Not all the results have been published. But the researchers shared that of the approximately 419,000 people who participated in the study, about 0.5 percent received a notification about an irregular heartbeat.
Stanford Medicine's principal investigator, Mintu Turakhia, pointed to that statistic to CNBC as an "important finding," as cardiologists have expressed concern that the Watch would produce a high rate of false positives, and send hoards of young people to the doctor's office unnecessarily.
That rate was higher with older users, at 3.2 percent, versus 0.16 percent in people between 22 and 39 years old.
Of those who received a notification, not all of them followed the protocol by contacting the researchers to ask for an electrocardiogram patch to confirm the diagnosis. Only about 450 did so, which is roughly one in five.
Medical experts shared a few theories on this. "It's possible that the folks who didn't follow through were primarily those who didn't have really bothersome symptoms," said Kumar Dharmarajan, a cardiologist and chief scientific officer at health insurance company Clover Health. Dharmarajan also made the point that the patch, which is worn on the chest, is not as enticing to wear as an Apple Watch, which might explain the drop-off.
The latest version of the Apple Watch, the Series 4, does however include an electrocardiogram sensor on the device, which has been cleared by regulators to screen for atrial fibrillation. But cardiologists would still recommend a patch for patients to make a diagnosis and determine treatment.