I wasn't implying that 80% of all tests were false positives. I'm talking about P(You don't have coronavirus | You test positive). 80% is far beyond what would be normal for these types of medical diagnostic tests. I would expect something like 1% or less for a really good test, and around 10% for a bad one, but I don't know if the test the Chinese were using is really bad in some way, which is why I'm asking for clarification there.

80%...in asymptomatic cases. So if most of the people who get tested DID show symptoms, the false positive rate generally could be far worse.

But, it would suggest downsides to more general testing.

But if your diagnostic criteria is showing symptoms later, then you are ejecting the entire population of carriers who might be, say, teeming with the virus but showing zero symptoms, for perhaps a genetic or "dumb luck" reason.