A 12-lead ecg can diagnose whether you have a-fib at the time of the ecg. The question for the doctor is whether you may have “paroxysmal” ie intermittent a-fib. So what if the Apple Watch said ‘a-fib’, but the 12-lead is negative. Most doctors are probably going to end up ordering a holter- monitor, to monitor the patient’s rhythm over 24-72 hours. Some proportion of those patients may have very short runs of a-fib, and would likely have remained blissfully ignorant (and well) without the Apple Watch. Instead, they may well end up anti-coagulated, because if that patient is not anticoagulated and gets a stroke later ( which may or may not be due to a-fib), the doctor is going to get sued. Whereas if the doc starts the patient on anticoagulants, and tells the patient that, “ there’s a risk of bleeding, but...”, that is much less likely to end up in a lawsuit.