Covid-19 screening tests would be the key to ending the crisis because they would allow to know if new cases appear in the country. Problem: current PCR-type tests are estimated to miss three out of ten patients. Why such a margin of error? Are these tests really reliable? How to do the samples well? Deciphering these “false negatives” in six questions.
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What is a “false negative” test?
“This is a PCR test (Polymerase chain reaction) performed on a person infected with Covid-19, the results of which are negative. The infected patient is therefore not detected by the test “, explains Jacques Izopet, of the Toulouse Purpan physiopathology center.
Individuals therefore develop the disease when they have tested negative. This is what happened to Julie, 16, who died of coronavirus at the Necker Hospital in Paris in late March. The adolescent had been tested negative twice, before being finally declared positive.
It also means that a potentially contagious person who goes undetected will continue to spread the virus without knowing it.
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Are these tests reliable?
“The PCR tests are reliable since they have been tested in the laboratory and have been very well validated”, maintains Jacques Izopet. In fact, the results of a positive PCR test are 98% reliable. This figure drops to 60% -70% when the test is negative.
The technology, in other words the test, is therefore reliable but it is the result that can be wrong. For Professor Izopet, “the results of the tests are simply conditioned by the nature and quality of the sample, and the time when it is done. Depending on the type of sample, you can have different virus concentrations”.
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How is it possible ?
Several elements can explain this. “The objective of a test is to recover infected cells, recalls the Toulouse infectious disease specialist. However, the viral load evolves over time in a given compartment: nasal cavities, bronchi … The natural course of the infection the fact that as one moves away from the observation of the first symptoms, the concentration of the virus decreases in the nasal cavities “.
The nasopharyngeal sample must be done well enough, explains the scientist. This means that it must “collect enough cells”, with the possibility that a sample taken “does not have a sufficient viral load”.
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Is this a risk?
Three in ten people go undetected because of poor samples, “but that is not cause for concern,” relativizes Jacques Izopet. “During the treatment of a patient infected with Covid-19, several tools can detect the infection: most studies show that there is a good agreement (more than 80%) between the results of PCR tests with the results of the first scanners, explains the professor. The clinical symptomatology, the results of the PCR test and the results of the first scanners are all elements that make it possible to detect an infection. The “false negatives” of the PCR tests are not in this not a real problem. “
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How should the sample be taken to be correct?
The nasopharyngeal sample is taken using a swab, a kind of large cotton swab, inserted into the nose. The sample must thus allow the virus to be sought in the secretions of the person tested. It is at this stage that improper handling can take place. Because the virus is not present everywhere: to be sure to collect it, it is indeed necessary that the swab is inserted far enough in the nose, almost to the pharynx, for the collection to be valid.
A video from the New England Journal of Medicine details precisely what to do: “Push the swab all the way down to the back of the throat, beyond the line of the eyes. There is a risk of making you cry a little the patient, to hurt him a little, but it’s almost a success factor. “
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When should a test be done to be most effective?
The amount of virus can vary depending on the patient and the stage of infection. For Jacques Izopet, the test must therefore be carried out “at the start of clinical management, from the first symptoms. The virus is present at that time in the nasal cavity”. But, “if the virus is not detected in the PCR test, there is no need to worry, he warns. We only know that at certain stages of contamination there will be a low chance of detect the virus at this level. “
An infected patient may therefore not test positive if the sample is taken too early during the incubation phase or at the very end of the disease. The sample then contains little virus which may go undetected.
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