Covid-19: which screening test to choose before New Year’s Eve?

The Christmas goal is behind us. After having sliced ​​the capon, congratulated its hosts on their gratin scallops or ended in nervousness a heated political debate with Uncle Jacky, the time has already come to rest the tireless question, this time for the New Year: “Can you get tested before Christmas Eve?” Stormed before Christmas, at a rate of just under a million daily tests, not counting the purchase of 200,000 self-tests per day (to the point that a good half of pharmacies were out of stock after Christmas), for 100,000 daily contaminations, laboratories and pharmacies have no reason to empty before New Year’s Eve.

Many French people therefore raise their nostrils to the sky and follow the recommendations of the Scientific Council. “For family gatherings such as Christmas meals, it is recommended to perform a self-test the same day or an antigen test the day before or the day of the event”, the Scientific Council thus indicated in its opinion published on 8 December. In total, more than 4 million screening tests are performed each week in France. “Laboratories, doctors and pharmacies test the equivalent of 10% of the French population every week, which has never been done”, underlined the Minister of Health, Olivier Véran, in front of the National Assembly before the holidays.

But if screening is strongly encouraged, it can sometimes be difficult to know which device to adopt depending on your situation. Release helps you see more clearly.

Nasopharyngeal PCR tests

PCR, for polymerase chain reaction (in English). This scientific term indicates how the sample is analyzed. The principle: look for traces of the virus’s RNA. To do this, a sample of mucus is taken with a swab inserted up to the nasopharynx, a cavity behind the nostril.

Advantages. The likelihood of false negatives or false positives is very low. The results of PCR tests are referred to as “sensitive”, where sensitivity is the ability of a test to detect a virus. In February, the High Authority for Health estimated the average sensitivity of nasopharyngeal PCR tests at 92%. The errors can be related to mistakes in the collection or handling of the samples. The appearance of new variants may affect the reliability of PCR tests, nasopharyngeal or not.

During the analysis, specific sequences of the genome of the virus are sought: if they have mutated, they may not be recognized. At least two sequences are sought to limit this risk. Test manufacturers should ensure that their device works on new variants.

Disadvantages. The scan can take at least a day, and it can take thirty-six or even forty-eight hours for the results to become available. A period of time that complicates its use for unvaccinated people needing a valid health pass, only tests of less than twenty-four hours entitling it. PCR tests are also more expensive when they are not reimbursed.

For who ? The PCR test is in particular the only test recommended in the event of symptoms persisting for at least five days and the only one accepted before a surgical operation. Positive results from other types of tests should be confirmed by a PCR test.

Antigenic tests

As with PCRs, antigenic tests are most often nasopharyngeal. Again, you have to scrub your nostril. But the method of analysis is different. This involves placing the sample taken on a strip that reacts to certain proteins produced by the virus in the body.

Advantages. It goes quickly ! Like a pregnancy test, the appearance of bars on the test indicates in a few minutes whether it is positive or negative, without further details.

Disadvantages. They are less precise than PCRs. According to a HAS opinion published in September 2020, “Antigen tests must have a minimum sensitivity and specificity of 80% and 99% respectively”. This corresponds to a risk of 1 in 5 of a false negative result and a risk of 1 in 100 of a false positive result (the specificity measures the ability of the test not to confuse Sars-CoV-2 with another virus). For their antigenic tests to be marketed in France, manufacturers must certify to the National Medicines Safety Agency (ANSM) that they comply with these two thresholds.

For who ? Nasopharyngeal antigen tests are recommended by HAS, as are PCRs, for people who have been symptomatic for less than five days. It is also the type of test recommended if you are in contact and do not have symptoms. It is faster and easier to access, but any positive result must be confirmed by a PCR test.

Self-tests

Self-tests are really antigenic tests but they work with a sample taken by the user himself. A swab always, but that one sticks less deeply in the nostril. A nasal test therefore, and not a nasopharyngeal test.

Advantages. As long as we have acquired kits in advance, we can make them whenever we want! Particularly as close as possible to Christmas Eve or New Year’s Eve, just before setting off to find family or friends… or not, depending on the result. Like antigenic tests, the results are very quick. The self-test can also be more reassuring for those who are afraid of having their nostril inspected by someone else.

Disadvantages. They are hardly precise. The rule imposed on antigenic tests in France also applies to self-tests: to be marketed, they must exceed 80% sensitivity and 99% specificity. However, since the self-tests are not administered by trained professionals, the risk of handling error is greater.

Studies on their reliability in real life are also scarce. HAS cites only one. In an opinion of August 6, it concludes with a sensitivity equal to that of the antigenic tests. However, the study focused only on self-tests performed “Under the supervision of a health professional”. It is also only when they are supervised that the self-tests allow a valid health pass to be obtained. The government remains very cautious about the value of their results, judging their reliability “Limited”.

For who ? These tests should not be used when the first symptoms appear. HAS recommends its use for people who have no reason to think they are infected. This is the test to be done to reassure yourself before Christmas and New Year’s Eve, explains the Scientific Council. It can be carried out, in a supervised manner, in a pharmacy or at the entrance of a place requiring the health pass. The self-tests are also used for massive screening campaigns, in particular for people working with an elderly population and in the National Education, where they are distributed to all sixth grade students.

Oropharyngeal tests

After the nostrils, make way for the throat. Unlike nasopharyngeal tests, the site of the sample is not at the back of the nose but in an area of ​​the back of the throat: the oropharynx.

Advantages and disadvantages. They can be both antigenic and PCR tests and therefore cover the strengths and defects inherent in these two large families of tests. In its opinion in favor of their use, in September 2020, the HAS described their results as “Acceptable”, but did not deliver any data. Earlier, the High Council of Public Health cited, in its own favorable opinion, a study (in English) on individuals having undergone an oropharyngeal swab and a nasopharyngeal swab: once analyzed by PCR, both gave the same result. in 76% of cases, and 85.2% in positive people.

For who ? HAS only recommends oropharyngeal tests as part of mass screening or testing of asymptomatic contact cases. It also recommends it for people for whom nasopharyngeal tests are more difficult to perform, such as young children, very elderly patients, patients with psychiatric disorders or those affected by a nasal deviation.

Saliva tests

Simply saliva in a tube. The sample, carried out without the intervention of a professional, is done without a swab. The contents of the tube will then be analyzed.

Advantages. Escape the swab! This is why these tests are mainly used for kindergarten and elementary school students.

Disadvantages. The risk of false negative is greater with nasopharyngeal tests. According to a HAS opinion in February, saliva PCR tests have a sensitivity of 85%, against 92% for nasopharyngeal PCRs. These rapid tests were the subject of an alert from the HAS in July, indicating that they “Present very heterogeneous results varying from 30% to 90%, without any real explanation”.

For who ? Younger children for whom the nasopharyngeal swab may be complicated to perform.

Are you positive? Contact case? here is isolation rules that apply to your situation.

Update Monday, December 27: addition of the latest daily figures for contamination and screening.