[Cookingboy]

I'm not sure that's how it works when it comes to accuracy of medical screening devices.

Say if there is a device that offers 99.9% accuracy but is used to screen a disease that occurs 1 in 100,000, then applying your logic would mean out of the 100 people who were identified positive, only 1 actually was accurate, thus makes the actual result of the device 99% false.

The 85% here is likely to refer to that out of the positive results, there is a 15% false-positive rate.

Medical screening device accuracies aren't calibrated to how rare the actual disease is.

[rfatnabayeff]

It's not about rarity of the disease. The problem with those percentages is that they're commercial bragging. The 99.9% accuracy is indeed very poor accuracy in case where there is 99.999% chance that one doesn't have a disease at all. It's just measuring the length of a bacteria with carpenter's tape ruler - the tool is too poor.