PCR, TDR, Elisa… What are the screening tests for Covid-19?


Faced with the epidemic rebound, medical biology laboratories are stormed by French people eager to be tested for Covid-19. PCR, RDT, Elisa and soon saliva tests? France 24 takes stock of the various screening tests, what they detect, and their reliability.

Screen more, to stop the Covid-19 epidemic, still very present in France. This is the objective posted by the government which lists, on the website of the Ministry of Health, the centers where it is possible to be tested. PCR tests which can now be carried out without a prescription and free of charge since a decree of July 24, because 100% covered by Health Insurance. Result: endless lines are emerging in front of medical laboratories that have never known such attendance.

Pharmacies, for their part, have been authorized, since July 11, to perform rapid diagnostic orientation serological tests (TROD) using a single drop of blood.

PCR test, TROD, but also TDR, Elisa and soon saliva tests … France 24 takes stock of the differences between these tests, when and why to do them, and their reliability.

“Am I infected today?” : the PCR test, a virological test

This is the famous test of the long cotton swab in the nose. The RT-PCR test [Reverse Transcriptase-PCR, pour “Transcriptase inverse-Réaction en Chaîne par Polymérase”], is a technique for removing deep nasal cells using a swab (the long cotton swab). These PCR tests make it possible to know if the person tested is suffering from Covid-19 at the time of the sample.

The objective of this test, in the long term, is to break the chain of transmission of the virus by diagnosing as much as possible the sick, presenting symptoms or having been in contact with an infected person. If the result is positive, the patient is placed in a fortnight.

If this could previously only be carried out by biologists, the decree of July 24th extended the performance of PCR tests to state-certified nurses and, “under certain conditions”, to students in the health profession, to caregivers, and firefighters.

The test result is usually available 24 hours after collection. Due to a significant increase in testing capacity upon exiting containment, it is currently possible to perform up to 700,000 tests each week across the country. While France is worried about a potential second epidemic wave, screening is intensifying. In the week of July 27 alone, some 527,000 tests were carried out nationwide. A record since the start of the pandemic in France.

“Have I been in contact with the virus in the past?” : serological tests

Via the extraction of a simple drop of blood or a complete tube, serological tests aim to detect, not the presence of the virus, but the presence of antibodies of the IgM and IgG classes, specific to SARS-CoV- 2. IgG antibodies are formed at least 14 days after coming into contact with the virus, while IgM can be detected about a week after infection. The goal ? Find out if the person tested has developed an immune reaction against the coronavirus and therefore contracted Covid-19 in the past, even without having had the slightest symptom.

There are several types of serological tests which, although their method differs, serve the same objective: the search for immunity to Covid.

  • Rapid diagnostic orientation tests (TROD)

For the TROD (rapid diagnostic orientation test), no swab in the nose, but a drop of blood placed on a strip, after a slight prick of the fingertip. In the presence of specific antibodies to SARS-CoV-2, colored bands appear on the strip.

Since July 11, and until October 30, pharmacists are allowed to perform these tests to find out, in just a few minutes, if the body has been in contact with the virus. But the National Order of Pharmacists reminds us: these TRODs “cannot replace medical biology examinations carried out in the laboratory”. Also, it is necessary after a positive TROD “to confirm the result by an Elisa serological test or TDR – reference tests.” Even by a PCR test, to check if the virus is still present in the body and therefore, if there is a risk of infecting other people.

  • Rapid diagnostic tests (RDTs)

These tests take the form of a medical biology exam. On the same principle as the TROD, the TDR makes it possible, after a blood test, to detect the presence of antibodies thanks to the appearance of colored bands on a strip. However, these can only be carried out in the laboratory.

  • Serological tests using the Elisa method

The Elisa diagnosis (for “enzyme linked immunosorbent assay”) makes it possible, following a blood test, to assay the antibodies using an automatic device. Like the TDR test, the serological test via the Elisa method must be performed in a medical laboratory.

For all of these serological tests, there is a double uncertainty. On the reliability of current tests, on the one hand, but also on the immunizing nature of the virus. Indeed, although no case of reinfection has been identified for the time being in France, “the presence of antibodies could not systematically protect against a new infection”, warns the Ministry of Health, adding that studies scientists are underway.

The EasyCov saliva test, a new virological test soon available?

More instantaneous, less painful … The saliva test could considerably change the experience of screening for Covid-19, by offering a saliva sample under the tongue, delivering a result in less than an hour. Like the PCR test, the saliva test will indicate whether the patient is infected with the coronavirus at the time of the sample.

The process, developed by a Montpellier company attached to the National Center for Scientific Research (CNRS), is called “EasyCov” and could also help relieve the clogging of laboratories, today stormed for PCR tests alone.

“The validation of the performance of saliva tests is underway by the French Society of Microbiology,” explained the Directorate General of Health (DGS), in Paris. It will then be necessary to wait for the High Authority of Health to examine and validate them “within a short period of time given the stakes”, specified the DGS, without giving a date.

Several steps are therefore still awaiting EasyCov before validation and marketing of saliva tests for the general public. All the more so as the scientific community is not unanimous on the reliability of such a process. Saliva tests “are not reliable!”, Launched the President of the Scientific Council, Professor Jean-François Delfraissy, on July 21 on BFM TV. “Contrary to the announcement effects that may have been made, we do not yet know the reliability of the saliva sample compared to the nasopharyngeal sample”.

In the meantime, it will therefore be necessary to take a deep breath and let the swab pass, the PCR test being, to date, the only reliable and recommended test for screening for Covid-19.


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