Why so many negative tests?

Q “Why are there so many negative tests? Is this a sign that we are testing too much? Is it because there is panic and people get tested without symptoms just because they come back from a trip? ” asks Josée Langlois, of Stoneham.
A The vast majority of tests performed to screen for COVID-19 are indeed negative, that is, they indicate the absence of the virus sought. In Quebec, for example, as of Friday, out of nearly 34,500 tests performed, only 2,021 had found the presence of the coronavirus, or 5.8% – and at the Canadian level, it was only about 2.5% of patients tested who are found to have COVID-19. Positive rates vary from country to country depending on disease progression and intensity of detection efforts – as of March 20, Italy had almost 20% positive and the United States, around 15% – but everywhere the tests are overwhelmingly negative.

However, we should not deduce that we are testing “too much”, explains Dr. Judith Fafard, medical consultant and microbiologist-infectiologist at the Laboratoire de santé publique du Québec. In general, screening is not done by just anyone: it is mainly people already showing “suspicious” symptoms who are entitled to it (with certain clienteles more at risk who have priority over others, but c is another question).

Precisely, “the symptoms of COVID-19 are not specific at all,” says Dr. Fafard. We are talking mainly about fever and cough, symptoms that can be caused by many other microbes. It can act from influenza, or from a coronavirus other than COVID-19 (there are four other different types that have been present in humans for a long time that cause colds), or from the respiratory virus syncytial, or other pathogens. And as these viruses are very common – literally millions of people contract them each year in Quebec alone -, especially during the cold season, it follows that by testing (roughly speaking) those who cough and have a fever , we necessarily pick up a lot of other things than COVID-19. Hence the high proportion of negative tests.

Another factor to consider is the state of the epidemic: one simply cannot detect many cases if there are not many in a given population. In this regard, it should be noted that Dr. Fafard expects to see the proportion of positives increase in Quebec over the next few days or weeks, as the disease spreads.

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COVID-19 raises a lot of questions. In order to respond to as many people as possible, science journalists have decided to join forces. The media members of the National Cooperative of Independent Information ( Le Soleil, Le Droit, La Tribune, Le Nouvelliste, Le Quotidien and La Voix de l’Est ), Québec Science and the Déclic Center team up to answer your questions. . You have some? Write to us . This project is made possible thanks to a contribution from the Chief Scientist of Quebec , who invites you to follow him on Facebook , Twitter and Instagram .

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