Looking at the difference between COVID-19 rapid tests & PCR tests

SAINT JOHN -

With Omicron spreading at lightning speed, some provinces – including New Brunswick – are shifting their testing strategies away from polymerase chain reaction, or PCR, and leaning more on rapid tests to determine cases of COVID-19 in communities.

“We’re seeing a shift away from universal PCR testing across the country, really just due to volume,” said Dr. Katharine Smart, president of the Canadian Medical Association.

“What’s very different clearly about Omicron is just how contagious it is, and the number of cases we’re seeing, and we don’t really have the PCR testing capacity really anywhere in the country to keep up with that.”

But because PCR tests have been the standard for so long, some may be wondering about the differences in the efficacy of the two tests.

The Canadian Medical Association says PCR tests are generally considered to be 90 per cent or more accurate, while rapid tests can vary from 50 to 90 per cent, depending on a variety of factors including whether or not the person being tested is symptomatic and the person who is doing the testing.

Toronto-based infectious disease physician Dr. Allison McGeer says the number of cases of COVID-19 circulating in the community has an impact on the rate of false positives – and because of Omicron and its surging spread, the chances of a false positive on a rapid test right now, are much, much lower.      

“Now we’ve gotten to the stage, not forever – just for the middle of this wave – where a positive test with the rapid test will almost always be real, and given that we have a shortage of PCR tests, having a positive rapid test is close to as good as having a positive test by PCR,” says Dr. McGeer.

“Hopefully three months from now when we’re back down to hardly having any cases again, that will not be true – but it’s true at the moment,” adds Dr. McGeer.

Dr. McGeer says while PCR tests are a little more sensitive – but that neither a negative PCR test, or a negative rapid test, are a guarantee that you’re not going to be infectious to somebody sometime in the near future.

Infectious disease specialist and medical microbiologist at the McGill University Health Centre Dr. Donald Vinh, says for those who are asymptomatic and test positive on a rapid test that means that you’re shedding enough virus at that point-in-time to be contagious.

“That’s what a positive result would be –keeping in mind that no test is 100 per cent, so if it’s positive it would suggest you are contagious,” says Dr. Vinh, “if it’s negative it doesn’t mean you can frolic carelessly – you still need to be able to maintain the regular public health measures.”

Dr. Vinh also believes New Brunswick is ahead of the curve with its new testing protocols, and that it makes sense to use a positive rapid test – which as the name implies, is much quicker, and more portable – as a sufficient result to report to public health for confirmation of COVID.

“You don’t want perfection to be the hole in your ship – in other words – if you have symptoms and you have a rapid test and it’s positive, the cherry on the sundae is go get a PCR test to get that confirmed, but it’s not going to really add added value at that point,” says Dr. Vinh.

“You have enough results with that rapid test to do something actionable which is to isolate,” adds Dr. Vinh.