qPCR doesn't use a gel, and you get specificity by using primers to the viral dna (which are what tell you the part you start and end amplification at), not res

qPCR doesn't use a gel, and you get specificity by using primers to the viral dna (which are what tell you the part you start and end amplification at), not restriction enzymes, but yes conceptually similar sort of idea. You set up reaction conditions so that at the end of the run, if there was any viral dna at the start, there's now a lot more for you to detect, while if there wasn't, you just see the background since nothing else should have been replicated. I believe the reagent issue with the original run of tests from the CDC had to do with the controls you set up to show that nothing amplifies if you don't have any virus (since you can't just run one tube and see if there's a signal, you've got to run a few for reundancy and run samples where you should and should not see signals to make sure you are seeing signals where you should, and the reaction worked, so your signal from the patient samples are accurate)