Antigen tests, on their box, usually say that if you test positive (there's enough virus to turn the strip, it's most likely correct. But if you test negative,

Antigen tests, on their box, usually say that if you test positive (there's enough virus to turn the strip, it's most likely correct. But if you test negative, it doesn't guarantee it's actually negative -- since there are so many factors that could have led to that negative result (insufficient virus, early in the infection, incorrect swabbing, reagent expired). This is explicitly stated on the box, so it's not "useless". During the early days of COVID, the antigen test is also a cheaper, faster "statistical" filter since full PCR tests are more expensive and less scalable. It's actually doing what it's meant to do.