Consumer fitness trackers and smartwatches allow many people to monitor their health directly from their wrists. Blood pressure measurement is a highly desired feature, especially for those managing cardiovascular health outside a clinical setting. While traditional blood pressure cuffs are the established standard, the convenience of a wearable device that estimates this metric is appealing. This article examines the core technology these trackers use, assesses their reliability, and explains how to interpret the data.
The Technology Behind Wearable Blood Pressure Measurement
Most consumer wearables estimate blood pressure using a cuffless method that analyzes changes in blood flow via optical sensors. This technique is called Photoplethysmography (PPG), which shines an LED light into the skin and measures the reflected light to detect volume changes in blood vessels. The tracker uses the raw PPG signal to calculate metrics like Pulse Transit Time (PTT). PTT is the time it takes for the pulse wave to travel between two points, and a shorter time generally correlates with higher pressure. Sophisticated algorithms then convert these time-based measurements into a blood pressure estimate.
Since these devices do not directly measure the force of blood against artery walls, they require frequent initial calibration using a medically validated cuff device. The cuff reading provides the algorithm with a reference point to establish the personalized relationship between the user’s PTT and their actual blood pressure. This calibration must be repeated regularly, sometimes every few weeks, because the relationship between PTT and blood pressure can drift over time.
Real-World Accuracy and Clinical Validation
The accuracy of wearable blood pressure estimation is the subject of ongoing scientific research. Some devices have shown promising results under controlled, ideal laboratory conditions, with studies reporting mean differences that meet international standards after proper calibration. However, the performance often degrades significantly when used in real-world, daily conditions.
Independent studies frequently find that cuffless devices exhibit a systematic bias, particularly with extreme values. For example, a calibrated smartwatch tended to overestimate low blood pressures and underestimate high blood pressures, biasing readings toward the initial calibration point. This lack of precision means most consumer trackers are not recommended for medical diagnosis.
A critical distinction exists between devices that are “FDA-cleared” and those that are merely “FDA-registered.” Clearance means the manufacturer has proven the device meets strict accuracy and performance standards for a medical purpose. Most current consumer trackers that estimate blood pressure are not FDA-cleared as medical devices for this function, meaning they have not been rigorously validated for clinical use.
Factors Affecting Measurement Reliability
A major challenge for cuffless trackers is the number of variables that can introduce error into the measurement. The primary method, PPG, is highly sensitive to motion artifacts; even small movements during measurement can distort the signal and lead to an unreliable reading. Unlike a cuff device, the optical sensor must contend with movement noise.
The fit of the device on the wrist is a major factor; a tracker that is too loose or too tight can compromise the quality of the PPG signal. Accuracy relies on a stable, recent calibration, and the device’s accuracy may diminish over time as the user’s physiology changes. If the user fails to recalibrate the device frequently, the estimated blood pressure can drift away from the true value.
External factors can also influence the reliability of the reading. Environmental temperature fluctuations and differences in skin tone or pigmentation affect how light is absorbed and reflected, altering the PPG signal quality. These variables make it difficult for the algorithm to consistently convert the optical data into a precise blood pressure reading outside of a controlled setting.
Using Trackers for Trends Versus Diagnosis
Given the limitations in real-world accuracy, the data from cuffless blood pressure trackers should be used for general wellness monitoring, not for clinical decision-making or diagnosis. These devices are useful for tracking broad, long-term trends, such as recognizing if blood pressure is consistently trending up or down over a period of months. This trend data can provide awareness and prompt a conversation with a healthcare provider. If a tracker flags a reading as concerning, the appropriate action is to confirm the measurement immediately with a validated, cuff-based device.