PCR’s are incredibly sensitive, so you want to be sure you can see the cycle count. PCR doesn’t check for the virus, It checks for viral RNA fragments, which can be shed for months after you are contagious. That RATs actually are looking for certain surface proteins, binding to them, then that is what is deposited on the positive line. The BinaxNOW tests will flip positive if there’s the N (Neuclocapsid) protein in your nose, which is a certain sign COVID is actively reproducing in your airways.