The virological RT-PCR test is the tool of choice for diagnosing Covid-19. It is based on the direct search for the virus in the ENT sphere (throat and nose) of the patient as opposed to the serological test which allows the detection of antibodies produced by the patient: an indirect marker of infection.

For symptomatic patients this RT PCR diagnostic test must be carried out as soon as possible following the onset of the first symptoms to increase its sensitivity.

The excretion of the virus at the nasopharyngeal level is considered at its maximum 48 hours before the onset of symptoms until about 5 days after the onset of symptoms.

Given the transience of the virus carriage, it might be that despite a strong clinical suggestion of Covid-19, the test result may be negative. In this case, taking a new sample should be considered.

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RT-PCR is a search for the genetic material of the virus (RNA)

Source: Société Française de Microbiologie

The virus is made up of a protein capsule that protects the genetic material which allows the virus to replicate in the host’s cells. This virus contains RNA, which is the target of the RT PCR reaction.enes with the chosen primers.

How is RT- PCR so accurate?

During the RT-PCR reaction, several specific genes for SARS-CoV-2 will be amplified ad infinitum.

Small, precisely selected sequences of nucleotides, called primers, will “delimit” the fragments of the genes of the virus that will be amplified: they make it possible to avoid false positive reactions with other coronaviruses.

This results in extremely accurate results since the amplification of several target genes with chosen primers is possible.

How is RT-PCR so sensitive?

In addition, the virus genes targeted with the primers are amplified over a billion times during the RT-PCR reaction. This technique is therefore analytically very sensitive.

If we take the variable carriage of the virus at the nasopharyngeal level into account, as well as the variability from the taking of the sample, the overall results are sensitive to 95% of RT-PCR (i.e. from 100 samples taken from patients with COVID 19, more than 95 are said to have a positive RT-PCR result).

The RT-PCR technique performed on oropharyngeal and nasopharyngeal swabs remains the reference diagnostic method. Priority should be given to qualified operators taking the sample, to optimize the quality of the sample collected, and to take into account the patient’s clinical signs for the interpretation of the test result.

Nose and throat sample

Sample collection is taken by swabbing from the throat and nose.

This sample is not painful but can cause an unpleasant sensation (tingling, sneezing, etc.).

It only lasts a few seconds and can be done on anyone, including children. It should be performed by a qualified operator to optimise the quality of the sample and limit discomfort.

As the virus mainly colonizes the nasopharynx, it is essential to take the sample from this location.

How is an oropharyngeal and nasopharyngeal sample performed?

It is desirable that the patient is seated with their head facing upwards, raised towards the ceiling. The patient must then lower their surgical mask to clear the mouth. The sampler swabs the tonsils (back of the throat) and then the patient places the mask back on the mouth, while leaving the entrance to the nose clear. The mask positioned in this way protects the sampler in the event of a cough reflex.

The sampler is positioned on the side of the seated patient (to limit his own exposure). The swab is gently introduced into the nostril and then advanced to the epithelium of the nasopharynx: a rotating movement will collect cells. The swab is then removed, placed in virus transport medium and sent to the laboratory.

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