PCR, antigenic, serological, salivary tests… Not easy to navigate while the coronavirus epidemic continues to gain ground. Small summary point on these tests: when to do them, why, in what time frame and for what efficiency.
- The PCR and saliva tests assess whether I am positive at the time of the test.
- The antigen test is effective during the first 4 days after the onset of symptoms to know if I am positive for Covid-19.
- The serological test aims to find out if I have already been infected and if I have developed antibodies.
What tests exist? What is the timeframe for getting the results? How do I know if I have already been infected? How much does it cost ? Does it hurt ? The questions surrounding the various Covid-19 tests are numerous and the answers are often unclear. To see more clearly, let’s compare the available tests.
The detection time: the PCR test
The PCR test aims to find out if I am infected at the time I take the test. The positivity is declared 2 to 3 days before the arrival of the symptoms until 7 to 10 days later. Antigenic tests, less reliable, make it possible to know if I am infected but with a loss of sensitivity after the 4th day of symptoms.
The serological tests do not determine my positivity but indicate if I have been contaminated and if I have developed antibodies. They must be done at least 15 days after the onset of symptoms.
The fastest way to give an answer: the rapid serological test for diagnostic orientation
This is one of the most heard criticisms of testing. The delays in results can seem interminable, especially for PCR tests. Yet, on paper, it takes 24 hours to get the results of this test. But already, we are far from the 20 minutes necessary for the rapid serological test of diagnostic orientation to give its result. The antigen test requires him to wait 30 minutes before knowing if you are positive or not. For the saliva test, it takes 40 minutes. The serological test carried out in the laboratory requires a window of 4 to 6 hours before rendering its verdict.
The most painful: the PCR and antigen test
The PCR test is collected by nasopharyngeal secretions. Concretely, it is a question of pushing quite deep what looks like a long cotton swab into the nostril to secrete mucus there. This causes tingling and a more unpleasant sensation and may cause some tears. The serological test requires a blood test which can be painful for those who cannot stand needles. On the other hand, the rapid serological tests for diagnostic orientation consist of a blood test at the end of the finger, which is less restrictive. Finally, saliva tests are the most harmless.
The most effective: the PCR test
As soon as viral RNA is present in the nasopharyngeal swab, the test is positive. It’s 100%. The saliva test, which also comes to know if the tested person is infected, has a sensitivity of between 70 and 78%. For the serological test in the laboratory, the efficiency is 99.8%, in other words almost perfect. The antigenic test and the rapid serological test for diagnostic orientation have a great heterogeneity in reliability depending on the suppliers. For the antigen, the sensitivity is of the order of 66-74% for significant viral loads.
The most expensive: the saliva test
It costs between 31 and 40 euros depending on the supplier. If it is less than the PCR test, which is charged 54 euros, it is reimbursed by social security. The rapid serological diagnostic orientation test has a variable price between 15 and 20 euros, while that which is carried out in the laboratory claims to pay 12 euros and is reimbursable for symptomatic patients and some cases of asymptotics (nursing staff, in particular) . The antigenic tests have not yet been priced and should be deployed very soon in the territory. The Haute Autorité de Santé recommends its use for the diagnosis of patients who have been symptomatic for less than 4 days and only in areas where the results of the RT-PCR tests cannot be obtained in less than 48 hours.
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