Several types of tests relating to the coronavirus are now available. RT-PCR virological tests, ELISA, TDR, TROD serological tests, self-tests. What is the difference between all these tests? What sample should I take from the “COVID-19 test methods”?
Since mid-July, the number of new infections has even increased significantly. The virus reproduction rate has risen to 1.3. In order to curb the epidemic, screening campaigns are increasing, including among asymptomatic populations.
This screening is based on carrying out virological PCR tests (making it possible to know if one suffers from the infection at a time T). But there are also four types of serological tests that can detect antibodies specific to the coronavirus in the blood, which indicate that one has already been infected in the past. The use of saliva tests is still under study.
The Minister of Health, Olivier Véran, announced that there was no longer a need for a prescription to be screened and that it was not mandatory to show symptoms. What are the different types of tests?
What is a virological PCR test for Covid-19?
The RT-PCR test is a non-invasive technique that allows you to perform a PCR (polymerase chain reaction) from an RNA sample.
The next step is to detect a strand of RNA belonging to the SARS CoV-2 coronavirus, then transform it into a strand of DNA that can be analyzed. These tests are offered in university hospital centers (CHU) and in town laboratories.
What is the difference between a virological screening test and a serological screening test?
The serological tests make it possible, by means of a blood sample, to detect the presence of antibodies of the IgM and IgG classes, specific to SARS-CoV-2.
Thus, we can know the immune status of a person. But these tests do not allow a formal diagnosis to be made.
Furthermore, scientists still do not know whether being infected confers protective immunity. According to the Scientific Council, the experience drawn from other coronaviruses suggests that immunity could last at least a few months. In France, no case of re-infection has been identified for the moment.
“There is no point in performing a serological test if you have had symptoms for two or three days. The risk of false-negative is very high at the onset of the disease, as antibodies do not form immediately. It is estimated that” around the 15th day, the tests are 80-90% reliable. This threshold reaches 95 to 100% after three weeks “, further specifies the biologist.
Automated serological tests (ELISA)
Standard serological tests using the enzyme-linked immunosorbent assay (ELISA) method require a blood test and must be performed in a medical laboratory. The serum (a constituent of blood plasma, devoid of red blood cells, and coagulation factors) is then analyzed using an automatic device that measures the antibodies.
Rapid diagnostic tests (RDTs)
RDTs are medical biology examinations performed in the laboratory. The serum is purified from a blood test, then placed on a strip. In the presence of antibodies, colored bands appear. These are unit tests.