Serology test covid-19 : How, When and Why?

✋ Serology testing cannot tell you if you have an active infection or to determine if you are immune or protected against COVID-19

COVID-19 (SARS-CoV-2) serology tests look for antibodies in a sample to determine if a person has ever had an infection with the virus that causes COVID-19. These tests are not used to diagnose active COVID-19, as they do not detect the virus itself, but evidence of the body's fight against the virus.

⚡ What are antibodies?

If you have been exposed to the virus that causes COVID-19, your body usually produces antibodies as part of the immune response to the virus.

Antibodies are proteins made by the immune system to fight infections and can help prevent future occurrences of these same infections. Antibodies can take days or weeks to develop in the body after exposure to a SARS-CoV-2 (COVID-19) infection, and it is not known how long they stay in the blood.

⚡ Type of sample for serology testing

This type of test uses a sample of blood, taken from a venous blood, to detect these antibodies. These are found in serum, the clear fluid that remains when proteins and clotting cells have been removed from the blood.

Or, depending on the type of test, a drop of blood from your fingertip, saliva, or nasal swab fluids can be used.

⚡ In which cases do I take a Covid-19 serology testing ?

According to HAS, a COVID-19 serology test remains indicated in four situations :

• The initial diagnosis of severe symptomatic hospitalized patients, in the event of a clinical picture or CT scan suggestive of SARS-CoV-2 infection and negative RT-PCR test
• The catch-up diagnosis of severe symptomatic patients hospitalized but unable to undergo an RT-PCR test for seven days
• The initial diagnosis of symptomatic patients with no sign of severity followed in town in the event of a clinical picture suggestive of SARS-CoV-2 infection and negative RT-PCR test
• The salvage diagnosis of patients with symptoms suggestive of SARS-CoV-2 infection (including prolonged symptoms of Covid-19) without signs of severity for whom an initial laboratory diagnosis has not been made.

⚡ What are the types of serology tests?

🔵 The tests can be broadly classified into 2 major groups :

➀ Detection of binding antibodies : These tests use purified proteins from SARS-CoV-2 (a non-viable virus). With specific reagents, individual antibody types, like IgG and IgM, can be differentiated.

➁ Neutralizing Antibody Tests : Used to determine the functional ability of antibodies to prevent SARS-CoV-2 infection in vitro. These tests monitor the inhibition of viral growth in cell culture when incubated with serum or plasma.

Example : Virus neutralization tests (VNT), Pseudovirus neutralization tests (pVNT), Competitive neutralization tests (cVNT)

⚡ Performance of serology tests

The performance of these tests is described by their "sensitivity" or ability to identify those that have antibodies against SARS-CoV-2 (true positive rate) and their "specificity" or ability to identify those that do not. no antibodies against SARS-CoV-2 (true negative rate).

You can consult, in the following links, the tests evaluated by the National Reference Center (CNR France) and U.S. FOOD AND DRUG..