As previously noted, these types of tests detect antibodies in the immune system produced in response to the COVID-19 infection. Antibody tests are not currently used to diagnose an active COVID-19 infection. Antibodies typically take several days or weeks to develop after a COVID-19 infection and may remain in the blood for several weeks or more after recovery. Blood samples are typically used for antibody tests.
Antibody (serologic) tests for the determination of SARS-CoV-2 exposure typically rely on the detection of either IgM or IgG antibodies that are specific for various viral antigens, such as the spike protein and its subunits as well as the nucleocapsid protein. Although serological testing is mainly used to analyze blood serum or plasma, it has also been operationally expanded to include testing of saliva, sputum, and other biological fluids for the presence of immunoglobulin M (IgM) and immunoglobulin G (IgG) antibodies. Antibody testing plays an important role in epidemiology and vaccine development since it can provide an assessment of both short-term (days to weeks) and long-term (years or permanence) for antibody response as well as antibody abundance and diversity.
IgM first becomes detectable in serum after a few days, lasts a couple of weeks upon infection, and is followed by a switch to IgG. Thus, IgM can be an indicator of early-stage infection, while IgG can be an indicator of current or prior infection. In addition, IgG can be used to suggest the presence of post-infection immunity.
However, the CDC does not recommend using antibody testing to diagnose COVID-19 infections. Depending on the time when someone was infected and the timing of the test, the test might not detect antibodies in someone with a current infection. Because antibody tests can have different targets on the virus, specific tests might be needed to assess for antibodies originating from past infection versus those from vaccination.
