What is the difference between PCR Tests and Serological Tests or Rapid Tests?

• “Test, test, test” is the key to controlling the spread of SARS-CoV-2 and its clinical manifestation, COVID-19, according to the World Health Organization.
• However, several months after the first coronavirus infection was reported in China, there is insufficient access to appropriate diagnostic tests worldwide and confusion among healthcare professionals and the public over the prioritization of testing and The interpretation of the results is so far present. (1)

The question that arises is what is the difference between PCR Tests and Serological Tests :

1 - PCR TESTS

• The reference test for screening for SARS-CoV-2 infection is done by RT-PCR (polymerase chain reaction) which highlights viral RNA in a nasopharyngeal sample obtained by swab.
• This test is very reliable provided it is performed correctly.
• Carrying out this examination requires equipment, reagents and significant expertise and is often carried out in reference centers and is not generalized to all laboratories, particularly in countries with limited resources.
• Delays in transporting samples to laboratories mean that the overall completion time for these tests often exceeds 48 hours.

2 - SEROLOGICAL TESTS

• Following an immunizing infection, patients produce antibodies against antigens (often proteins) specific to the pathogen. The first to appear (less than a week) are IgM which persists for a few weeks or months, then IgG which appears very few days later but which will persist for years.
• Serological tests are primarily used to determine whether a person has previously been in contact with SARS-CoV-2.
• Specific IgM and IgG antibodies begin to be detectable within 4-5 days of infection :
  • 70% of IgM positive on days 8-14.
  • 90% of total antibody tests positive on day 11-24.
  • IgG reactivity would reach more than 98% after several weeks, but the duration of this response is not yet known. (1, 2)
• The principle of serological tests carried out on a blood sample, essentially of the ELISA (enzyme-linked immunoassay) type, is based on antigen-antibody recognition. Their use in practice is quite simple: the test includes viral antigens which are brought into contact with the patient's blood. In the presence of antibodies, the antigen-antibody reaction is visualized by coloring.
• Quickly after the start of the pandemic, rapid unit tests were developed: they only require two small drops of blood and are therefore much simpler to carry out and without risk of contamination for health personnel, as is the case. case for PCR tests. However, this test format is generally a little less sensitive than traditional tests carried out in medical analysis laboratories. (1)
• In France, the National Academy of Medicine recommends that individual serological tests be used as a priority in people at high risk of severe disease and people suffering from chronic illnesses, as well as in those exercising exposed professions, in particular medical personnel and nurse, etc. (3)

In short, new tests must be properly validated before use, because “an unreliable test is worse than no test.”

References :

• 1 - BEECHING, Nick J., FLETCHER, Tom E., and BEADSWORTH, Mike BJ. Covid-19: testing times. 2020.

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• 2 - Kelvin Kai-Wang, TSANG, Owen Tak-Yin, LEUNG, Wai-Shing, et al. Temporal profiles of viral load in posterior oropharyngeal saliva samples and serum antibody responses during infection by SARS-CoV-2: an observational cohort study. The Lancet Infectious Diseases, 2020.

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• 3 - HOPPENOT, ISABELLE, 2020, COVID-19: PCR tests and serological tests are complementary. VIDAL [online]. 2020. [Accessed April 16, 2020]. Available from: https://www.vidal.fr/actualites/24747/covid_19_tests_pcr_et_tests_serologique_sont_complementaires/

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