Testing of a reliability variable

Des tests de dépistage d’une fiabilité variable

Photo: Renaud Philippe The Duty
The INSPQ is currently performing validation of multiple sets of serological tests. Some of these tests must be carried out in the laboratory, others are rapid tests.

The research for the development of tests to diagnose the COVID-19 was in full effervescence these last few months. It has resulted in the design of hundreds of different tests around the world, some of which are used in Quebec. These tests are distinguished primarily by their reliability and by their speed to make a profit.

The PCR test (polymerase chain reaction), which is to detect the genetic material of the virus, namely SARS-CoV-2, in samples collected in the pharynx with swabs is today still the reference standard test was the most reliable and the most used in Quebec.

Most public health laboratories and academics of the world have adopted one of the recipes developed by european laboratories and have adapted it to their own needs. This was the case of the public health Laboratory of Quebec (LSPQ), which, at the beginning of the epidemic, performed all the testing of the COVID-19.

Then, several hospitals in Quebec have been equipped with PCR tests commercial developed by private companies, but which have been thoroughly evaluated by the LSPQ, which has helped to increase the volume of tests performed.

“The PCR tests used by the world to screen the COVID-19 are not of sufficient sensitivity to be perfect. Nearly 20 % of people who have undergone this test and received a negative result however, have developed the COVID-19. Usually, when the doctor suspects that his patient is infected, it will repeat the test, ” says Dr. Matthew Oughton, infectious diseases physician and microbiologist at the jewish general Hospital.

“These false negatives can be due to the fact that the test was taken too early while the virus has not had time to replicate enough. They can also be attributed to the fact that taxes have not been adequately performed. A good sampling is to get a sample of it in the pharynx through the nose or through the mouth and the back of the throat. If the buffer is not pressed in deep enough or if it is not left long enough at the collection site, the sample collected may not be of good quality and not to reveal the presence of a virus, even if they are present, ” he explains.

At certain times of the epidemic, the shortage of some components of these PCR tests performed in the laboratory has led to delays in obtaining the results which are normally disclosed within 24 hours after collection.

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Rapid Tests

However, rapid tests made on-site or in different places, such as a pharmacy or a clinic, outside of the laboratory, have also made their appearance. “These PCR tests produce results more quickly and is technically more simple than the laboratory tests which require the expertise of laboratory technicians well trained. But their greater simplicity and speed have a price. These tests are often very expensive. Generally, they are equivalent, and in some cases, they are even higher than the lab tests. They may even be more sensitive, and therefore able to detect lower amounts of virus in a sample, ” says Dr Oughton.

The canadian company Spartan has developed a PCR test for rapid which it had received a conditional approval at its validation, which failed however. It is a small device into which it is introduced the levy, and which makes the verdict in half an hour, says the microbiologist-infectiologist Judith Fafard, a medical consultant to the Institut national de santé publique du Québec (INSPQ).

There are also antigenic testing rapid detect in a sample of saliva, pieces of the virus itself. Very simple, these tests are like a pregnancy test. But for the moment, they are not very sensitive and for this reason, none has yet been approved in Canada.

Some private clinics in Quebec offer to a serological test to diagnose the COVID-19. This test aims to detect antibodies against the SARS-CoV-2 in a sample of blood. “These tests are presented as enabling a rapid diagnosis, but this is to forget that when a person is infected, it takes a few days before her body produces antibodies and that they can be detected. If this test is done during the first week of infection, it is very likely that the test will prove negative, ” says Dr. Fafard.

More to come

The INSPQ is currently performing validation of multiple sets of serological tests, some of these tests must be carried out in the laboratory, others are rapid tests. “To do this, it was necessary to look in freezers of hospitals in the sera of patients who suffered from other respiratory diseases, but who have never had the COVID-19 to verify that the tests weren’t reacting with the antibodies generated by other respiratory diseases, such as infections caused by coronavirus and common. Audits are also carried out on sera from patients who had the COVID-19 to ensure that the tests can detect this infection, ” explains Dr. Fafard.

“These serologic tests tell us if the person has been infected. But is that the presence of antibodies protects that person against the acquisition of a new infection ? We don’t know yet “, she says.

“These tests are still very popular to diagnose infection retrospectively, and to estimate the proportion of the population that has been infected by the virus,” says Dr Oughton.

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