[andrewseanryan]

Would I be correct with the following:

If the false positive rate is higher than the expected rate of disease in a given community, then the majority of positive tests will be false positives.

Does this relate to COVID in any way? Since the rates among affected communities seem to be growing rapidly. Would appreciate your thoughts.

[usrusr]

Looking at growth rate with false positives is a bit of a mindbender: if you limit your testing to the potential contacts of a positive (false or not), you could get a "false R0" virtual epidemic from testing alone, if and only if you test more contacts per positive than 1/false positive rate. Unfortunately, actual hospitalizations and and deaths rule out a virtual epidemic so this is not a hope to cling to.

[AnthonyMouse]

> Unfortunately, actual hospitalizations and and deaths rule out a virtual epidemic so this is not a hope to cling to.

Not necessarily. In theory all the deaths could have some other cause, i.e. some fraction of people with a different underlying fatal condition had false positive tests for this coronavirus and then died of the other condition.

That's probably not what's happening, but it's theoretically possible. (It's also probable that some of the reported deaths are that, but who knows what percentage.)

[hanche]

If the false positive rate is p and the false negative rate is q, and the infection rate is r, then you will have p·(1-r) false positives (as proportion of the tested population) and (1-q)·r true positives. Your hypothesis p>r is not enough to settle which of those two numbers is bigger.

(Edited to fix a silly mistake: The phone rang while I was posting, so I ended up being hasty.)

Edit the 2nd: Even in the simplified case q=0, you can't easily tell.