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Understanding COVID-19 rapid antigen tests: local experts share guidance

A rapid antigen test, while a great tool for COVID-19 screening, should be used in combination with a lab-based testing for those considered high-risk contacts, according to local epidemiology experts in Kingston. Photo by Mika Baumeister.

Compared to booking a COVID-19 test at a testing facility, attending the appointment, and then waiting for the results, the quick answers of a self-administered rapid test can seem to offer instant clarity. However, the accuracy of rapid test results has come into question recently. With COVID-19 infections on the rise in the Kingston, Frontenac, Lennox and Addington (KFL&A) region, the need to understand the various testing methods and how to respond to their results is more crucial than ever.

On Friday, Nov. 12, 2021, Kingstonist spoke with Joanne and Dave Gervais of Glenburnie, who were dismayed by having received a negative for COVID-19 result from a rapid antigen test, which turned out to be a false negative. In fact, Dave was positive for COVID-19, and ended up passing the infection to Joanne as well.

“In my view,” Dave stated, “[the rapid test] is not a suitable substitute for the PCR test.”

Dr. Gerald Evans, Chair of the Division of Infectious Diseases and Professor with Queen’s Department of Medicine, couldn’t agree more.

“The rapid tests are not an equivalent to, or a substitute for the lab-based PCR tests, for sure. And they’re not meant to be,” Dr. Evans noted. “The PCR tests are the gold standard; they’re lab-based, they have to be performed by a healthcare professional, and they have the ability to produce a result which is accurate. PCR test result accuracy is much higher than the rapid tests.”

In a video released on Thursday, Nov. 18, 2021, Dr. Piotr Oglaza, Medical Officer of Health for KFL&A Public Health, gave an overview of how the PCR tests administered at official COVID-19 testing sites work.

Dr. Piotr Oglaza, KFL&A Medical Officer of Health, shares insight on PCR tests and rapid antigen tests in a video released by Public Health.

“The PCR test is [analyzed at] a lab that uses very complicated machinery, that does this chain reaction — and that’s what the PCR stands for, Polymerase Chain Reaction — and it amplifies that piece of the genetic material in the virus, which are called RNA. It amplifies a piece of that genetic material that’s found on the swab to then produce a result, and that result can be positive, can be negative, and it’s really very accurate.”

For rapid test options, Dr. Evans explained that there are two main types in use currently: the molecular test, which operates on a similar principle to the PCR tests, but uses a slightly different methodology, and a rapid antigen test, which instead of looking for genetic material, looks for specific proteins. These proteins are larger molecules produced by cells infected with the virus, which, if detected in the sample, would indicate the presence of COVID-19.

“They’re very, very accurate when they’re positive,” said Dr. Evans. “A positive result on a rapid antigen test is pretty reliable. Where it has a drawback is when you get a negative test [result].”

Dr. Evans empathized with those who, like Dave and Joanne Gervais, were eager to embrace the hope that a negative result from a rapid antigen test seemed to offer. “Especially in a situation with lots of uncertainty, like COVID transmission, we really want something definitive, but that’s not the function of the rapid tests. A difficulty is that when we take a test, we carry this expectation that the results of the tests are going to be a strict yes or no. But unfortunately that’s not how a lot of medical testing works in real life,” he explained.

Instead, Dr. Evans urges those using a rapid antigen test to consider it as a screening tool. “A rapid test is a useful tool for screening or surveillance, not for a diagnosis. Think about the screening questions required for school and other activities. When we answer those screening questions that we’re all so familiar with by now, we know that it’s not giving us an actual answer or diagnosis of whether we have COVID-19 or not, it’s giving us a sense of the risk, of the likelihood, and whether or not we need to adjust something – not going to an event, or seeking a PCR test for a true confirmation, for example.”

Dr. Evans noted that a key to a wise interpretation of a negative rapid antigen test result is understanding other factors that may be affecting what he referred to as “pre-test probability” for infection. “Someone who is symptomatic and gets a negative rapid test result, yeah, they’re not going to want to just take that as a true diagnosis, because they have a different pre-test probability for having COVID-19 than someone who does not have any symptoms and also gets a negative result on a rapid antigen test.” He explained that in those examples, the symptomatic individual should seek out a PCR test to verify the results, whereas the asymptomatic individual would not need to do so.

Other factors that would influence pre-test probability are known close-contact with an individual who has COVID-19, or known exposure in a location that has been identified as a source of outbreak. Even asymptomatic individuals who fall into one of those higher-risk categories are advised to confirm rapid test results through a PCR test administered by a testing facility.

This recommendation was reiterated in Dr. Oglaza’s video presentation.

“[A rapid antigen test] is a far less sensitive test compared to the [PCR] test. It’s mainly used to enhance screenings or if someone is already undergoing some screening for symptoms. Additional levels of protection can be achieved by also adding that rapid antigen testing, and if the test is negative, then the person is okay to go in; we’ve seen that in workplaces. We’ve seen that in other settings as part of the very comprehensive, thorough screening processes,” he said, “but that test is not appropriate or recommended by both the World Health Organization as well as our own Ontario Ministry of Health in diagnosing people who are symptomatic, or testing those who are high risk [of being positive for] COVID-19.”

Dr. Evans expressed that he doesn’t want the public to feel that there’s no point in performing rapid antigen tests. “I don’t want to dis the rapid tests. They have a role to play. They can give a false negative, but that risk is still fairly low. If your pre-test probability of being positive is low, getting a negative result is still going to be accurate about 85 to 90 per cent of the time. If you know that your pre-test probability is higher because you have symptoms, high-risk contact, or something like that, and you get a negative, that negative test result would only be 75 to 80 per cent accurate and you want to get it confirmed with a PCR test, but that’s still decent accuracy.”

“So a key thing for people taking a rapid test to be aware of is that they need to view the test and its results through the lens of their pre-test probability of having COVID-19,” Dr. Evans summarized. “Rapid tests can be thought of not as a diagnosis, but as an extension of screening. In many ways a more sophisticated and objective extension of screening, but in the case of a negative result, not something that we should lean on as a definitive conclusion.”

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