Christopher Labos: Some Things You Should Know About Rapid COVID Tests

Article content

Starting this week, Quebecers can obtain rapid test kits from pharmacies so they can be tested at home. Compared to going to a test center where lineups start to spread out waiting hours, the convenience of a home kit is obvious. The tests can be done anywhere and are easy to perform. Everything you need is in the test kit. They are less expensive than PCR tests and provide results in 15 minutes. On the surface, the appeal of these tests should be obvious, and yet their implementation here has been slow compared to other countries, especially in Europe, where they have been widely used for months.

Article content

These rapid tests have been feared to be less accurate than standard PCR tests. But to truly assess its usefulness we have to look at both its sensitivity and its specificity. A sensitive test would correctly identify all with COVID-19 as positive. A specific test would correctly identify all without COVID-19 as negative. Ideally, you want a test to be sensitive and specific, but practically speaking, you often have to sacrifice one for the other. The problem with rapid tests is a matter of sensitivity. In other words, some people with COVID-19 do not test positive.

There are several reasons why this can happen. Rapid antigen tests detect a protein that is part of the virus and can tell you if you have an active infection. But if you test too early in the course of infection, there may not be enough replicating virus in your nose for the test to detect it. Similarly, if you test too late after symptoms start, then the viral load may be dropping and also undetectable. Reviews of various rapid antigen tests show that they work best in the first week of infection. Technique also matters. If the swab collects an inadequate sample, the test may be inconclusive or falsely negative. Therefore, although the procedures can be overwhelming for people who do not know them, explanatory videos Rapid tests should go a long way toward demystifying the procedure, which is actually quite simple.

Article content

The other problem that can affect the sensitivity of the test is if someone has symptoms. Because it takes a certain amount of virus to be detected by the test, asymptomatic people are more likely to have mild infections or be too early in the course of the disease for the test to detect them. At Cochrane Review Regarding rapid antigen tests, the sensitivity was higher in symptomatic individuals than in asymptomatic individuals (78% vs. 58%), while the specificity was high for both groups (99% vs. 98%). In summary, the tests are good at ruling out disease, but less effective at ruling it out, especially in asymptomatic individuals.

A final drawback is that the precision of these tests changes depending on the epidemiological situation. If a different branch of statistics is used, called Bayesian statistics, it can be seen that the probability of false positives and false negatives can change depending on how much COVID is circulating in the community. When the prevalence of people with positive COVID increases, the positive predictive value of the test increases while the negative predictive value decreases. With a test positivity rate of 7.4 percent as of Monday, the probability that a rapid test is incorrect is very low , but not zero.

Article content

A negative quick test is not a license for carelessness and should not be used as a justification to let go of caution and start throwing big parties. But if used correctly in conjunction with all other public health measures, they can help reduce the spread of the virus. We must remember that they are not perfect, but a test does not have to be perfect to be useful.

Christopher Labos is a Montreal physician and co-host of the Body of Evidence podcast.

All our news related to the coronavirus can be found at montrealgazette.com/tag/coronavirus .
For information on vaccinations in Quebec, Click here .
Sign up for our email newsletter dedicated to local coverage of COVID-19 at montrealgazette.com/coronavirusnews .
Help support our local journalism by subscribing to the Montreal Gazette here .