Two types of testing exist for SARS-CoV-2, the coronavirus which causes COVID-19. RT-PCR tests (or nucleic acid tests) are useful in diagnosing an active infection. Serology tests are useful in diagnosing previous infections. For those who may have been at risk, or who think they may have contracted SARS-CoV-2 previously, a serology test may be able to provide confirmation of recent or prior infection.
SARS-CoV-2 (COVID-19) Tests and Antibodies 101
Types of SARS-CoV2 Tests
Active Infection | (Asymptomatic or Symptomatic)
Test-Type: Nucleic Acid/PCR Test (Nasopharyngeal Swab)
How it Works: Nucleic Acid/PCR Tests look for genetic material (RNA) that is specific to the virus. These tests identify unique regions of genetic material that cannot normally be found in humans.
When it Works: This test is only effective if there is an active viral infection. After the infection is over, the test will return a negative result.
Test-Type: Serology/Antibody Test (Blood or Saliva Sample)
Serology testing for SARS-CoV-2 is at an increased demand in order to better quantify the number of cases of COVID-19. These cases includes those who may have been asymptomatic or who were unable to receive a confirmatory RT-PCR screen while sick. Serology testing works by identifying if a person has developed antibodies against SARS-CoV-2. Antibodies, part of a normal immune response towards pathogens like the coronavirus, circulate in blood and saliva for an extended period of time after infection. These antibodies may help prevent reinfection or may result in a less serious reinfection.
How it Works: As the body fights infections it produces antibodies which can help prevent future re-infections. Serology/Antibody tests look for these antibodies. Your body generates five types of antibodies with different purposes.
When it Works: This depends on the antibody. IgM antibodies can be detected earliest, but are often imperfect markers as they can show frequent false-positives. IgA is the next earliest and is best detected in saliva samples. IgA is a good marker of response and is intended to provide protection from re-infection by blocking virus in the lungs before it enters cells. IgG follows shortly after IgA and is best detected in blood samples. IgG is another good marker of response and is intended to fight virus that makes it past IgA antibodies.
*Indicates that these can be used in COVID-19 testing
The IgA antibody class comprises up to 15% of antibodies in an individual. Unlike IgG, while a small percentage of IgA antibodies can be found in serum, the majority are found in mucosal secretions – including in sweat, saliva, tears, the gastrointestinal tract, and the respiratory epithelium. IgA antibodies are produced in the lamina propia and are transported across the epithelium before being secreted into the mucosal layer . IgA antibodies function primarily through neutralization of the pathogen, preventing the pathogen from interacting with host cell receptors and therefore inhibiting infection . As a result, the IgA antibody class provides one of the first lines of directed defense for preventing re-infection by a known pathogen.
Although IgA responses have not been widely studied in the COVID-19 pandemic, our studies indicate that subjects appear to be mounting a robust salivary IgA response to the SARS-CoV-2 virus. IgA can be most conveniently measured in saliva. We have observed the presence of virus-specific IgA antibodies in subjects more than 60 days past confirmed infection, with no indication that the IgA response was decreasing.
Since the oro-pharyngeal mucosa is frequently the first site of exposure to the SARV-CoV-2 virus, the development of a salivary IgA response may provide an early and sensitive indicator of viral exposure.
 C.S. Kaetzel, J.K. Robinson, K.R. Chintalacharuvu, J.P. Vaerman, M.E. Lamm, The polymeric immunoglobulin receptor (secretory component) mediates transport of immune complexes across epithelial cells: a local defense function for IgA, Proceedings of the National Academy of Sciences. 88 (1991) 8796–8800. doi:10.1073/pnas.88.19.8796.
 N.J. Mantis, N. Rol, B. Corthésy, Secretory IgA’s complex roles in immunity and mucosal homeostasis in the gut, Mucosal Immunol. 4 (2011) 603–611. doi:10.1038/mi.2011.41.