Carried out by blood test, the serological test makes it possible to know if we have had Covid by looking for the presence of antibodies developed against the Sars-CoV-2 virus. Among these tests, so-called “TROD” rapid tests. How reliable are they? What does a positive test mean? What timeframe to have the result? For whom before vaccination? Where to do it?
[Mise à jour le mardi 8 juin à 12h09] A serological test, for blood test, may be offered at a Covid vaccination center to screen people who have already been infected with the virus and who then only need to receive a single dose of the vaccine. “For the million French people who have already contracted COVID, with or without symptoms. Even though it was over 6 months ago, only one dose of vaccine is needed. Of rapid serological tests may be offered to you in vaccination centers “ Olivier Véran announced on June 2 on TF1. But what is a serological test? What does it say? How long does it take to get the result? Explanations.
“The Covid-19 serological test is a test for detect in the blood antibodies developed by the body during contact with or vaccination against the SARS-CoV-2 virus“Says Dr. Lionel Barrand, biologist. Clearly, it allows you to know if you have had the Covid or not. There are two categories of antibodies:
- those that appear in the days following infection (IgM)
- those that appear later (IgG).
“When antibodies are found in blood tests, this makes it possible to confirm that the subject has encountered the virus, without specifying the date (retrospective diagnosis). On the other hand, serology is not a screening test for infection and does not allow us to know if the person is contagious“, he indicates.
Reliability depends on techniques. “Generally the sensitivity and specificities are greater than 90% when the test is performed at least 2 to 3 weeks after infection or vaccination, continues the doctor. Don’t do it too soon (risk of false negative since the antibodies have not yet been able to develop) or too late (the antibodies decrease over time and may disappear after a few months / years). Immunochromatographic rapid tests (TROD) are so-called “qualitative“(positive or negative result, without assay) and can be carried out in a pharmacy or in a laboratory. They are less efficient than the so-called serological tests”quantitative“(dosage of the amount of antibody), carried out with ELISA technique (enzyme linked immunosorbent assay) au laboratoire“.
It is possible to perform a Covid serological test without prescription, as with any biological analysis. “For non-prescription health professionals the analysis is 100% covered by the Health Insurance; but for non-prescription health professionals the analysis will not be covered and will be chargeable.“, specifies the biologist.
The Covid serological test can be performed in pharmacy and / or laboratory. “Serologies in pharmacies are carried out using a drop of blood taken from your fingertip. They are a little less efficient and less precise than those produced in the laboratory. In the laboratory, we take a blood sample, which will then be centrifuged to separate the serum from the red blood cells. The serological analysis will focus on the blood serum and will be more precise“.
► In the vaccination center : “Of rapid serological tests may be offered to you in vaccination centers “ Olivier Véran announced on June 2 on TF1.
No, it is not necessary to be on an empty stomach.
“It is necessary to count between 15 and 30 minutes for the TROD rapid tests in pharmacies and a few hours for ELISA serological tests in the laboratory“, emphasizes Dr Barrand.
“A positive serological test means that there are specific antibodies against SARS CoV 2 and therefore that the patient has been in contact with the virus (infection) or that he has been vaccinated “, emphasizes our interlocutor. Note that there are many rapid tests that do not detect that the antibodies linked to the infection and not to the vaccination. “There may be false positives for this type of test, related to interference or when they are not performed under good conditionsThe protection provided by these blood antibodies is not absolute. It is therefore necessary to continue to apply barrier measures.
A negative serology can have several reasons. “This means that no specific antibody against SARS Cov 2 has been detected:
- either because it’s too early,
- either because it’s too late,
- when there has been no contact (virus or vaccination)
- or because the patient does not develop antibodies after infection.
- or because it is (more rarely) a false negative. In this case, the test must be interpreted according to the clinical context.“.
On June 3, 2021, the Haute Autorité de Santé recommended the use of rapid serological tests (TROD) in parallel with the first dose of vaccine to identify people infected with SARS-CoV-2 whose infection has not been diagnosed and do not give only one dose instead of two. “It is estimated that 23% of the population has been infected while only 8% have been detected by PCR or antigen test. It would therefore be useful toidentify anyone who has been infected without knowing it : this would simplify the vaccination schedule for the people concerned, and better protect the population as a whole by vaccinating several million people faster additional in the slots and with the doses thus released “ she explains.
► HAS recommends the performing this test in immunocompetent people with no known history of SARS-CoV-2 infection, especially in people most likely to have an asymptomatic form of the disease, especially young adults. The test will have to be done before the first dose (not between the two doses because the injection can falsify the results). The result will be obtained in 10-15 minutes. The test will be free.
According to the HAS, the fixed reference price for reimbursement of the act of performing a serological test in the laboratory is 9.45 euros for an ELISA test or for a rapid diagnostic test. “With the blood sample, the fixed prices associated with the analysis and in the absence of a medical prescription, the test costs approximately 25 euros“, specifies our interlocutor. In pharmacies, rapid tests are sold between 15 and 25 euros.
Thanks to Dr Lionel Barrand, biologist in Strasbourg and President of the “Les Biologistes Médicales” union.