Before leaving to celebrate Christmas with their (reduced) family, many French people are preparing to go through the screening box, with a test against Covid-19, most often antigenic.
Most of them will rush to pharmacies where they will think they know in a quarter of an hour on which foot to dance.
However, they must keep in mind that it is important not to relax the barrier gestures and that no result is 100% reliable.
"It is not an immunity totem," insists Olivier Véran, the Minister of Health.
One of the 80 antigenic tests officially accepted on the market has even just been withdrawn due to its propensity to diagnose false positives.
The National Agency for Medicines and Health Products Safety (ANSM) asked pharmacists at the beginning of the week to no longer use the VivaDiag Rapid Test SARS-CoV-2 Ag test from the Chinese company VivaChek and to recall the patients tested. positive the week before the ban to submit them to RT-PCR tests ... So, how can you be confident?
Insufficient sensitivity for some tests
Especially since two studies recently carried out by the team of Professor Jean-Michel Pawlotsky from the Henri-Mondor hospital in Créteil (Val-de-Marne) on sensitivity (ability to detect a positive case) and specificity (efficacy to detect a negative case) of these rapid tests showed disparities between the different products.
If the sensitivity is not sufficient, the risk is to obtain a negative result while the person tested is positive.
This is a false negative.
If the specificity is not sufficient, the reverse is true: the result may be positive while the person is negative.
It is a false positive.
Of the nine brands tested, only three were declared “compatible with their use in the early diagnosis of infection”.
Although all of the tests studied had specificity of at least 99%, “six did not demonstrate sufficient sensitivity to be used in the indications for rapid Covid-19 antigenic tests.
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A significant number of "fakes"
And according to a study carried out over the past three weeks by the group of medical analysis laboratories Biogroup on 1666 patients who came to carry out an RT-PCR test twenty-four hours after an antigen sample, the “false” are legion.
Of 1327 presumed negative, 111 were in fact positive.
But above all, 214 of the 339 positives (63%) would have turned out… negative.
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