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How effective are rapid tests? | Coronavirus


People using the tests must be symptomatic for it to be useful, repeated the Minister of Health Christian Dubé, during a press briefing to announce a tightening of health measures.

Pharmacists who distributed the rapid tests on Monday indicated in support that the efficacy rate of rapid self-tests is sometimes less than 50% in asymptomatic people and that they can produce false negatives.

This statement is partly true, but it is not sufficiently qualified, believes David Juncker, professor and director of the Department of Biomedical Engineering at McGill University and holder of the Canada Research Chair in Bioengineering.

There are of course risks of false negatives with rapid self-tests. [si on fait le test et qu’on n’a pas de symptômes], but these false negatives are usually in people who have a low viral load and therefore have a low risk of transmission, he explains.

On the other hand, he adds, the risk of false negatives for a high viral load is extremely low.

However, rapid self-tests are very effective in determining whether a person – asymptomatic or not – has a high viral load and, therefore, if they are contagious at the time of the test.

Rapid tests administered at a screening clinic, according to Juncker, produce about 3 false negatives in 1,000 tests. Of course, when people take a rapid test at home, there is a higher chance of sample error, which can increase the chance of false negatives. But it’s on the order of a few percentage points, he emphasizes. As people use them they get better.

Self-tests done at home can detect infected people who would never have thought to be infected because they are asymptomatic, adds Roxane Borgès Da Silva, professor at the School of Public Health at the University of Montreal. .

Much of the transmission is attributed to people without symptoms, so it is essential to use strategies like frequent rapid testing to fill in the gaps where molecular testing is impractical to deploy., write the authors of this article.

Focus on the number of cases missed by rapid tests versus molecular tests [tests PCR] is not productive or useful. Rapid self-tests can be more easily used and work best when people are most contagious, a key point that is overlooked when compared with molecular tests., can we read in Tea Lancet.

Self-tests aren’t perfect, but if 50% more people are contagious, isn’t that a good thing? asks Mr Juncker.

It is very disappointing [ce que dit le gouvernement au sujet des autotests], and that seems counterproductive to me in these situations. Many experts advise using them just before a gathering, to test if someone is contagious., says Juncker.

This biomedical engineering professor continues to believe the government should recommend that people get tested before family gatherings, even if they have no symptoms, because this is where there are high risks of transmission asymptomatic.

I understand that there is a lack of rapid tests and that we should not use them to the best of the best, continues Ms. Borgès Da Silva, but she also believes that self-tests should be used by Quebecers, especially if gatherings of 10 people are still allowed.

David Juncker further specifies that these rapid self-tests should be taken shortly before assembling and that the result is only good for a few hours. A test taken on Christmas Eve can be negative and then turn positive the next day as a person’s viral load increases.

He adds that a person with symptoms similar to COVID-19 and who receives a negative result in a self-test should still avoid participating in a rally. He suggests that this person retest 24 hours later to make sure the test was not taken too early and that the result is indeed negative.

Finally, he recalls that having a negative result does not mean that you have to forget all the other health measures.

Here’s how to do the rapid tests at home:

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