Apple Watch High Blood Pressure Detection: Technology and Validation
How the high blood pressure alert works technically
The analysis runs passively in the background over a longer period: the algorithm continuously evaluates the data over 30 days. Only if signs of high blood pressure appear consistently over weeks does the user get a notification. This longer observation period is meant to avoid false alarms from short-term fluctuations or measurement noise. Important: the watch does not measure specific systolic or diastolic values in mmHg. It also does not provide a medical diagnosis it simply notifies the user if the detected patterns suggest chronically elevated blood pressure. The alerts are intended to encourage users to have their blood pressure checked, since hypertension is often silent and can go unnoticed.
The fact that this PPG-based approach can work is also supported by independent research. Studies show that AI algorithms can detect elevated blood pressure from PPG curves with useful accuracy. For example, a deep-learning model using short PPG recordings (1015 seconds) on the UK Biobank dataset achieved a ROC-AUC of ~0.82 for detecting hypertension (and ~0.87 for markedly elevated hypertension). In another study with long-term wrist PPG recordings, a ResNet-based model also achieved a very high specificity (>95%) when distinguishing normal blood pressure from hypertension.
Scientific validation and clinical studies
Apple validated the high blood pressure alert extensively before release. The algorithm was first trained on very large datasets: according to Apple, PPG and health data from several studies totalling over 100,000 participants were used in development. The finished algorithm was then tested in a prospective clinical study with more than 2,000 participants. In that study, adult participants (without known hypertension) wore an Apple Watch for about 12 hours per day over roughly a month and simultaneously measured their blood pressure twice daily with a standard cuff. This allowed a direct comparison to see whether the watch warned the same people who showed persistently high values on the standard blood pressure device. The result: Apple Watch notifications proved to be comparably reliable to the conventional cuff at detecting hypertension patterns. In other words: the watch was able to detect the same signs of chronic high blood pressure in participants who were also identified by daily cuff measurements.
Proving this level of accuracy was key for regulatory approval in September 2025 Apple received FDA clearance for the Hypertension Notifications. Apple and independent experts stress, however, that the watch does not replace a doctors visit or validated medical devices. The new feature is meant primarily as an early-warning and screening tool. If the watch issues a high blood pressure alert, Apple explicitly recommends users recheck their blood pressure carefully with a validated cuff within the next week (for example, at home in the morning and evening) and discuss those readings with a doctor. This approach aligns with current American Heart Association guidelines, which require repeated measurements over several days to diagnose hypertension. To record and evaluate these measurements properly we recommend the BloodPressureDB app.
Cardiologists welcome the new Apple Watch feature as an opportunity to identify hidden high blood pressure earlier, but they also note that it does not replace continuous blood pressure monitoring and wont catch every blood pressure rise. Overall, the thorough validation from big-data training to a controlled 2,000-patient study shows that this technology is scientifically grounded. It could help uncover many previously undiagnosed cases of hypertension at scale (Apple estimates over 1 million in the first year) and thus help prevent cardiovascular disease.
Criticism
Apple has now published the validation study. We reviewed it and came away somewhat disappointed: the share of cases in which high blood pressure is correctly detected is smaller than expected.
Stage 1 hypertension is detected in less than 30% of cases, while Stage 2 hypertension is detected in about half of the cases.
The unequal distribution is also notable: detection is worse in people under 60 and in those with a BMI under 30. That the feature reports high blood pressure more often in older people with higher BMI is not surprising no elaborate algorithm is needed for that.
You have to look at it this way: the feature is an additional capability of the Apple Watch, and because the watch is so widely used it will certainly identify many previously undiagnosed cases of hypertension. Thats an impressive achievement by Apple. But: no one should buy an Apple Watch for that reason alone. The feature is not a reliable safeguard.
Sources
- Apple (product announcement)
- Apple validation study
- Time Magazine
- Hindustan Times
- Fierce Biotech
- American Heart Association (abstract)
- ArXiv (study on PPG and blood pressure)
Author Horst Klier has been intensively involved with high blood pressure since 2002 initially from personal experience and, since 2009, as the developer of BloodPressureDB. Thanks to his app and specialist platform used by millions as well as numerous publications, he is now regarded as a recognised blood pressure expert. As the author of several health guides and professional articles, he makes complex information understandable and practical.
