Covid-19: PCR, saliva, serology …, the tests in five questions

Posted on Sep 15, 2020 at 11:15 a.m.

“Test, test, test! »Since March, WHO recommends that all states massively screen people with coronavirus in order to be able to better control the spread of the epidemic. Initially lagging behind, France has managed to increase its testing capacities, and more than a million people are now screened weekly, to the point of overloading laboratories, overwhelmed by demand.

What should I know about these screening tests? Are there others that are faster or less painful, more or less reliable? The point on what to know.

1. What are the characteristics of the PCR test?

The government has embarked on a massive screening strategy for Covid-19 using virological tests, known as “PCR”. 100% reimbursed and achievable in medical analysis laboratories, these tests make it possible to say that a patient is infected when they are carried out.

Based on a search for the genetic material of Sars-Cov-2, these tests require taking a sample by deeply inserting a swab (long cotton swab) into the patient’s nose. It takes a few hours to analyze the samples, but often, for logistical reasons, it takes several days to send the results to the patients.

The “specificity rate” of PCR technology is estimated at around 99%, which means that “false positive” cases are extremely rare (cases of improper handling, accidental contamination for example). PCRs can also detect even very small amounts of virus, to avoid “missing” infected people.

However, in real conditions, the average sensitivity of PCRs strongly depends on the sample, hence the possibility of having “false negatives” when the sample was not taken correctly or was taken too early. or too late in the course of the infection. Paradoxically, the fact that PCR tests detect tiny amounts of virus is, also for some, their disadvantage: because patients can be positive even when their viral load is very low and they may not be contagious.

2. Are saliva tests less reliable?

Saliva tests are a promising alternative to conventional PCR tests. These tests are simpler and less unpleasant than nasopharyngeal swabs, and could be done at home, which would make it possible to resolve some of the problems of laboratory saturation.

However, saliva is considered a less reliable medium for detecting the virus, unlike swabs from the nose, which are taken in areas where the virus multiplies the most. In France, the Haute Autorité de Santé (HAS) will soon indicate whether it is still useful to integrate them into the screening strategy. With the onset of winter, pediatricians, who also fear the arrival of the flu, whose symptoms are often similar, want their use in any case.

3. Will there be rapid screening tests soon?

Rapid screening tests for the coronavirus, known as “antigenic tests”, will be deployed from this week, the Minister of Health Olivier Véran promised last Tuesday. The Assistance publique-Hôpitaux de Paris (AP-HP), which has ordered 100,000 copies, has already started using them.

Like the current reference test, the PCR, the antigenic tests are carried out from samples in the nostrils, by swab. But while RT-PCR requires heavy laboratory analysis to detect the genetic material of the coronavirus, the antigen test spots virus proteins within minutes (around 15 to 30 minutes). This makes it possible to quickly sort people who are probably contaminated, in airports or at the reception of hospitals for example. A positive result must however be confirmed by PCR, which remains the reference test.

Rapid tests could also soon be deployed directly in offices thanks to Loop Dee Science, a young company from Caen. The latter designed a small case allowing a result in 30 minutes where the tested patient is.

4. What is the difference from a serological test?

Lighter than RT-PCR tests (a blood sample is sufficient), serological tests do not have the same objective: they aim to detect antibodies to determine after the fact whether an individual has been in contact with the virus, and s ‘he is therefore a priori immune. But since it is not known whether the antibodies provide complete and lasting protection against the virus, these tests are mainly used to give an indication of the proportion of people who have been infected in the population.

Also reimbursed at 100% with a prescription, these serological tests can be carried out by medical analysis laboratories, in town or in hospital. They are also available in a quick version, which only require a drop of blood and can be performed by a healthcare professional or pharmacist. But they are only considered indicative, and therefore to be confirmed by a laboratory test.

5. Is there a test planned to distinguish influenza from Covid-19?

To “anticipate the arrival of seasonal flu in this context of an epidemic”, a double test detecting both influenza and Covid-19 could soon be implemented. In a press release, the Syndicate of Biologists has announced that it is working with the Health Insurance on this possibility.

“It will be necessary, for a certain number of cases, to be able to determine precisely what the patient is suffering from, the clinical symptoms being able to be almost the same”, justifies the SDB. This PCR-flu test would be reimbursed jointly with that of the Covid.

Leave a Comment