In plain English: These are, at first glance, strong findings that underscore the watch's AFib-flagging accuracy. It's a good look for Apple.

Of course, the full picture is a lot more complicated. "This technology has enormous potential for good, and it would be foolish to say otherwise," says cardiac electrophysiologist Rod Passman, director of the Center for Atrial Fibrillation at Northwestern Memorial Hospital. "At the same time, those of us who work in this field are looking at each other saying: Do you see the can of worms we might be opening here?"

Apple

What he means is that even if the Apple Watch's heart-monitoring features are accurate, it's not obvious that screening for AFib in millions of undiagnosed people will do more good than harm. The lack of clarity is why the US Preventive Services Task Force, an independent, volunteer panel of national experts in disease prevention, concludes that current evidence is insufficient to assess the balance of screening's benefits and harms. "The main concerns are that people will be unduly alarmed, anxious, and seek medical attention and treatments they don't need," says Marcus, of UCSF, who led the first study to show that the Apple Watch's optical sensors could be used to discriminate AFib from a normal heart rhythm, using a third-party app called Cardiogram.

In that study, Marcus and his colleagues demonstrated that the big issue with using the Apple Watch to detect AFib isn't so much its accuracy—it's who's wearing it. In a large, generally healthy population, even accurate diagnostic tests can produce relatively large numbers of false positive results, which can frighten patients and overwhelm healthcare systems. Medical providers describe such a test as having a low positive predictive value. Millions of people wear Apple Watches, and most of them don't have AFib. "So almost by definition, if we’re screening for a disorder across this large population, then it is very likely that the positive predictive value is going to be quite low," Marcus says.

To complicate things further, it's not clear what benefits will come from screening with the Apple Watch. "We don't know what to do in an otherwise healthy 65-year-old who experiences one brief episode of AFib a year, let alone a healthy 30- or 40-year-old," says Passman. Short-lived arrhythmias that only crop up occasionally are exactly the kind of thing the Apple Watch is likely to detect. Meanwhile, Passman says, researchers have yet to prove that knowing you have AFib, as a young person, is useful information.

In gray-area cases such as those, a doctor could risk exposing a patient to therapies they don't need—anticoagulation meds, for example, come with a significant risk of bleeding. "We may only be harming them," Passman says. "The cart is definitely before the horse here."

The great irony of the Apple Watch's new heart-monitoring features is that they are well-positioned to address many of the questions they raise—eventually. Especially if the company continues to collaborate with academic researchers in the public eye, as they have with the Apple Heart Study. Never before has an arrhythmia-screening device been so accessible to so many people.

The issue is not so much that Apple was irresponsible in releasing these features, Marcus explains. The company saw an unmet need, approached the FDA, got approval—all while enrolling test subjects in actual, large-scale studies. Likewise, he says, "it would be a missed opportunity to see [the features] simply as a good or bad thing."

The science of AFib screening is far from settled, and with the release of the Apple Watch's new heart-monitoring features, the potential for discovery is high. In the short term, they could wind up doing more harm than good. But years down the road, they could also make solid progress in demystifying heart health.

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