Covid-19 serology testing plays a critical role in fighting and containing coronavirus. In order to help our customers understand their options, we’ve outlined important things you need to know about Covid-19 serology testing below.
What are serology tests and why are they used for Covid-19?
Covid-19 serology tests look specifically for the presence of antibodies. Some tests detect immunoglobulin G (IgG), while other tests detect all three main immunoglobulins, IgA, IgM and IgG. These antibodies are typically produced within 14 days of an initial infection and the IgG antibodies are then retained by the body to help prevent re-infection. The majority of immunoglobulins in the blood (70 to 80 percent) are IgG.
According to the US Food and Drug Administration (FDA), a patient who tests positive for the presence of IgG may have “at least some immunity to the coronavirus.” Unfortunately, as of this writing, the medical community has not reached consensus on either how long Covid-19 immunity lasts or whether it truly means an individual will be protected from re-infection, versus being susceptible to re-infection but perhaps of a milder variety.
What is an immunoglobulin?
Immunoglobulins are proteins produced by plasma cells in response to bacteria and viruses the body deems harmful or foreign. The first time a patient is infected or exposed, the immune system produces immunoglobulins in order to bind to and neutralize the threat. With subsequent infections or exposures, the immune system recognizes the previously encountered threat (in this case Covid-19), which triggers rapid production of antibodies to fight re-infection.
There are five classes of immunoglobulins: M, G, A, D, and E. In typical serology testing, only the three primary classes – immunoglobulins A (IgA), M (IgM), and G (IgG) – are measured.
What are the differences between IgA, IgM and IgG tests?
Immunoglobulin A (IgA) provides protection to the mucosal areas of the body, such as respiratory and gastrointestinal tracts. These antibodies make up about 15 percent of the body’s total immunoglobulin population and can be found in blood, saliva, tears, secretions, and breast milk.
Immunoglobulin M (IgM) antibodies are produced as the body’s first response to an unfamiliar infection. These provide short-term protection before declining as immunoglobulin G (IgG) production begins.
Immunoglobulin G (IgG) makes up the majority of immunoglobulins in the blood (about 70 to 80 percent). Like IgM antibodies, IgG antibodies are produced during an initial infection before decreasing and stabilizing so that the body may retain them to use in the future. Unlike IgM antibodies, these provide long-term protection and prevent re-infection.
What do Covid-19 antibody test results mean?
A positive result suggests that a patient has likely been exposed to the virus and thus produced the antibodies necessary for an immune response to coronavirus. If positive, patients should look to confirm the infection with further clinical evaluation and testing.
While a negative result suggests that a patient does not have detectable antibodies, it is contingent on a number of factors, including how soon the test was taken after cessation of Covid-19 symptoms. It should not be used to rule out a current infection, or prior exposure to the virus.
Regardless of test results, patients should not rely exclusively on serology tests for a diagnosis of prior infection or guarantees of immunity. We recommend that all patients make decisions about treatment and the return to normal activities in accordance with their primary care physicians and with guidance from public health authorities.