The SARS-CoV-2 IgG assay is intended for qualitative and semi-quantitative detection of IgG antibodies to the S1 receptor binding domain (RBD) of the SARS-CoV-2 spike protein. The measurement of IgG levels can provide insight to an individual's adaptive immune response to a SARS-CoV-2 infection or vaccination. Although the assay is designed to assess the level of an individual's immune response, studies are still needed to determine the index level threshold that confers protective immunity as well as how long the adaptive immune response may last post-infection or via vaccination.
What is an antibody?
An antibody (also known as an immunoglobulin) is part of our body’s response to a foreign molecule or pathogen (also known as an antigen) such as a virus or bacterium. The immune response is valuable as it helps to fight off infection. Protective antibodies can in some cases provide long-term immunity, so we do not become re-infected with the same viruses or bacteria. As such, antibodies are vital for our health.
Yet, we don’t always develop antibodies—or the right or sufficient antibodies—to fight off all infectious diseases. For example, people infected with HIV or hepatitis C virus do not usually develop protective antibodies.
There are five primary classes of antibodies. Of those, IgG, IgM, and IgA are often used as part of infectious disease serology testing.
- IgG antibodies are found in all body fluids. They account for 75 to 80 % of all antibodies. IgG antibodies are very important in fighting bacterial and viral infections and when present indicate that someone has previously been exposed to a particular antigen and has developed an immune response. In general, these antibodies usually appear 10-14 days after symptom onset.
- IgM antibodies are found in blood and lymph fluid and are the first type of antibody made in response to an infection, usually detectable within 3-7 days from the start of the infection.
- IgA antibodies play a crucial role in the immune function of mucous membranes, such as the respiratory epithelium, but is also found in the blood. These antibodies also appear early in the infection process (3-7 days).
Antibody testing provides insight into an individual’s immune response after exposure to the SARS-CoV-2 virus but is not intended to diagnose an active infection. For those vaccinated, the measurement of IgG levels can be used to assess whether an individual has mounted an immune response to the COVID-19 vaccine. Although this test is designed to provide information on one’s immune response, studies are still needed to determine the level that indicates protective immunity as well as how long the immune response may last.
Important note:
This test should not be used to diagnose or exclude an acute SARS-CoV-2 infection. If acute infection is suspected, direct testing by molecular methods for SARS-CoV-2 is necessary. False positive results for the test may occur due to cross-reactivity from pre-existing antibodies or other possible causes.
SARS-CoV-2 Antibody (IgG), Spike, Semi-Quantitative
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