Study Questions Smartwatch Accuracy in Detecting Undiagnosed High Blood Pressure

Last fall, the U.S. Food and Drug Administration cleared the Apple Watch Hypertension Notifications Feature, a cuffless tool that uses the watch’s optical sensors to measure blood flow patterns and detect possible hypertension. While the feature was not designed to diagnose high blood pressure, it nevertheless represents a step toward wearable-based population screening.

In a new analysis led by investigators from the University of Utah and the University of Pennsylvania and published in the Journal of the American Medical Association, researchers envisioned the real-world impact of this technology if deployed broadly across the U.S. adult population.

“High blood pressure is what we call a silent killer,” said Adam Bress, senior author and researcher at the Spencer Fox Eccles School of Medicine at the University of Utah. “You can’t feel it for the most part. You don’t know you have it. It’s asymptomatic, and it’s the leading modifiable cause of heart disease.”

How Smartwatches Detect—or Miss—High Blood Pressure

Apple’s previous validation study found that approximately 59% of individuals with undiagnosed hypertension would not receive an alert, while roughly 8% of those without hypertension would receive a false alert. Current guidelines recommend employing both an office-based blood pressure measurement and an out-of-office blood pressure measurement using a cuffed device to confirm a hypertension diagnosis. For many people, blood pressure readings can vary between the doctor’s office and the home.

Drawing on data from a nationally representative survey of U.S. adults, Bress and his colleagues assessed the ways in which Apple Watch hypertension alerts could alter the likelihood that adults without a prior diagnosis actually have high blood pressure. The analysis focused on adults aged 22 years or older who were not pregnant and were unaware they had high blood pressure—the population eligible to use the feature.

The analysis revealed important variations: among younger adults under 30, receiving an alert increases the probability of having hypertension from 14% (according to NHANES data) to 47%, while not receiving an alert lowers it to 10%. However, for adults 60 and older—a group with higher baseline hypertension rates—an alert increases the probability from 45% to 81%, while the absence of an alert only lowers it to 34%.

The data indicate that as undiagnosed hypertension becomes more common, an alert is more likely to reflect a true case of the condition. In contrast, the absence of an alert becomes less reassuring as prevalence increases. For example, the absence of an alert is more reassuring in younger adults and substantially less reassuring in older adults and other higher-prevalence subgroups.

The study also found differences across racial and ethnic groups: among non-Hispanic Black adults, receiving an alert increases the probability of a hypertension diagnosis from 36% to 75%, while not receiving an alert lowers it to 26%. However, for Hispanic adults, an alert increases the probability from 24% to 63%, while its absence lowers the probability to 17%. These differences reflect known disparities in cardiovascular health that are largely driven by social determinants of health, Bress said.

Should a Smartwatch’s Hypertension Feature be Used?

With an estimated 30 million U.S. Apple Watch users and 200 million worldwide, the notification feature represents a promising public health tool, but it should supplement—not replace—standard blood pressure screening with validated cuff-based devices, researchers advise.

“If it helps get people engaged with the healthcare system to diagnose and treat hypertension using cuff-based measurement methods, that’s a good thing,” Bress said.

Current guidelines recommend blood pressure screening every three to five years for adults under 40 and no additional risk factors, and annually for those 40 and older. The researchers caution that false reassurance from not receiving an alert could discourage some individuals from obtaining appropriate cuff-based screening, resulting in missed opportunities for early detection and treatment.

When patients present with an Apple Watch hypertension alert, Bress recommends clinicians perform “a high-quality cuff-based office blood pressure measurement and then consider an out-of-office blood pressure measurement, whether it’s home blood pressure monitoring or ambulatory blood pressure monitoring to confirm the diagnosis.”

The research team plans follow-up studies to estimate the actual numbers of U.S. adults who would receive false negatives and false positives, broken down by region, income, education, and other demographic factors.

The study was supported by the National Heart, Lung, and Blood Institute and involved researchers from the University of Utah, the University of Pennsylvania, the University of Sydney, the University of Tasmania, and Columbia University.