Covid-19: ELISA, TDR, TROD serological tests, self-test, what difference?
By Coralie Lemke (Read all of her articles)
Posted on 05/20/20 at 5:02 p.m.
ELISA, TDR, TROD tests, self-tests, Sciences et Avenir helps you see more clearly in the jungle of serological tests evaluated by the French National Authority for Health (HAS).
The French National Authority for Health (HAS) has issued an opinion on the many types of serological tests available for Covid-19.
JODI HILTON / NurPhoto / NurPhoto via AFP
Serological tests validated by the health authorities should soon be launched. The French National Authority for Health (HAS) has reported on the usefulness of these tests in the fight against Covid-19. It details the different types of serological tests available on the market and recommends in which settings health professionals can use them. Elisa, TDR, TROD, self-tests, all test for the presence of antibodies in the blood. In the case of SARS-CoV-2, this means that a patient has already been infected and that he could benefit from immunity at a future exposure, provided that this is proven by science. Sciences et Avenir takes stock of these different devices, their operation and their reliability.
ELISA tests
An ELISA plate used in the laboratory. The virus is detected by a colorimetric revelation caused by the antigen-antibody reaction. Photo credit: Ole Spata / DPA / dpa Picture-Alliance via AFP
The name ELISA comes from the English “enzyme-linked immunosorbent assay”, which means “enzyme linked immunoabsorption assay”. It designates tests carried out in laboratories using a blood test then analyzed using reagents. ELISA works on the reaction between the antigen (contained in the test) and the antibody (in the blood), the combination of which is identified by an enzyme which colors and establishes the result. HAS recommends prescription ELISA tests to confirm a diagnosis of Covid-19, to conduct epidemiological studies, or to test personnel, even asymptomatic, in medico-social institutions or working in confined environments (nursing homes, boarding schools, door – planes, etc.).
Rapid diagnostic tests (RDTs)
Rapid diagnostic tests (RDTs) work with a single drop of blood. Photo credit: JODI HILTON / NurPhoto / NurPhoto via AFP
This type of test is done by drawing a drop of blood from the fingertip, which is then inserted into a device similar to a pregnancy test (see photo above).
The passage of blood through different reagents makes it possible to establish or not the presence of antibodies. A result is available in fifteen minutes. It is indicated in the same cases as the ELISA test. The advantage of TDR tests? Cities with only medical biology laboratories without a highly developed technical platform will be able to use them to screen their population. “The HAS recommends using it with the same populations as automated tests: in the catch-up diagnosis of symptomatic patients, in the event of a negative virological test but of symptoms suggestive of COVID-19, and with nursing staff or establishments of ‘collective accommodation that has been in contact with the virus and for epidemiological investigations. “
Rapid Diagnostic Orientation Tests (TROD)
TROD involves taking a drop of blood and placing it in a device similar to a pregnancy test. (This is a TROD to detect hepatitis C). Photo credit: FRANCOIS NASCIMBENI / AFP
These rapid diagnostic orientation tests are similar to rapid screening tests (RDTs). TROD “can be carried out in more places and by any health professional (doctors, midwives, nurses, pharmacists …) or even trained members of certain associations who could do it in a medical office, in a pharmacy, at home. ” That said, these so-called “rapid” tests, which are lighter, serve only as an indicator and cannot replace a real serological test to establish a diagnosis of Covid-19.
“TRODs are diagnostic orientation tests, not tests allowing to formally make the diagnosis of COVID-19, reminds HAS. Therefore, they cannot replace medical laboratory biology examinations. is necessary after a positive TROD to confirm the result by an ELISA or TDR serological test – reference tests. ” The HAS recommends using them in a more restricted field than RDTs, for “epidemiological studies, patients suspected of being affected but distant from biological laboratories or even the staff of certain reception institutions.” But not in the hospital. They must be accessible to a greater number of people but cannot be sufficient to establish a diagnosis.
Self-tests
These tests are available in pharmacies but HAS warns of their reliability. It is “premature” to recommend them because too little scientific data attest to their proper functioning. In addition, the results are difficult to interpret without the help of a health professional. “If the taking of the sample is simple (the patient performs it alone, at home, by pricking the fingertip), it is not the same for reading and interpreting the result. Without support, the patient takes the risk of drawing the wrong conclusions from this test, “explains the French National Authority for Health (HAS). These tests include several reading tapes.
At this time, the level of antibodies required to be immune has not been established. It is also unclear how long this immunity could last. In its opinion of May 2, the French National Authority for Health (HAS) recalls that “the protective nature of antibodies”, sought in this type of blood test, has not been demonstrated, and, more importantly, that “even in using a test which meets the requirements of the specifications, if the virus has circulated little within a population, the result of the test has an increased risk of being erroneous “. For the moment, in France, the National Reference Center (CNR) has not approved any serological test. In the United States, on the other hand, the authorities suspended all regulatory obligations at the start of the pandemic. There are now 160 tests on the American market without their effectiveness having been validated before their commercialization.
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