Coronavirus: five questions about serological testing

Several studies are currently assessing the reliability of serological tests in France. Ultimately, they should make it possible to screen people who have been infected with the coronavirus, but also to draw up a map of French immunity.

A gradual deconfinement will begin in France on May 11. Massive screening campaigns should accompany it, in particular with serological tests. Laboratories and companies are currently trying to develop their own, and the first could be announced this week.

In France, “several teams from the Institut Pasteur, CNRS, Inserm and University of Paris have mobilized to develop different serological tests and have therefore carried out a pilot study to assess the reliability of 4 detection tests” , explains the Institut Pasteur in a press release.

What is a serological test?

Serological tests make it possible, thanks to a blood test, to determine if a person has developed antibodies against the coronavirus, “in other words, if this person has triggered or not an immune response against the virus”, explains Haute Health Authority (HAS). If so, it will mean that she has been infected, and is therefore supposedly immune.

These tests, in addition to individual screening, may provide an idea of ​​the population’s potential immunity. “What will be fundamental is to know how many people have antibodies, what level of antibodies, and are the antibodies going to be protective. This is a real big question”, explained Jean Sibilia, Dean of the Faculty of Medicine of Strasbourg, about test campaigns in the Grand Est.

Which tests are reliable?

HAS has published specifications framing the methods for evaluating the performance of these serological tests. It recommends that they be evaluated by the National Reference Center for respiratory infection viruses. The various tests developed must indeed be validated, in order to avoid the dissemination of poor results.

The government has regularly called for caution on this matter. “Everyone wants it to go as quickly as possible, but before offering a massive test we must ensure the validity and sensitivity of these tests”, explained in early April Frédérique Vidal, the minister in charge of Research and Higher Education, “that is, to be sure that when the test is negative, it is negative, and when it is positive it is really positive.”

Currently, two types of serological tests coexist. The ELISA test which detects the presence of antibodies fighting against SARS-CoV-2 which are “practiced in the majority of medical analysis laboratories, describes the Academy of Medicine.” There are also rapid orientation tests tests (TRODs) presented in a unitary format, detecting antibodies from a single drop of blood in a few minutes “.

How do laboratory tests work?

The enzyme linked immunosorbent assay (ELISA) test is very sensitive and is used to detect and quantify substances “, explains the American medical information site NCBI, which considers it as” the gold standard of immunological tests “. In the case of the coronavirus, its role is to detect antibodies of classes IgM and SARS-CoV-2 specific IgG. Apart from these two antibodies, “little data is currently available on the immune response directed against SARS-Cov-2”, explains HAS.

Laboratories are trying to set up other forms of screening. The Institut Pasteur is currently developing tests called “S-Flow” or “LIPS”, which detect via other elements and traces of the virus the presence of antibodies in patients.

How do rapid tests work?

TRODs tests are carried out “by doctors / pharmacists who are not biologists, nurses”, explains the HAS. They are carried out outside the laboratory, “under the responsibility of the person who carries them out, without any result report”. There is also the possibility of self-tests, directly performed by the patient.

For these rapid tests, a droplet of blood collected on a disposable pipette is placed on a strip, explains Dr. Catharina Boehme, director of the NGO Find which lists all the diagnostic tests in the world. “Above the drop of blood deposited, you put a few drops of a buffer which is supplied with the test, and which contains reagents. Then, in a few minutes, the plasma will form one or two colored lines depending on the presence of ‘antibody”.

But this test, although rapid, has its limits. As HAS points out, “the interpretation of the results of unit tests can prove problematic and generate a large number of false positives or false negatives in this coronavirus context”, especially when the test is carried out by a person non-medical. “The explanatory leaflet must be very accessible in order to support the patient in his approach,” says Catharina Boehme.

For example, if serological tests are used too early, before the production of antibodies, “the patient could in fact still be a carrier of the virus and contagious,” Dr Michael Skinner of the United States told early April in Imperial College London.

What are the results for?

These tests, whatever their reliability, are still confronted for the moment with a big unknown factor: immunity. “There is no evidence demonstrating protective immunity against coronavirus induced by antibodies produced against SARS-CoV-2,” said HAS. The immunity contracted after being infected with the coronavirus remains in effect to this day supposed.

So even if a person is tested positive, in the sense that they have caught the coronavirus and developed antibodies, it is not currently known until when they will remain so. “If we are positive we will not immediately become negative the next day,” however explained Frédéric Altare, immunologist and research director at Inserm.

The HAS also specifies that “serological tests do not make it possible to decide whether the person is contagious or not”.

David T

David T is a french Medical student, during its free time, he is writing for The Medical Progress and helping us to understand better the Coronavirus.

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