COVID-19 tests look for the presence of one of three molecules: viral genetic material (nucleic acid), viral proteins, or human proteins called antibodies. As the first two are looking for parts of the virus, they allow us to detect if an individual is currently infected. The third can tell us if someone has previously been infected – antibodies are a sign that the immune system has responded to a specific disease.
These develop after your body has dealt with the exposure/ infection. Antibody tests are taken by blood test. They do not show current infection, they show previous infection. We are still waiting on more evidence to help identify how long antibodies for COVID-19 last for. Whilst some companies offer an antibody test this will only tell you if someone has been previously infected at some point. A positive antibody test DOES NOT tell you an employee is not infectious (1).
To complicate matters because PCR tests can amplify minute viral fragments, one can have a positive PCR test many weeks after being infected. The person may not be infectious at the time despite the positive PCR test (2).