Here’s why negative test results don’t necessarily mean you’re not infected

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Whether or not the coronavirus screening technique is effective, an infected person may get negative results. This happens when the nasopharyngeal sample has not been performed properly or if the virus has migrated into the deep respiratory tract, according to France Info.

Although the technique of so-called PCR (polymerase chain reaction) biological tests intended to detect the coronavirus is well mastered, its negative results do not always mean that the person tested is not contaminated, reports France Info. The explanations relate to the sampling technique and the characteristics of the disease.

According to Bruno Pozzetto, head of the virology department at the Saint-Etienne (Loire) university hospital, where 300 tests of Covid-19 are performed each day, the quality of a test depends 90% on that of the sample.

“You have to push the swab down into the back of the throat, beyond the line of the eyes. There is the risk of making the patient cry a little, of hurting him a little, but this is almost a success factor, ”explains the doctor.

A good sampling technique, unpleasant for the patient, has been shown on video by the New England Journal of Medicine.

However, even if the sample was taken completely correctly, much still depends on the time of screening, because the viral localization changes during the disease.

On the 7th day the infection goes down

According to Bruno Pozzetto, after the cap of the 7th or 8th day, the infection enters the lower respiratory tract. At this stage, it is necessary to pass to bronchoalveolar washes, carried out under endoscopy, and the nasopharynx is no longer the optimal area for taking a sample.
During this operation, a few tens of milliliters of physiological saline are introduced to the bottom of the bronchioles. This serum is then re-aspirated before being placed in a sterile jar.

“It is the most noble and clean sample to test for a pulmonary infection, because it is sure that it will not be polluted by the bacterial flora present in the nasopharynx, the tracheae and the beginning of the bronchi”, explains the specialist.

The nasopharyngeal swab is sufficient in 99% of cases

Nicolas Lévêque, head of the virology department of the Poitiers university hospital, points out, however, that in most cases the nasopharyngeal swab is more than sufficient.

“The nasopharyngeal swab is sufficient in 99% of cases for banal forms of the disease, especially since it is easier to handle in the laboratory,” said the specialist, while adding that “if the test is negative despite strong suspicion infection, maybe we need to go further. ”

It is then advisable to carry out x-rays and scanners of the lungs beyond the broncho-alveolar washing.

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