Céline Géraud, edited by Manon Fossat 1:59 p.m., January 04, 2022
To cope with the very strong demand for self-tests, especially during the Christmas holidays, the government has authorized supermarkets to sell them until January 31.
To avoid another shortage, test manufacturers are therefore working extra hard.
Only their effectiveness depends on many parameters.
An unprecedented wave of contamination has hit France in recent weeks and the number of positive Covid-19 cases is exploding.
Many French people first test themselves thanks to self-tests.
Faced with very strong demand, the government even authorized supermarkets to sell them until January 31.
Only their efficiency and reliability depend on several parameters.
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According to the Haute Autorité de Santé (HAS), an antigenic self-test performed by a healthcare professional detects the virus in 80% of cases.
But once it is practiced at home, this percentage drops sharply.
According to a study by Swiss researchers, the reliability is only 65.3% for people with symptoms and only 44% for asymptomatic.
In other words, you have a more than one in two chance of getting a false negative test, especially in the first few days during the incubation period when the viral load is lower.
"Extremely random"
For Professor Jean-Paul Stahl, infectious disease specialist at Grenoble University Hospital, reliability also depends on the sampling method.
"The problem is the sample. How much is taken, have we gone deep enough to remove properly, in the right place ...", he explains.
"These are all parameters that we do not control when it comes to a self-test, since it is the individual himself who takes his own sample. So it is extremely random."
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On the other hand, if the self-tests are carried out several times a week, their reliability is mechanically enhanced.
Always in addition to a PCR test, or an antigen produced in a pharmacy.