pharmacies, or, depending on your province, can be picked up for free at participating locations.
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The test can be self-administered using a nasal swab and, usually within about 15-20 minutes of swabbing your nostril, you can expect to have a result showing whether you have COVID-19.
Rapid tests are “less sensitive” compared to lab-based polymerase chain reaction tests, or PCR tests, but they get you the results more quickly. PCR tests can take days, while rapid tests serve up your results in under an hour — which has its own benefits, according to Miller.
“If we can use these so that a lot more people are testing themselves — again, as a supplement to the existing public health measures and personal sort of safety measures that we know reduce risk of transmission — that can only be a good thing,” Miller said.
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Rapid antigen tests work by looking for proteins that are created when someone is infected with COVID-19, infectious disease specialist Dr. Gerald Evans said. Those proteins, he added, disappear “fairly quickly” after someone recovers from the virus.
Molecular tests — including PCR tests and point-of-care nucleic acid tests, which is a different kind of rapid test — look for something called a “nucleic acid signature” from the virus.
While molecular tests are really helpful in circumstances where health officials need to know definitively if someone is positive, whether it be for contact tracing purposes or because they’re symptomatic, antigen tests are a fast way of checking if you’ve got COVID-19 regardless of whether you have symptoms or a close contact with the virus, according to Evans.
“If it’s positive, you’re not going to go to school, you’re not going to go to work, you’re not going to hang around with other people and stuff,” he explained.
Are rapid tests reliable?
Rapid tests aren’t quite as precise as the PCR — but that doesn’t mean they don’t work really well, said Sue Paish, co-chair of the Testing and Screening Expert Advisory Panel.
“If these rapid tests are administered effectively and appropriately, those results are reliable,” she said.
The reliability of rapid tests vary in part because the tests themselves aren’t all the same. Different companies make their tests slightly differently, and some of them aren’t approved by Health Canada, Miller noted. The ones that provinces are handing out for free, however, are approved.
“Doing some background checking to ensure that the test you’re using or purchasing has been approved by a regulatory body — I would say that that’s the best way to ensure that the tests that you’re using are as reliable as possible,” Miller said.
Studies point to a pretty big range in the efficacy of rapid tests. One CDC study showed one kind of antigen test, the Sofia antigen test, was about 80 per cent accurate when it comes to showing which symptomatic people have COVID-19, and 98.9 per cent effective at indicating which symptomatic people don’t have the virus.
Accuracy was much lower when it came to asymptomatic people, with just 41.2 per cent accuracy when it comes to the positive results. However, the negative results were accurate 98.4 per cent of the time when used on asymptomatic people.
Because rapid antigen tests can be less accurate, the study suggested getting a follow-up PCR test if you’re symptomatic and get a negative result, or asymptomatic and test positive.
The takeaway reflects what the FDA also found in its own research: while it said antigen tests’ positive results can be highly accurate, “there is a higher chance of false negatives, so negative results do not rule out infection.”
Testing isn’t the exact science that you see on television, Evans said.
“These tests are not 100 per cent perfect. They don’t always get every true positive and every true negative, and they need to just keep that in mind,” he said.
“But what’s useful about them, and why the rollout really actually is a great thing, is because it is going to detect positive individuals and then those people can take the necessary precautions to isolate and not go on to transmit it to other people.”
When should rapid tests be used?
The calculation about whether antigen tests are better or worse than PCR tests depends much more on why you’re taking the test, according to the experts.
“If you’re getting the test done because you don’t feel well or you had contact with someone with COVID, then you want that diagnostic (PCR) test because you want to have a low number of false-negative tests,” Evans explained.
But rapid antigen tests are “for a different purpose,” Evans said.
“You want to use a rapid test if you’re feeling well and you don’t have symptoms. If you’re in a circumstance of, say, going to school, perhaps a workplace where a lot of people will work together.”
Miller added that you’ll want to make sure that you take the test at the right time, too, as it will provide only a “snapshot” of whether you’re infectious at the moment you took the test.
“It’s possible that you could take a rapid antigen test today, let’s say in anticipation of an event you wanted to go to tomorrow, go to that event, and still be capable of transmitting,” Miller said.
So if you’re thinking of using a rapid test as an additional line of defence before gathering with family for the holidays, Miller said you should “ideally” take that antigen test as close to the event’s start time as you can.
But these are just another tool in the toolkit if you’re following all other public health advice, rather than a silver bullet against COVID-19, Miller warned.
“If people are going to be doing all of the things that public health is advising them to do to enhance the safety of gathering with family over the holidays, adding this on top of that can only be a good thing,” Miller said.
“But a negative test shouldn’t change those other behaviours.”View link »
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