Recently the UK government tried testing everybody in Liverpool for Covid-19. Some of this testing was done using fast turnaround rapid antigen tests.

An interesting discussion from Andrew Gelman: according to some people, mass antigen testing is obviously a good thing. If, however, it is so clearly good, why isn't it being done more widely already?

The answer hinges on small differences in sensitivity and specificity that will massively effect the number of false positives and false negatives. All of the science here is new and rapidly evolving and even the reference test (PCR) has a fairly low sensitivity, depending on which paper you read.

Gelman concludes:

"So, again, I’m not sure what to think. I’d think that even a crappy test if applied widely enough would be better than the current setting in which people use more accurate tests but then have to wait many days for the results. Especially if the alternative is some mix of lots of people not going to work and to school and other people, who do have to go to work, being at risk."