To know if you have had the Covid and then only receive a single dose of vaccine, you have to do a serological test. Without a prescription, the test is not reimbursed. How to interpret its results? Where to do it? In pharmacy? Laboratory? What reliability? Can we do it if we had the Covid in 2020? What price ? Refund ? Is it valid for the health pass? Guide.
[Mise à jour le vendredi 16 juillet à 15h50] A serological test allows to know of a past infection. Carried out via a blood test, it detects specific antibodies produced by the body and directed against the SARS-CoV2 virus. It does not allow you to know if you are infected with Covid when you do so, but to find out if we have had the Covid a few weeks or months ago. He answers the question: “Has the patient ever encountered the virus and made any antibodies?“. If so and within the framework of vaccination Covid, only one dose of the vaccine is then necessary on presentation of the test result. This single dose makes it possible to validate the health pass for these people. It is carried out on blood samples (blood sample in case of a test ELISA or drop of blood taken from the fingertip during a TROD) on laboratory and in some pharmacies. But what is a serological test ? What does it say? In how long do we have the result ? Explanations.
A serological test allows to know of a past infection. “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.
There are two main categories:
- Automated ELISA tests: they can only be done in a medical biology laboratory. They are carried out on a blood sample, generally by blood test, and provide information on the presence of different types of antibodies (IgM and IgG most often). These tests fall under the category of medical biology examinations;
- Unit tests are made from a drop of blood taken at the tip of the finger, in the laboratory, but also in pharmacy, in a doctor’s office or at home. With these tests, it is not a question of an assay, but of the detection of a given amount of antibody. “The answer is then binary: if the blood contains less antibodies than the minimum threshold, the result is negative; if it contains more, the result is positive” explains HAS. In unit tests, there are RDTs (rapid diagnostic test), TROD (rapid diagnostic orientation test) and self-tests (no blood self-test has been validated in France).
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 dosage) and can be carried out in a pharmacy or in a laboratory (or in a vaccination center, editor’s note). They are less efficient than the so-called serological tests”quantitative“(determination of the quantity of antibodies), carried out with ELISA technique (enzyme linked immunosorbent assay) au laboratoire“.
From June 18, in accordance with the instructions of the Directorate General of Health (DGS), people who have already been infected with Covid-19 can, on presentation of proof (eg: PCR or positive serological test) validate their vaccination schedule with a single dose of vaccine, regardless of the date of infection, including if it took place at the start of the health crisis (2020).
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.
A 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 pharmacies against Covid : “The implementation of these tests in pharmacies is still not recorded while talks are continuing with the Minister of Health to define their use” informed Philippe Besset, president of the Federation of Pharmaceutical Unions of France (FSPF), during his live on June 18, reports the Moniteur des Pharmacies.
Free tests in a vaccination center from 12 to 55 years old.
► In a Covid vaccination center : rapid serological tests called “TROD” can be offered in the vaccination center for people who have made an appointment to be vaccinated (but it is better to inquire beforehand to find out if the center offers them). On their arrival, this test makes it possible to know in about ten minutes whether they have antibodies to the Covid virus and therefore whether only a single dose of vaccine is necessary for their protection (instead of 2 in the classic vaccination schedule of RNA vaccines from Pfizer and Moderna). As part of their free use in the vaccination center, serological TRODs are offered for people aged 12 to 55 inclusive, indicates the Ministry of Health. People over 55 may also request to have a serological TROD performed at the time of the pre-vaccination interview in a vaccination center, doctor’s office or at home. It will be up to the health professional in charge of the pre-vaccination interview to decide whether or not to prescribe this TROD, in particular according to the patient’s state of health and his vulnerability to the disease.
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 vaccination center, editor’s note) 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 18, the Directorate General of Health (DGS) indicates that people who have already been infected with Covid-19 can, on presentation of a supporting document (eg: PCR test or serological positive) validate their vaccination schedule with a single dose of vaccine. “This instruction is now valid regardless of the date of infection, including if it took place at the start of the health crisis.” On June 3, 2021, the French National Authority for Health 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“ she explained.
► HAS recommends the performing this test in people with no known history of SARS-CoV-2 infection, especially in people most likely to have an asymptomatic form of the disease, and more especially young adults. The test must be done before the first dose (not between the two doses because the injection can falsify the results). The result is obtained in 10-15 minutes. The test is free at the vaccination center.
The serological test can prove a past infection of Covid. Alone, it does not allow you to have the health pass. It is not part of the documents validated by the government in France and by European states for travel. The serological test is useful for people who have already had the Covid because its presentation attesting to a past infection (presence of antibodies) at the time of vaccination makes it possible to receive a single dose of vaccine which is sufficient to validate the health pass, 7 days after injection.
“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 (in the laboratory, editor’s note)“, specifies our interlocutor. The price can go up to a forty euros in certain laboratories that we contacted. In pharmacies, rapid tests are sold between 15 and 25 euros. A Covid serological test can only be taken care of if the patient has a prescription. According to HAS, the reference price set for the reimbursement of the act in the laboratory is then 9.45 euros for an ELISA test or for a rapid diagnostic test.
► Free at the vaccination center : As part of their free use in the vaccination center, serological TRODs are systematically offered for people aged 12 to 55 inclusive, indicates the Ministry of Health. People over 55 may also request to have a serological TROD performed at the time of the pre-vaccination interview in a vaccination center, doctor’s office or at home, but they must have a prescription the health professional in charge of the pre-vaccination interview who decides whether or not to prescribe this TROD, in particular according to the patient’s state of health and his vulnerability to the disease.
Thanks to Dr Lionel Barrand, biologist in Strasbourg and President of the “Les Biologistes Médicales” union.