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[quote=Anonymous][quote=Anonymous]I have not heard this much grumbling about false positives outside of a pro-Trump forum. Are false positives really a concern? [/quote]

That noted Trumper, Monica Gandhi:

https://www.washingtonpost.com/outlook/2021/04/19/schools-covid-testing-cost/

Written in April, 2021:

But there are downsides to systematic testing that have been insufficiently considered, including costs, lost learning time, logistics and stress for those subjected to such a regime. False-positive results — which say you are infected when you aren’t — pose particular problems. Overall, this kind of widespread testing fails cost-benefit analysis: It will drain already insufficient public school resources while doing little to improve safety. And with nearly 80 percent of teachers and school staffers vaccinated with their first dose, the argument for testing grows even weaker.

First, classrooms have thankfully been found — in studies examining schools in multiple states — to be places of limited disease transmission, compared with communities at large. The rate of transmission within schools from individuals who test positive has been estimated to be on the order of 0.5 percent to 0.7 percent (and this includes people exhibiting symptoms).

A rate that low implies that a testing regimen would need to identify roughly 200 infected people to prevent one person from transmitting the disease in school. It would take an awful lot of tests to achieve those numbers. In New York City, where more than 234,000 asymptomatic students and staff members across approximately 1,600 schools were tested last fall, the overall rate of positive tests was only 0.4 percent. That suggests that — even during a time of high community spread — about 40,000 tests among asymptomatic individuals would need to be performed to prevent one in-school transmission.

And how accurate are these tests? Rapid antigen and saliva PCR tests, which are frequently used in schools, can have a false positive rate of 1 or 2 percent. That may sound low, but statisticians know that, when testing in a setting of low prevalence of disease, even a single-digit false-positive rate can be extremely problematic.

The current prevalence rate for the coronavirus in the United States is roughly 15 cases per 10,000 people per week. (Prevalence in schools tends to be similar to, or lower than, that in the surrounding community.) If you give 10,000 people a test that produces false positives 2 percent of the time, that means you might get 215 positives: 15 true positives and 200 false positives. In other words, [b]more than 90 percent of the positive test results will be incorrect.[/b][/quote]

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