Both antigen tests and PCR tests are very useful when you are testing for COVID-19, but there are key instances when one may be better suited for the situation than the other. After you have had COVID-19 for days and want to confirm you’re likely not infectious a rapid antigen test would be the best test to take to confirm this. If you have access to an antigen test, good practice would be to take the test toward the end of the isolation period to check if you still have high amounts of the virus in your system. If you do test positive you should continue to isolate, if you don’t you can stop isolating but continue to wear a mask around others until day 10 after symptoms began.
The antigen test is not as sensitive as a PCR test and is the inferior test when it comes to telling you if you are infected, but because of this it only picks up the virus at higher levels which are also probably the points when you are more infectious. Antigen tests seem to capture when people are at their most infectious and having two antigen tests two days in a row that are both negative is a good indicator that you are no longer infectious.
So, while a PCR test is best at telling if you have an infection before symptoms begin or that you have an infection when you are asymptomatic, an antigen test will be best at telling you when you are most infectious and when you should isolate during that time. Both tests are very useful for different situations and should be utilized accordingly.