How to put an end to the increasingly long delays to know if we have Covid-19?
This is one of the main challenges facing health authorities, while the epidemic is growing “exponentially” in France.
One of the solutions could be through the deployment of different screening techniques.
While saliva tests are still pending, rapid antigenic tests should be deployed "from Wednesday" in Île-de-France, Olivier Véran announced this Tuesday morning on France Inter.
Their result can be known in a few tens of minutes, but they are less reliable in some cases.
Here's what you need to know.
How does an antigen test work?
Today, more than one million virological tests by PCR (or RT-PCR) are carried out per week in France, ie more than 140,000 per day on average.
A quick antigen (or “antigen”) test is also done with a swab that is slipped through the nose.
"Alas", recognized Olivier Véran, about this "not pleasant" examination.
We can also take a "lower respiratory tract sample", indicated the High Authority of Health (HAS) in a brochure on the various tests published on May 18, 2020.
But unlike a PCR test, where we look for the genetic material of the virus (viral RNA), this time it is the presence of viral proteins that we will detect.
The particles collected from the back of the nose are then mixed with a reagent on a strip or in a tube, and a colored band appears if the result is positive.
The operation is manual and there is no need to go through a machine.
Whatever the method, these tests "answer the question: is
the patient affected by Covid-19 or not
", underlines the High Authority for Health.
How long does it take to get the result?
In "fifteen to twenty minutes", announced Olivier Véran.
Some manufacturers promise for their part a delay of 30 minutes maximum.
The result can then be communicated immediately.
"It's very fast and it's the same principle as for a pregnancy test, except that there we detect viral proteins and not a hormone", details the virologist Yves Gaudin, of the Institute of Integrative Biology of the Paris-Saclay cell (I2BC).
This would avoid the long hours or even the days of waiting currently experienced.
While the examination of a PCR itself only lasts a few hours, the delays before obtaining the results are sometimes very long due to the traffic jam in laboratories and screening centers.
They are "less than 36 hours in 80% of cases," said Olivier Véran on Tuesday.
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“In both cases, the sample itself is extremely fast.
We will still have to queue in the laboratories, but what will change is that we will save a lot of time for the rendering of the result ”, specifies the infectious disease specialist Benjamin Davido, of the Raymond-Poincaré hospital, in Garches (Hauts-de-Seine).
Is he reliable?
The main concern of antigenic tests is that they "only detect the virus when it is present at a high titre", indicated in April the immunologist Eric Muraille, attached to the Free University of Brussels, in a text published on The Conversation.
“When we do PCR, we amplify the viral genome.
So we can detect it as soon as there is only a little.
A rapid antigenic test involves the detection of virus proteins but without an amplification phase.
If there are few at the start, we will have a very weak signal and we risk missing the infection.
Suddenly, the proportion of false negatives is likely to be greater, ”summarizes Yves Gaudin for his part to the Parisian.
By way of example, the company Coris BioConcept indicates that the antigenic test that it developed had a sensitivity of 60% compared to a PCR test.
"In the event of a negative result, it is prudent to confirm the rapid antigen test by an RT-PCR test", even believes Eric Muraille.
What can they be used for?
To identify more quickly people with a high viral load and therefore, probably, more contagious.
“The deployment of antigenic tests can only come as a complement to the screening policy currently being carried out.
It will make it possible to increase screening capacities and improve the implementation of contact tracing at the national level ”, indicates the Ministry of Health.
“While they are less sensitive than PCR tests, they are also less expensive and much faster.
They are therefore better suited to mass screening ", also advances Eric Muraille to the Parisian, stressing that China had" used them a lot ".
For the moment, antigenic tests should be deployed “from this Wednesday” in Île-de-France, announced Olivier Véran.
They will be useful "in certain areas, such as airports," said the Minister of Health.
Passengers from 29 countries (including India, Israel and Brazil) who have not been able to be tested before departure abroad must indeed submit to it upon arrival.
From August 17 to 23, 16,171 tests were carried out at airports, mainly in the Paris region.
570 were found to be positive, according to the weekly epidemiological point of Public Health France on August 27.
According to our information, the Assistance publique - Hôpitaux de Paris (AP-HP) has ordered 100,000 tests from the South Korean group SD Biosensor Inc and an order must still be taken to authorize its deployment.
"If this first experience is conclusive," the ministry "plans to generalize this new screening technique" in addition to the others, we are told.
The queue to be tested in a laboratory in Paris.
LP / Delphine Goldsztejn
But knowing more quickly if a person is positive will not necessarily allow the success of the strategy "trace, test, isolate", wanted by the government.
"What is more important than the result of the test is the behavior of the individuals who will accompany it and in particular the respect of barrier gestures and the fortnight (which should be shortened to one week, Editor's note)", concludes Yves Gaudin.