To put this into perspective, let’s use a round number of 1 million watch owners. We know that about 1 percent—or 10,000 people—will have AF, and 990,000 will not have AF. If the watch is wrong 10 percent of the time, that means nearly 100,000 people will be falsely diagnosed with AF.

Sending hundreds of thousands of wrongly diagnosed people to the doctor scares me. In addition to needless anxiety and costs, this is hazardous because while some doctors will simply reassure the patient, many other doctors will order tests. Since all medical interventions come with risks, many people will suffer harm from unnecessary tests and procedures.

“When you endeavor to make healthy people healthier, you always risk making them worse.”

Another snag in heart rhythm screening stems from a poor understanding of AF. Despite decades of research, doctors still argue about the causes of AF and its treatments.

In 2017, Harvard researchers published a paper exposing a core deficit in AF knowledge. In their review of 34 studies of hundreds of thousands of patients with AF who were nottreated with clot-blocking drugs, they found wide variation in the rate of stroke, which means we don’t really know the stroke risk of having an AF diagnosis.

Even if the Apple Watch identifies true AF, we can’t be sure the treatment will do more good than harm. While anticoagulants reduce the rate of stroke when used in patients with multiple risk factors—like high blood pressure, diabetes, and older age—these patients are typically diagnosed with AF using ECGs done in doctor’s offices, usually because they are experiencing symptoms.

I worry that the Apple Watch will discover lots of short-duration AF or AF occurring in younger people with fewer risk factors. Given their lower risk of stroke, it’s unlikely that anticoagulant drugs will deliver similar benefits. But anticoagulants do increase the risk of bleeding.

The truth is that preventive health is far more complicated than identifying irregular rhythms from a watch. When you endeavor to make healthy people healthier, you always risk making them worse.