Thread by @michaelmina_lab

To: Scientists, Doctors and anyone writing OpEds or Making Policy Decisions about tests

PLEASE NOTE:

The number of False Positive tests does *NOT* go up when prevalence of #COVID19 goes down.

The number of False Positives is the SAME regardless of high vs low prevalence.

1/

This idea keeps getting thrown around in OpEds and by decision makers - worried false positives sky rocket once disease drops.

But if you have 10 False Pos per 100,000 tests when incidence is high, you’ll have ~10 False Positives per 100,000 tests when incidence is low.

2/

What changes is not the number of false pos results, but the fraction of all pos that may be false

Of course even if false pos rate is 0.0001 (about that of newer rapid Ag tests), if there is zero Covid, then sure, 100% of positives will be false. But it’s still only 1 in 10,000

So you know at the outset the burden of false pos you may face based on the test.

And if 1 in 1,000 or 1 in 10,000 is too many, then simple solution - confirm w another rapid test! If the first was False Pos, you’ll know in ten minutes.

A small burden compared to the benefits

Here’s another tweet about it by @dr_barrett

https://twitter.com/dr_barrett/status/1387957925264732163

And just to be clear - i'm talking about community testing for public health in this pandemic

Of course if prevalence of disease is 100%, there would be no false positives. But that's not COVID... Prevalence in population is usually <<1%