On January 5, while Fitbit was promoting its latest fitness tracking watch at CES in Las Vegas, a class-action lawsuit was filed against the company on behalf of users of the Fitbit Charge HR and Fitbit Surge. The claim: That the devices misread heart rates by “a very significant margin, particularly during exercise.”

At Consumer Reports, we were surprised because we had tested both of the devices, and found the heart rate readings to be quite accurate. We decided to retest these models to confirm that we should continue to recommend them. And to learn more about their performance, we added some elements to our standard fitness-tracker test protocol. The result: Both the Fitbit Charge HR and Fitbit Surge passed our tests handily, accurately recording heart rates at everything from a leisurely walk up to a fast run. (The details are outlined down below under “How We Tested.”)

This should be a reassuring finding for many Fitbit customers. Lots of fitness trackers can indicate how far you walk and how many steps you take. But only a few are designed to measure heart rate, and you pay a premium for them. The Charge HR sells for about $150, while the Surge goes for around $250, compared to just $100 for simpler models in the Fitbit lineup.

What the Lawsuit Says

The lawsuit says that users have found inaccuracies in the Fitbit devices after measuring their pulse manually or with other equipment. And the lawsuit also cites independent testing. “We went and had a board-certified cardiologist put some folks through different levels of exercise and test the different models,” Jonathan Selbin, the lead partner for the plaintiffs, says. “And, sure enough, particularly at high intensity levels, they are wildly inaccurate.” The lawsuit claims that readings were off by an average of 24.34 beats per minute (bpm), and a whopping 75 bpm in extreme cases. 

Purple Fitbit Charge HR on woman's wrist for an article on Fitbit heart rate accuracy
The Fitbit Charge HR is one of two Fitbit devices involved in a class-action lawsuit concerning the company's heart rate monitoring feature.

Fitbit says it stands behind its heart rate monitoring technology, which the company calls PurePulse. However, the company wouldn’t say how it conducts its own testing. “For proprietary reasons and because we operate in an extremely competitive environment, we do not disclose our specific validation study methodologies and results,” a spokesperson told us. 

On Fitbit’s PurePulse page, the company seems to leave itself some wiggle room: “Like all heart-rate monitoring technologies, accuracy is affected by physiology, location of device, and different movements.” And how you wear the device may also matter. Fitbit uses optical heart rate monitoring, or photoplethysmography. Unlike an ECG (electrocardiogram) that reads electrical activity, an optical heart rate monitor detects the pulse by shining a light through the skin to see blood flow. Fitbit suggests that users wear its products snugged up a couple of inches above the wrist to get the most accurate results. However, in the real world of gadgets, not everyone reads instructions. 

How We Tested

To retest these devices, we recruited a male and female volunteer and put them on a treadmill. As a reference, we used the Polar H7, a chest-strap monitor with proven accuracy. The two subjects each used the Fitbit Charge HR and Fitbit Surge.

When we tested these devices previously, subjects wore them at the wrist. This time around, we had the testers wear two copies of the same model of fitness tracker, one placed on the wrist, and the second one a few inches higher.

We also expanded the range of heart rates. In our previous testing, we had captured our data when the subjects were at rest, and when they were undergoing moderate exercise. But the problems cited in the lawsuit had allegedly cropped up during harder workouts. For our new test, we recorded our subjects’ heart rates at four levels of intensity: at rest, a walking pace (110 bpm), a jogging pace (130 bpm), and a running pace (150 bpm). All tests were conducted twice. A total of 64 heart rate measurements were recorded.

The new testing confirmed our earlier results: Both the Charge HR and Surge were very accurate when compared to the reference Polar H7 ECG monitor. During nearly every trial, the variance between the chest strap and the Fitbit devices amounted to no more than three heartbeats per minute.

However, there was one exception: When our female tester wore the Fitbit Charge HR on her wrist and got up to higher intensity levels, the margin of error crept upwards. During one run, when the chest strap read 150 bpm, the Fitbit Charge HR read 144 bpm. During the second run, the device read only 139 bpm. That problem went away when she wore the Charge HR on her forearm. (And the Fitbit Surge was accurate no matter how it was worn.) 

We’ll continue to recommend these two products, but we’ll also be watching the case closely. For more coverage of fitness trackers, see our ratings.