On-demand antigen testing is not for tomorrow

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On-demand antigen testing is not for tomorrow




People are waiting to carry out tests at Covid-19, in Paris.


© Stephane LAGOUTTE
People are waiting to carry out tests at Covid-19, in Paris.

Contrary to what Macron suggested on Wednesday, these tests will not be available to the general public for several weeks at best.

Will we soon be able to know in less than 30 minutes and without having to queue for hours in front of the laboratories if we are infected with the Covid? Recognizing “Difficulties” in the current organization of screening by PCR, the Head of State dangled this pleasing prospect Wednesday in his televised intervention: “We have an innovation […], these are called these antigen tests. You will always have to take this famous sample – either in the nose or the mouth with this famous swab, […] but it is in 15 to 30 minutes that we have the result ”welcomed Emmanuel Macron. And the same to add to the risk this time of going a little fast in the work: “We can also do it in pharmacies, in different points. So that, we will have more points of support, not just in laboratories. That’s going to be a real change. “

What is an antigen test?

While the PCR test is designed to identify the genetic material (RNA) of the coronavirus, the antigen test focuses only on detecting its structural proteins. As such, it falls under rapid diagnostic orientation tests (TROD). Like PCR, it requires a swab sample from deep in the nasal cavity. It is the continuation which is faster. The drops taken are brought into contact with specific antibodies of Sars-Cov 2, capable of fixing the antigens indicative of an infection. The presence of these results in a change in color of the test, in less than 30 minutes.

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Are these tests reliable?

The comparative evaluation of six rapid antigenic tests, conducted this summer by Jean-Michel Pawlotsky’s team (Inserm, Henri Mondor, Créteil) confirmed the “Overall satisfactory performance” of four of them to screen for Covid. The ability of these latter tests to detect infected individuals is close to 80% during the first three days of symptoms. However, after this period, their performance does not exceed 60%, ie a detection capacity much lower than the reference PCR test. Clearly, antigenic tests detect people with a very high viral load, the most contagious, but often miss people with a low viral load.

This mixed result has led the Haute Autorité de santé to recommend that their use be limited to specific cases. In one opinion posted on September 25, the body said it was favorable “To their use and reimbursement by health insurance”, with the condition that it is for diagnostic use, that is to say in symptomatic patients. She also recognizes their usefulness “As part of large-scale screening operations targeting populations in which the risk of infection is greater than in the general population”, such as universities or slaughterhouses, the idea being to be able to identify clusters. On the other hand, in the current state of her knowledge, she does not see the interest in asymptomatic contact cases.

When will the antigenic tests in pharmacies be done?

Contrary to what Macron suggests, it is not for tomorrow. Contacted, the Directorate General of Health is walking on eggshells. The ministry is only talking about “Deployment supervised by AP-HP and ARS Ile de France at airports and health students” for “Collect data”. Then we will have to wait “The opinion of scientific authorities” for “Consider a generalized deployment of these antigenic tests in the coming weeks”. This means that it may take a long time before a quick screening in the local pharmacy. For the DGS, “The RT-PCR test must remain the reference. It’s essential”.

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