I have COVID symptoms. What kind of test should I take?
At-home rapid antigen tests such as BinaxNow or QuickVue measure the viral proteins in SARS-Cov-2 that increase as the virus replicates in the body. Because of that, rapid tests only pick up an infection when the virus is making more of itself. Luckily, that period of time coincides pretty closely with the time when a person is most contagious.
“The rapid tests really do pick up when you are most infectious to other people,” said John Wherry, director of the Institute for Immunology at the University of Pennsylvania Perelman School of Medicine.
On the other hand, PCR (polymerase chain reaction) tests — which take longer to process in the lab, sometimes many days — are looking for any trace of the virus itself. That means they might be able to detect infection earlier on in the illness. But it also means PCR’s can continue to detect trace amounts long after a person has stopped being contagious.
Source : https://whyy.org/articles/how-should-we-be-using-at-home-rapid-tests-for-omicron/180