Covid-19: when to take a PCR or an antigenic test?

Published by Elodie D. · Published on 12 November 2020 at 12h25 · Updated on 12 November 2020 at 17h16
To control the Covid-19 epidemic, France runs over 2 million PCR tests per week, and antigenic tests for a few days, with a quicker result. When and where to take these? How do they work and why is one of them quicker as the other requires 24 to 48 hours to get results? Here is a guide to help you out.

Olivier Véran recalled it on Twitter: “tests are a precious weapon to fight against Covid”. And as the Health Minister was pleased to exceed a million PCR tests in early September, France counts on records in Europe with over two million PCR tests in a week – according to Santé publique France’s epidemiologic report released on Thursday November 5.

But laboratories are overwhelmed and the government noticed the system was no longer holding because of the waiting time – first the waiting time to get an appointment, meaning the virus could have disappeared from one’s body; and the waiting time between the test and the result – was having French stay at home as long as results were not in.

Faced with this situation, a PCR test-prioritization system has been set up as early as September for people with symptoms, contact cases and the medical staff. And French recently started to be able to take a quick “antigenic” test in pharmacies in their companies.

Alright, but what test to take? A PCR test or an antigenic test?

PCR test, “the reference test to detect the virus
The PCR test is the reference test according to the Haute Autorité de Santé (HAS – High Health Authority). It consists in spotting the SARS-COV-2 RNA – naming the coronavirus originating Covid-19. It requires the RNA present in the sample be purified by adding different solvents, which takes between one and two hours. Then, the RNA has to be transformed into DNA with a reverse transcriptase enzyme (RT) and this DNA is amplified by a polymerase chain reaction (PCR) by heating it up to 95°C several times. A long process that has to be carried out in a laboratory.

PCR test is to be taken if:

  • You are sick with suggestive symptoms such as cough, fever, runny nose or even loss of taste or smell;
  • You are contact-case, the earlier you can if you live with someone positive, or 7 days after your last contact with the patient if they do not live with you;
  • You have to undergo surgery or any surgical act;
  • You have to fly to countries outside the European Union.

Antigenic test, quicker but less reliable
Antigenic tests are simpler because they rely on the detection of proteins produced by the Sars-CoV-2 virus called “antigens”. It only takes 15 minutes for the reactor to validate or not the presence of the proteins in the sample. Unfortunately, this test is only performed in symptomatic patient, upon their request. It can be performed in asymptomatic case as part of collective screening campaigns, but cannot be made in people aged 65+ or people at risk (suffering from chronic disease, obesity, and so on).

It can be taken if:

  • You are sick with suggestive symptoms and no laboratory can have you take a PCR test within 48 hours, your GP can ask you to take an antigenic test;
  • You are going to your company and a screening campaign is going on. You do not have to take the test;
  • You are back in France, directly at the airport.

But if you have been in touch with someone called “contact case” by France’s health insurance, do not take a test before knowing if that very person has tested positive or note. Please comply with the health guidelines in the meantime. If the test is positive, you will receive a text message from France’s social security system. When clicking on the link, you will get a subscription for a PCR test, a subscription for free masks and a period of sick leave if you cannot work from home.

Practical information

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