Samsung says that its newest Galaxy Watch will be able to measure blood pressure, which could be a big deal for doctors and consumers alike — but it’s still unclear how useful the feature will actually be.

High blood pressure, or hypertension, is linked to increased risk of heart disease, stroke, kidney disease, and other conditions. Unfortunately, not only do many people have high blood pressure without knowing it, accurately measuring blood pressure is tricky because it changes so much at different times of day and in response to different emotional and physical events. “If more people wearing watches are more aware of their blood pressure in general, that would be great,” says Ann Marie Navar, a cardiologist at Duke University. An easy way to continuously monitor blood pressure would help patients check their stats between doctor’s visits. It could also allow researchers to better study the relationship between blood pressure and health.

“We need to see a lot more data”

Whether the Samsung watch can do this is still an open question. The company has been short on details, but it appears that the watch will use an optical sensor to read blood pressure using a technique called photoplethysmography (PPG), according to Wendy Mendes, a psychophysiologist at the University of California San Francisco who helped Samsung develop the tool.

The feature has actually been around for a year on the Samsung Galaxy S9. The Galaxy S9 phone has an optical sensor that shines light onto tissue (in this case, the fingertips) and then detects how much light bounces back. That amount of reflected light is related to changes in blood volume in the tissue and that information is sent to Samsung’s My BP Lab app, where a proprietary algorithm interprets the results to estimate blood pressure.

On its own, PPG can technically only measure changes in blood pressure. That’s why it’s crucial that users calibrate the watch’s estimate with an actual blood pressure reading at the doctor’s office or one of those kiosks at Walgreens. “Calibration is really important,” says Mendes. “It’s like a scale. If I couldn’t calibrate a scale, I might only be able to predict when it goes up and down.”

During a traditional blood pressure reading, a doctor puts a tight inflatable bladder, or cuff, around the arm to block the flow of blood. As the cuff releases, the doctor measures the points at which blood starts flowing again. This is the most common way of measuring blood pressure without actually sticking a needle in the artery, and is more clinically accepted than a technique like PPG.

“It’s a little too early to celebrate.”

“This is where I as a clinician and a researcher have a lot of concerns,” says Navar. “We need to see a lot more data on the accuracy of PPG compared to the standard ways we’re used to measuring blood pressure with external inflatable bladders.” A lot of physiological factors could affect the strength of the PPG signal, she says. It could be inaccurate in people who don’t have hypertension, or people who have thicker or thinner arms.

For its part, Samsung has released very few details, and did not respond to CNBC inquiries about whether the device integrates with third-party wireless cuffs, how often it needs to be calibrated, or how it validated the data. (In a statement to The Verge, Samsung said that it will share details at a later date.) Mendes said that her group is working on writing up data from the validation study, which involved 120 subjects and compared the app against three different, medical-grade sources of blood pressure. Another smartwatch, created by Omron, uses the cuff method to measure blood pressure and was approved by the US Food and Drug Administration in December. There is no word, however, on whether Samsung also plans to apply for or receive clearance from the FDA.

The FDA is responsible for regulating medical devices, but has stated that “low risk general wellness products” do not need to face the same scrutiny, according to Barbara Binzak Blumenfeld, a lawyer that specializes in new device approval and navigating FDA regulations for the firm Buchanan Ingersoll & Rooney. “For example, a product that claims to promote a healthier, more active lifestyle by monitoring heart rate would be considered a general wellness product,” she wrote in an email, “whereas a product like the Apple Watch’s EKG, which is intended to do more than promote a healthy lifestyle, needs to be a medical device.”

This split between wellness and medical products is frustrating to cardiologists like Bruce Alpert, who sits on the International Organization for Standardization committee for blood pressure monitors. “Those of us on the United States and the international committees are very frustrated because the term ‘blood pressure’ to us is always a medical term,” he says. “Of the vital signs — like heart rate and temperature — it’s the only one that has any future predictive value for disease severity or prevalence.”

“There are enormous numbers of new technology that involve the stiffness of the artery or the way the artery behaves and a lot of people have tried to translate that into real blood pressure,” Alpert continues. “All those are valid from a physiology standpoint and so there are many different ways to skin the cat. It’s just really a question of: Have they gone through, in the development of their technology, appropriate testing to make sure that in fact what they are calling blood pressure is truly accurate blood pressure?”

Navar agrees. “I really hope that one of these gets through an FDA clearance process, but as of now, it hasn’t been robustly tested in a way that I would say is suitable to replace an external bladder-inflated measurement,” she says. “It’s a little too early to celebrate.”