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since everyone in LA just learned about false negatives ... spoiler alert: they exist for every test!
whatever COVID test you use, it's measuring the amount of virus in whatever place was swabbed -- your cheek, lower nose, upper nose, throat etc -- but sometimes there isn't enough virus at that very moment to show up on a test, even though you are infected
as the virus replicates, it eventually will reach a level where it shows up on a test. but that doesn't mean that up until that point that you haven't been infecting other people, even if you have negative test results in hand
there are also sometimes false negatives because the test isn't swabbing where the virus is most concentrated in your body. your upper nose -- the infamous brain poke -- is probably the best, but a lot of our tests are swabbing other, less painful places out of convenience
typically, these PCR tests are super accurate, but this lag time between infection and testing positive as well as the uncertainty about where to swab has created the opportunity for false negatives -- and often a false sense of a security
there aren't clear numbers on the false negative rate. some studies suggest 20%. some 30%.
recommended reading: "In one study from China, between 11 and 40 percent of tests conducted on 213 hospitalized, COVID-positive patients ... came back negative." https://fivethirtyeight.com/features/take-negative-covid-19-tests-seriously-but-not-literally/
recommended reading: "In one study from China, between 11 and 40 percent of tests conducted on 213 hospitalized, COVID-positive patients ... came back negative." https://fivethirtyeight.com/features/take-negative-covid-19-tests-seriously-but-not-literally/
this is for PCR tests, which tend to be the most accurate test. antigen tests -- the rapid ones you get at urgent care, pregnancy test-style -- are generally much less accurate and have an even higher rate of false negatives, no matter where you're swabbed 
the benefit of rapid antigen tests is that because they require less equipment, you can test people multiple times and test a lot more people, increasing the chance that you'll catch more illnesses overall. it's a good strategy at a population-level, not so much for individuals
there's a school of thought that a much higher of volume of less accurate testing is better than a limited amount of very accurate testing. and LA Mayor Garcetti seems to be saying this too -- that poor asymptomatic testing is better than none at all https://twitter.com/skarlamangla/status/1347358558380118016?s=20
the problem lies in the fact that people want to use their test results to regain a sense of normalcy in their lives, as a passport to doing something that would be unsafe if one of them had COVID. but not taking precautions and then relying on a negative test result is illogical
it definitely doesn't work given the FDA's warnings about the Curative tests, but it also doesn't work perfectly for other tests. LA officials have been saying forever that you can't use your test results to abandon pandemic precautions, and that's still true!!
the best way to know you're virus-free would be to quarantine for two full weeks -- no grocery shopping, no contact with any humans at all, indoors or outdoors. after that, if a test offers you peace of mind, fine, but your protection is coming from the quarantine, not the test
