After other Canadian provinces, which have been doing it since February, it is Quebec’s turn to recommend this new technique: rub the swab of rapid antigenic tests in the mouth, in addition to the nostrils.
Almost a quarter of false negatives with Omicron
According to infectious disease diagnostic specialist Cédric Yansouni at the McGill University Health Centre, “we have known for a long time that the virus does not appear at the same time in the mouth and the nostrils. The mouth comes first, but we did the test in the nostril because the viral load is higher there, ”he explains to The Press.
Except that with the Omicron variant, with a higher viral load, this delay also appears to be longer.
Consequence: “There may be one or two days of symptoms without the virus being very present in the nostrils”, details the scientist. With a significant risk of false negative tests … in people who are nevertheless infected. Which will only know of their virological status on the occasion of a possible new test, or… never, if their condition improves. But in the meantime, they will have been able to spread the virus.
4 million tests per week in France
In recent weeks, the number of tests in France has risen sharply, with currently 4 million screenings per week. And more than 130,000 contaminations per day on average.
But according to a study pre-published on MedRxiv at the end of January, “the rate of false [tests] negatives decreases from 22% to 5% when rubbing a BTNX rapid test swab first in the mouth and then in the nose, rather than just in the nose.”
“It brings the false negative rate to about the level of the Delta variant.
Glenn Patriquin, lead author of the study at Dalhousie University
A how-to video
The new technique proposed is therefore to rub the swab inside the two cheeks, then against the back of the tongue, and finally to practice with the same swab the two “usual” passages in the nose, one per nostril.
A video (in English) has been released as a “how to”:
In February, Nova Scotia and then Ontario adopted this technique in Canada. Soon to be followed by Saskatchewan in March.
Only for antigen testing
The instruction, on the other hand, only applies to antigenic tests (including self-tests), known as “rapid”: with PCR tests, it is true that they are much more sensitive, no increase in efficiency has been observed.
In France, for the time being, no recommendation in this direction has been given: the only swabs provided in the mouth are in the context of tests… PCR, for children. A choice which is perhaps the cause, at least in part, of a high rate of false negatives with the Omicron variant…