If someone is testing themselves to make sure they will not spread it, they expect a level of certainity higher than the subjective "feel sick" - say you're vis

If someone is testing themselves to make sure they will not spread it, they expect a level of certainity higher than the subjective "feel sick" - say you're visiting grandma who has asthma, and you take the bus everyday. I'd want to be as sure as I can be that I can't share it. This isn't saying everyone needs to be testing at all times and tests should be 100% effective - some situation require testing, and it'd be nice if it caught asymptomatic covid as well.