This is a really good question. If a test has an NPV of 99% usually we would think that is great as it means if you have a negative result you have a 1% chance of having Covid. However, a 1% chance of having Covid is actually very high.
https://twitter.com/GenerallyUnwell/status/1340308187451371521
Case rates where I am at the moment are 200 per 100,000, so 0.2%. So a totally "random test" would give an NPV of 99.8%. How low should the rate be to "rule out"? Maybe 20 per 100,000? That would be an NPV of 99.98%. Clearly we are getting into decimal place madness with NPVs
Thats why I don't use them, and because they are going to vary between places and times. My preference is to talk about how much a negative result reduces the chances of Covid. Given the event is rare, the specificity is nearly 1, this is simple to get from the sensitivity.
In these circumstances, a test with 50% sensitivity will halve the chances of disease, a test with 35% sensitivity will reduce it by one-third, a test with 70% sensitivity will reduce it by just over two-thirds.
Does that help?
Postscript - at the moment a negative lateral flow result will never be low enough to be "safe" - I've said it in the BMJ, the Royal College of Pathologists have said it, WHO said it in September, the manufacturers of Innova say it, the CMO says it and now SAGE too have said it.
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