Currently there are two tests in use around the world*, one evaluates whether you are currently infected and the other evaluates whether you have been. The most widely used for diagnosis of COVID -19 infection is the PCR lab test. It is looking to see if you have DNA or RNA specific to the Corona virus. The newer rapid antibody test is looking for evidence that you HAD COVID-19 and have made antibodies (IgG/IgM) to the virus. The PCR test is used to provide the correct treatment and to isolate infected people. The rapid antibody test (serology) can assess if those without symptoms have an active or recent infection. The importance of this identifier is two-fold it can confirm the need for isolation to avoid spreading the infection and it may indicate a period of immunity from re-occurrence.
The serology test will pick up both IgM and IgG antibodies to the virus. IgM is the first to respond but the trick is timing the taking of the test to coincide with enough of the antibody to make the test positive. The optimal is between 2-14 days up to 21 days from initial exposure. The IgG test will show if you have had the virus and now have some antibodies to the virus that may provide some protection against it.
Both tests have their shortcomings; for example the PCR test relies on getting enough of the virus on the swab to give a positive result and not a possible false negative. The challenge with the serology test lies in the timing of the test. If the test is given too soon there may not be enough antibodies in the patient to turn the test positive. For now all indications seem to be that assessing symptoms along with a combination of both tests PCR then IgG/IgM would offer the best information as to the state of infection.
*Health Canada is in the process of evaluating COVID-19 rapid IgG/IgM antibody test applications in an effort to expedite Class III device licenses- similar to the FDA issuing EUA (Emergency Use Authorization).