Self-tests: against Omicron, beware of false negatives

It’s a helpful reminder when self-tests land, for a few days, on supermarket shelves. According to a press release from the United States Food and Drug Administration (FDA), antigenic tests that can be performed at home and that detect SARS-CoV-2 virus proteins undoubtedly have reduced effectiveness against the Omicron variant.

Previous work done with inactivated viruses had not concluded that there was a significant drop in efficiency, the organization notes. However, the most recent results obtained from samples containing active viruses turn out to be mixed: Omicron is detected well, but not in all cases. The sensitivity of the tests, that is, their ability to identify people infected with Sars-Cov-2, appears to have decreased.

The FDA reminds that these are preliminary results that will need to be confirmed. It also specifies that it will continue to allow rapid tests. However, since the latter were already working before the arrival of Omicron with a lower efficiency than RT-PCR tests (which identify the genetic material of the virus), certainly more false negatives can be expected.

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The fear is the same on this side of the Atlantic. To be marketed on the French market, antigenic tests must have a sensitivity of at least 80%. But this is data from manufacturers and in real life, this percentage may drop much lower as indicated by the High Health Authority (HAS) by the end of 2020. This led to returns of between 68% and 97% for patients with suspected infection. Since then, other studies have highlighted the low efficacy of antigenic tests in asymptomatic subjects. Ultimately, this type of test is less effective than RT-PCR in identifying recent infections, the FDA stresses.

With a view to New Year’s Eve, but also to the return of the children to school in early January, many French people will undoubtedly turn to antigen testing in the coming days. Especially since we are in a phase of a strong epidemic outbreak. “People should do a rapid test a few hours before leaving home, given Omicron’s very short transmission time,” advises Tim Spector, professor of genetic epidemiology at King’s College London.

But even if the result is negative, they must remain vigilant. Because the self-test is not 100% reliable. And handling errors, such as misuse of the swab on the nose, when using it are relatively common. “Repeat the tests that occur later, with respect to an interval of 24 hours between tests, will increase the certitude that you are infected”, indicates a communiqué des Centers pour le contrôle et la prevention des maladies aux U.S.

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“By repeating the sample, for example, 1 to 2 times a week, we increase the chances of detecting the virus early in the disease,” says the Normandy regional health agency. And for those who have doubts, there is still the possibility of performing several antigenic tests at the same time.



Christophe Donner ITV in ParisChristophe donner


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