Covid: antigenic tests, do you need to take two samples from the nose and throat?

By Laurent P. Posted on January 7, 2022 at 4:17 p.m.

Faced with the Omicron variant of the Covid, making antigenic tests less reliable, should a double sample be taken, one in the nose, the other in the throat, to ensure real results? Response elements !

A question that torments many French people … variant Omicron from Covid is now the majority in France, but less easily detectable via antigenic tests available in pharmacies and supermarkets, in self-test, until January 31, is it necessary to do a double debit as some researchers recommend, one in the nose, the other in the throat, to ensure the reliability of the results?

The answer is yes, according to the professor Jennifer L. Rohn, cell biologist at the University of London: “After a series of negative lab tests, I finally took a tip I saw on Twitter and took a swab from my throat and nose (which is no small feat with this stick)“, she explains in a tweet. And to add, while her results are positive:”If you think you have Covid-19, consider adding the sample taken from the throat“.

Same observation for the doctor Swapneil Parikh, researcher and specialist in internal medicine: “Important tip on antigen testing to detect Omicron. In the past two days, a few of my patients have tested negative with a self-test antigen taking only nasal samples, but a positive result taking samples from the throat and nose.“, he indicates in a tweet noted by our colleagues from Why Doctor?.

One double debit particularly useful in cases of contamination with variant Omicron… And for good reason, “the Omicron variant first emerges in the throat and only then in the nose“explains the Belgian virologist Marc Van Ranst with our Flemish colleagues from VTM News. And to continue: “If you take samples from your nose early in the disease process, the virus is not there yet. But when you combine it with a throat swab, you’re more likely to test positive.“.

One saliva test which would also be more effective in the detection of this strain … According to a study published in pre-print on medRxiv on December 24, this seems to be the case: this consisted of observing different people who had contracted different strains of the virus and having them do several saliva and nasopharyngeal tests. And the results are clear: the PCR tests by nasopharyngeal swab are 100% effective in detecting the virus for the Delta strain, and 71% for the variant Omicron.

For saliva tests, the results are reversed: “for the Omicron variant, the saliva tests were 100% reliable“, explains the study, against a reliability of 86% for the salivary in the detection of the Delta strain. double debit is therefore a good way to ensure contamination at the variant Omicron. But a study that will still have to be validated by pairs.

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