you have been advised to do so by a health professional PCR tests are done by healthcare workers who take a swab from the back of your nose and throat.      

Why do pcr results take so long –

  To account for this, if there are doubts about the accuracy of a particular test result, your doctor may ask you to be re-tested. You need to wear a mask at the clinic and remove it to provide a sample. I got an alert notifying me they were ready the next day. It has had to prioritize tests from hospitalized or symptomatic people and send other specimens to private labs, a process that can stretch the wait time for results to up to seven days.