Whenever you want to be sure about a possible infection with the coronavirus. Especially if you are experiencing flu-like symptoms. In addition, testing is strongly recommended before meeting family and friends, visiting hospitals and nursing homes, or returning to work.
If infection has occurred within 72 hours prior to testing, the viral load may be too low to detect the virus when the test is performed. In this case, a negative test result does not represent a definite exclusion of Covid 19 infection, but rather a health status at the time the test was performed. This situation is the same for both rapid antigen tests and PCR tests.
Therefore, it makes sense to wait 72 hours after a known risk contact before performing the test.