No, accuracy doesn't tell us anything about usefulness. If the test is cheap and it brings in people for a better test and thus saves lives it is useful. The only real danger is if people decide that since the watch says there is nothing wrong so they can ignore other symptoms.
It is something like chest pain: most of the time chest pain is not a symptom of a heart attack, but it is best indicator we have so you go to the emergency room when you have chest pain. Doctors there can evaluate your situation.
It is something like chest pain: most of the time chest pain is not a symptom of a heart attack, but it is best indicator we have so you go to the emergency room when you have chest pain. Doctors there can evaluate your situation.