The Apple Watch is reported to be the world’s best-selling smart watch, although sales have slowed lately.

Apple didn’t comment, but the tech giant uses true stories of people whose lives have been saved because of the heart tracking function of the device in its marketing, and anecdotally I have heard plenty of those too. What I haven’t heard however, is how many cases of false positives there are.

In many cases when patients present their data to healthcare professionals, clinicians prefer to try to recreate it using their own equipment, rather than simply trust what the wearable has captured.

There are several reasons for this, says Dr Yang Wei, associate professor in wearable technologies at Nottingham Trent University – and they’re all very practical.

“When you go to hospital, and you measure your ECG [electrocardiogram, a test that checks the activity of your heart], you don’t worry about power consumption because the machine is plugged into the wall,” he says.

“On your watch, you’re not going to measure your ECG continuously because you drain your battery straight away.”

In addition, movement – both of the wearable itself on a wrist, for example, and general movement of the person wearing it – can “create noise” in the data it collects, he adds, making it less reliable.