Covid-19 Diagnosis Test
Currently the only FDA-approved test to diagnose a COVID-19 disease in the United States uses PCR test technology to detect COVID-19 nucleic acids. It is a sophisticated test method which uses a nasal or throat airway sample collection. It is very sensitive and specific and can diagnose an early infection to COVID-19 virus. However, if you are not collecting a test sample during the time that the virus is replicating, 60% of the positive cases will be missed.
Expanding Testing
With a need to understand the larger picture of COVID-19 exposure, the FDA has implemented an Emergency Use Authorization (EUA) which allows CLIA certified labs (who meet a certain criteria) to submit to FDA and continue to market their tests pending full FDA approval.
Antibody Testing
The latest of these EUA approved tests are antibody tests. Antibody test results cannot diagnose COVID-19 disease, but may prove useful to determine a prior exposure that was otherwise asymptomatic, missed or not tested for when active.
What tests are available?
There are three COVID-19 immunoglobulin (Ig) or antibody tests available – each will test for IgA, IgM, and/or IgG antibodies. The majority of labs are using IgM/IgG or IgG only tests. For this reason, we will focus on these two tests. Additional testing is also being developed using other antigen-antibody responses.  
Antibodies are produced in response to an antigen associated with the COVID-19 virus. Most antigens are proteins and current tests will look at an antibody response to either the COVID-19 Spike (S) protein, Membrane (M) protein or the NucleoProtein (N) protein.
Each of these proteins also has a different level of specificity and sensitivity to the COVID-19 virus. Ranking in order of most to least sensitivity; the most sensitive has the strongest response to IgG :
  • N protein is the most sensitive and greatest response
  • M protein
  • S protein produce the weakest responses
You also want the lab’s testing methods to be as specific as possible to the COVID-19 virus. This means the antibody test is less likely to cross-react with another non-COVID-19 coronavirus or influenza virus.