If that is the case, the major potential for the device is to pick up arrhythmias in otherwise healthy people. That is still a big selling point. Picking up abnormal function earlier could theoretically lead to improvements in health, such as reductions in strokes.

But just because something seems like a good idea does not mean it is. No screening test is perfect. In the simplest sense, whenever we consider the results of medical tests, they can be "positive" or "negative".

In general, we would like people who are sick to have a positive screening result, and people who are well to have a negative result. Unfortunately, people who are sick sometimes have a negative result. Those are false negatives. People who are well sometimes have a positive result. Those are false positives.

Both of these outcomes are worrisome. A false negative might leave someone who needs medical help with a mistaken sense of assurance. Given that relatively few people have serious, undiagnosed arrhythmias with no symptoms (if people did, we would be screening for this more often), this is not the major concern. False positives are worrying because they cost us time and money as well as cause emotional distress.

The US healthcare system is already busy, if not overloaded. No physician wants to field calls from patients who have no problems. Such patients will require visits and further testing, and will potentially receive interventions. They will generate bills and harms without benefits.

The watch also has an "irregular rhythm" notification feature, which alerts people to potential problems. There is every reason to believe it will generate many false positives. Before granting clearance, the FDA reviewed data collected by the Stanford Heart study for 266 people who got such a notification. Most of the notifications were wrong.

The study was not peer reviewed, so we do not know for sure, but this was also a population for whom atrial fibrillation might be more common than in those who might use the watches. People who buy the latest Apple watch will most likely be younger, healthier, wealthier and more plugged into the healthcare system; and less likely to remain undiagnosed.

This is one of the major problems with such a device. The people most in need of it, those who might benefit from tests and distance monitoring, are the least likely to get it. If we truly believed this was a medical test beneficial to the general population, insurance should pay for it. No one is suggesting that should happen.

In fact, many experts do not think it makes sense to have universal cardiac monitoring of the general public. The US Preventive Services Task Force has issued a "D" recommendation for screening asymptomatic adults at low risk. The group does not think there is enough evidence to recommend screening of adults at intermediate or high risk. It does not even think there is enough evidence to recommend screening adults 65 or older, who are at higher risk, for atrial fibrillation.

The task force bases these recommendations on good research. A large randomised controlled trial of echocardiographic screening for many heart problems did not demonstrate that such screening offered any benefits in reducing death or the risk of heart attacks or stroke in middle-aged people. And these are scans much more robust than will be available with the new Apple Watch.

None of this prevented the American Heart Association from heralding this new function, although it is not clear where the group's enthusiasm comes from.

Dr Ivor Benjamin, the association president, appeared at the official announcement of the watch and praised the advancement for tools that "help fight heart disease." (The AHA does not officially endorse the watch or any other specific products.)

I happen to own an Apple Watch. I find the other functions useful and fun. I even enjoy aspects of the activity monitoring. But I am under no illusion they will help me lose weight or exercise more or improve my heart health. I own one because I want it, not because I need it. That is the same criterion you should use, too.

Aaron E. Carroll is a professor of pediatrics at Indiana University School of Medicine.

New York Times