Sleek, high-tech wristbands are extremely popular these days, promising to measure heart rate, steps taken during the day, sleep, calories burned and even stress.
And, increasingly, patients are heading to the doctor armed with reams of data gathered from their devices. "They're essentially asking us to digest the data and offer advice about how to avoid cardiovascular disease," says cardiologist Euan Ashley, associate professor of medicine at the Stanford University Medical Center and Stanford Hospital and Clinics in northern California. And, being somewhat near Silicon Valley, he says he gets a lot of tech-savvy patients bringing fitness-tracker data to appointments.
The problem, he says, is that he just didn't know how reliable that data was. So, he and colleagues decided to study seven of the most popular devices and compare their accuracy to the gold-standard tests that doctor's use.
They looked at two metrics: heart rate and calories burned. For heart rate, the fitness trackers were compared to findings from an electro-cardiogram, or EKG. It turned out the devices were "surprisingly accurate", says Ashley. "Most devices most of the time were 'off' by only about 5 percent."
However, when it came to measuring how many calories a person burned, the findings were way off, says Ashley, showing a degree of inaccuracy that ranged from 20 percent to 93 percent, meaning 93 percent of the time the worst-performing device was wrong. Researchers compared the findings of the wrist devices to a sophisticated system of calculating metabolism which measures oxygen and carbon dioxide in people's breath.