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A class action lawsuit against Fitbit may have grown teeth following the release of a new study which claims the company's popular heart rate trackers are "highly inaccurate."
Researchers at the California State Polytechnic University, Pomona tested the heart rates of 43 healthy adults with Fitbit's PurePulse heart rate monitors, using the company's Surge watches and Charge HR bands on each wrist.
Subjects were then hooked up to a BioHarness device that produced an electrocardiogram (ECG), to record the heart's rhythm against the data being produced by Fitbit's devices.
Comparative results from rest and exercise, including jump rope, treadmills, outdoor jogging and stair climbing, showed that the Fitbit devices miscalculated heart rates by up to 20 beats per minute on average during more intensive workouts.
"The PurePulse Trackers do not accurately measure a user's heart rate, particularly during moderate to high intensity exercise, and cannot be used to provide a meaningful estimate of a user's heart rate," the study document stated.
The study was commissioned by Lieff Cabraser, the law firm behind the class action suit that is taking aim at three Fitbit models that use the PurePulse heart monitor, including the Fitbit Blaze, Fitbit Charge HR and Fitbit Surge. The suit claims the devices do not accurately measure heart rate, and the study was included as part of an amended complaint filed last week.
In a statement, Fitbit called the study "biased" and "baseless," saying it lacked "scientific rigor."
"It was paid for by plaintiffs' lawyers who are suing Fitbit, and was conducted with a consumer-grade electrocardiogram – not a true clinical device, as implied by the plaintiffs' lawyers. Furthermore, there is no evidence the device used in the purported 'study' was tested for accuracy."
However, a separate study by Ball State University in Indiana and journalists at NBC-affiliated TV station WTHR released in February, also showed that the Fitbit Charge HR missed heartbeats, marking an average heart rate error of 14 percent.
"Calculating a heart rate that's off by 20 or 30 beats per minute can be dangerous — especially for people at high risk of heart disease," the report explained.
Fitbit did send a written reply to WTHR, saying that its devices "are designed to provide meaningful data to our users to help them reach their health and fitness goals, and are not intended to be scientific or medical devices."
Fitbit also highlighted to CNBC a separate study from independent consumer-advocacy organization Consumer Reports which has found "the heart rate readings to be quite accurate."
But the most recent study by California State Polytechnic University, if correct, could mean some users are putting themselves at risk, said Dr. Allan Stewart, director of aortic surgery at The Mount Sinai Hospital.
Stewart says it's a big assumption to believe that the study, which was paid for by a legal firm suing Fitbit, is correct. But if it is, he says, "athletes may be at a minimum improperly training and, at worst, potentially elevating their heart rates to an unsafe level where a massive heart attack is entirely possible."