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For example, Oura’s new nighttime blood pressure feature tells users whether their blood pressure is dipping overnight — when it doesn’t dip, that’s a significant sign of cardiovascular risk.

Another new feature continuously detects “shifts and patterns in blood pressure that may indicate cardiovascular strain,” said Oura vice president of product, Jason Russell. The feature alerts users “when their biometrics suggest that their body might need attention, and it gives clear guidance on what actions to take to address it.”

Companies argue that the risk to consumers from such features is low, but medical device consultant and regulatory attorney Jason Brooke said it depends on how you scope the risk. He gave the example of a person who is overweight and has been told about slightly elevated blood pressure in the past but never received treatment. After seeing advertisements, they might turn to a wellness device to try to manage their conditions.

“There may be an extended period of time where if these products aren’t really doing anything for the individual, that they could have been doing something different,” Brooke said. “And potentially over time, they become diabetic, or have high blood pressure, or are obese. It’s that prevention aspect that potentially loses out.”

Do cuffless devices work?

A top concern is that cuffless blood pressure measurement in wearable devices is unreliable.

The products from Whoop, Oura, Samsung, and others do not directly measure blood pressure and instead estimate it by analyzing data collected by a wearable’s photoplethysmography sensor, which measures the pulsing volume changes in arteries as blood circulates. While this technology is commonly used to compute heart rate, using it for blood pressure can produce variable results depending on several factors, including the rigidity of a user’s arteries. An important study comparing cuffed readings to a cuffless wearable called Hilo, found it could not reliably measure overnight blood pressure dips or a patient’s response to blood pressure medication. (Hilo’s manufacturer, Aktiia, disputes the findings.)

Cohen led a scientific statement from the American Heart Association recommending against using cuffless devices for blood pressure diagnosis or management “until these devices demonstrate greater precision and reliability.” She said the statement is a reflection on cuffless blood pressure products in general — not only those intended for clinical use cases.

“The whole technology isn’t that accurate yet,” Cohen said. “The wellness devices are likely going through that approach because they know that… They might as well just be honest that it’s not super accurate, that it’s more providing whimsical inferences of what a blood pressure could be on the right day and time.”

Alta Schutte, a professor at the University of New South Wales in Australia, helps maintain Stride BP, a website listing sufficiently validated blood pressure monitors. She hopes to add a listing of cuffless devices when they are accurate enough, “but it’s not there yet for a reason,” she said.

Because of these limitations, the FDA has made stringent requirements for cuffless blood pressure devices intended for medical uses. The FDA-cleared Hilo wearable, for example, requires daily calibration — the company hasn’t released the product in the U.S. because it thinks this method is untenable for users.

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Apple, meanwhile, does not display blood pressure readings in its FDA-authorized feature. The Apple Watch analyzes 30 days of data and alerts users of possible hypertension. The company’s data shows the feature catches just 41% of cases.

Since the release of Apple’s product last fall, there’s been an outcry from experts concerned that it might result in false reassurance for the 59% of people who have hypertension and aren’t flagged. This concern extends to devices that display blood pressure values of questionable accuracy as well. Someone could falsely believe their numbers are healthy and not seek medical care. Though products might specify that they are not intended to diagnose or rule out medical conditions, users still may lean on readings inappropriately.

“As a consumer there’s going to be a very low level of literacy in understanding how the technologies work and whether the output is accurate or not,” said Eugene Yang, a cardiologist and professor at the University of Washington who studies cuffless technology. He added, “If something is put on the market, then [consumers] come in probably with some assumptions that this has been tested appropriately.” If they didn’t believe that, why would they purchase a device?

Contributing to consumer confusion is the lack of rules or norms around how companies should disclose information about the function of wellness features and their performance.

The FDA’s wellness guidance does instruct companies not to display physiologic measurements like blood pressure that “mimic those used clinically,” unless they are “validated.” During a February presentation on the new guidance, an FDA official said that “there are no specific requirements for validation” and that it could include manufacturer testing or peer-reviewed studies. The official said that “FDA encourages making validation results public” and that the agency may ask manufacturers for data.

The validation requirement has created some confusion in industry about what exactly needs to be done. David Simon, an associate professor at Northeastern University School of Law, called the mandate “quasi-regulatory” — something the FDA could enforce if it sees the need but which otherwise offers just a guideline for manufacturers to follow.

One obstacle, industry insiders say, is that there is currently no International Organization for Standardization protocol for validating cuffless blood pressure wearables. Many manufacturers, including Samsung, have chosen to rely on modified protocols for cuffed devices, which are tested on seated, relaxed users. An ISO protocol for cuffless blood pressure monitoring that takes into account more complex conditions has been drafted but not finalized and disseminated.

Despite FDA’s nudging, companies have disclosed little about how their wellness products were validated and how well they work.

Oura has promised to publish more information about its blood pressure features in early June, and Russell, the product VP, said the company conducted “extensive validation” using direct arterial measurements as ground truth. A Samsung spokesperson shared that the company used an ISO standard for cuffed devices but did not offer data about the performance of its blood pressure product.

Whoop has said it trained its blood pressure model using data from over 11,000 members and then validated it against clinical readings. The company’s head of health care products Alex Vannoni recently said it was “highly accurate.” Representatives for the company did not respond to questions about whether performance data would be made public.

A new noisy world

There’s a broad feeling among industry observers that the FDA’s wellness guidance update would not have happened if Whoop hadn’t pushed the boundaries. Now patients, clinicians, and developers are left to navigate a new landscape.

Brooke, the regulatory attorney, said it’s unfortunate that FDA created policies geared toward companies that want to avoid regulation, because there are many willing to do the hard work “to do things the right way,” he said.

Cohen, who helped write the AHA scientific statement, said that even before blood pressure wearables were common, she found herself at loggerheads with a patient who insisted a watch purchased online showed normal blood pressure and that elevated readings in the office must be “white coat” hypertension that only shows up in medical settings. A 24-hour reading from an ambulatory blood pressure monitor showed alarming levels of hypertension.

“My big concern is it’s going to create a bigger gap of physician distrust,” she said.

Josep Solà, the co-founder and chief technology officer of Aktiia, said even in the wake of the updated FDA guidance, he believes it’s important for the company to remain focused on clinical applications of its cuffless technology. Still, he would not rule out a wellness offering. He said that the new products from well-known brands like Apple and Samsung help build credibility for his company.

“The negative is that they’re adding a lot of noise to the market,” Solà said, “and more people will also come to the market with wellness things that just make nothing.”