Antigenic self-test or PCR test: how to choose, on a case-by-case basis | Slate.fr – Slate.fr

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With the approach of Christmas and theincrease in the number of Covid-19 cases, it is important to continue to be tested: whether it is because you have symptoms, that you have been exposed to the virus … or simply because you plan to go to a high-risk environment –Like a closed room full of people.

To detect SARS-CoV-2, the virus responsible for Covid, we now have access, among others, PCR tests (or RT-PCR tests, for “reverse transcription polymerase chain reaction “, carried out in the laboratory) but also to rapid antigenic tests (which may exist in the form of self-tests).

What is the difference between these two types of tests? And which one should you use?

How it works?

PCR tests are used to diagnose infections with SARS-CoV-2. This type of test looks for the virus genetic material [en l’occurrence de l’ARN et non de l’ADN comme pour notre espèce, ndlr].

RT-PCR first converts viral RNA into DNA, so that it can be read by our cells, and “amplifies” this genetic text, making billions of copies. This allows them to be detected reliably.

Because this test can leverage tiny amounts of viral genetic material, it is considered the gold standard for detection. It can spot infection at earlier stages than other tests, including rapid antigenic tests.

These antigenic tests:

• are faster that the PCR tests (fifteen to twenty minutes, against a result for the next day for the RT-PCR);

• can be done at home, instead of standing in line and waiting for a swab, which then needs to be analyzed in a lab.

But they are less sensitive than a PCR test because there is no amplification process.

What efficiency?

While both tests are more likely to correctly detect an infection when a person’s viral load is high, PCR tests are more sensitive than rapid antigen tests.

A australian study comparing the sensitivity (i.e. a correct diagnosis of SARS-CoV-2 infection when you have it) of a type of rapid antigen test versus a PCR test showed that 77% of the antigen test results positive corresponded to the results of the PCR test.

This proportion reached 100% when people were tested within a week of symptom onset.

The Therapeutic Goods Administration provides a list of approved rapid antigenic tests, the results of which agree with the PCR test in 80-95% of cases, provided the test is performed within one week of symptom onset. Some of these tests are classified as having very high sensitivity, with 95% agreement with PCR tests. [La liste de tous les tests et autotests dont les performances correspondent aux prérequis définis par la Haute autorité de santé en France est consultable ici, ndlr]

What test to do when?

Take a test RT-PCR in the laboratory if:

• You have symptoms of Covid,

• You have known exposure to someone with Covid,

• You have performed a rapid antigen test and obtained a positive result: confirmation by PCR is required,

• Your health service asks you to come out of quarantine or isolation,

• You are required to contact a health service to obtain permission to go to a given location.

PCR is the test of choice in these situations because it is more precise in diagnosing an infection.

Consider a rapid antigenic self-test if:

• You plan to visit a sensitive site (for example, an elderly care facility),

• You plan to come into contact with a person at high risk for Covid (for example, an elderly person or a person on immunosuppressive treatment), and you want to protect them,

• You have symptoms of Covid but cannot go to a PCR testing center,

• You are going to an event where a lot of people are going to mingle, especially if it is taking place indoors, where the risk of transmission is considerably higher,

• You want to quickly check if you have a SARS-CoV-2 infection,

• You are part of a regular Covid surveillance program (some workplaces require this, especially in situations where the population is not fully vaccinated).

The rapid antigen test is considered a screening tool among others. In other words, it can indicate that you might be infected, but a PCR test is needed to confirm the result.

While a negative rapid antigen test result does not guarantee that you are not infected, it still offers your contacts better protection than not testing. [Si vous avez un résultat positif, il vous faut le confirmer immédiatement par un test PCR et vous isoler, ndlr]

How often should I do rapid antigenic tests?

It depends on what you are taking the test for. If you are part of a monitoring program, take the test when asked.

If you have no symptoms, getting tested two to three times over a week can help improve the sensitivity of the test because the viral load varies. The sensitivity of the test will be highest when the viral load is at its highest. [«Le caractère itératif –c’est-à-dire répété– de l’utilisation des autotests permet d’augmenter leur fiabilité par la répétition régulière des prélèvements», souligne le ministère de la Santé français, ndlr]

What about Omicron detection?

This new highly mutated variant appears to be still detected by PCR tests and rapid antigenic tests.

Normally, a PCR test will show whether or not you have a SARS-CoV-2 infection, but not which variant you have contracted. This is not his goal. Sequencing of the viral genome is necessary to find out.

However, some PCR tests look for a specific genetic sequence that is missing in the Omicron variant (called the S gene target defect). These particular PCR tests can therefore not only detect an infection, but also give a clue that it is probably Omicron.

This article is republished from The Conversation under a Creative Commons license. Read theoriginal article.