Unveiling the Truth Behind Blood Oxygen Sensors in Smartwatches

While devices like Fitbit Sense 3 demonstrate promising features, skepticism about their accuracy remains.

Washington Post journalist Geoffrey A. Fowler spent a week testing the latest Apple Watch 6, focusing on its blood oxygen sensor. His detailed analysis revealed inconsistent readings: the watch fluctuated between showing 100% oxygen saturation and alarming low oxygen levels that could indicate lung conditions such as emphysema, which he does not have. Within minutes, the sensor reported an 88% saturation (low) and then a 95% saturation (typical for healthy individuals).

Throughout the week, Fowler compared Apple Watch 6 readings with those from a medical-grade pulse oximeter, which has a measurement error margin of about 2%. Apple does not disclose the sensor’s accuracy in its watches. The results varied significantly. Upon reporting these inconsistencies to Apple, the company offered a replacement watch. The new device generally showed errors between 2–3%, but occasionally discrepancies reached 7%, a margin considered critical for health monitoring.

Geoffrey also pointed out that blood oxygen sensors in both Apple Watch 6 and Fitbit Sense 3 have not received FDA approval, meaning they are not classified as medical devices. Both companies clarify on their product pages that these features are intended solely for tracking fitness progress and daily activity. However, marketing often promotes these sensors as tools for health monitoring. In contrast, Apple’s ECG sensors, introduced in 2018, have undergone the necessary regulatory approvals.

The difficulty in accurately measuring blood oxygen levels stems largely from the device’s placement on the wrist, which is less reliable than fingertip measurements. This positioning increases the likelihood of false readings or errors. Fitbit Sense 3 addresses this by restricting SpO2 scans to nighttime, providing average readings over sleep periods, which proved more accurate than instantaneous measurements, albeit less convenient.

The article highlights that neither company offers clear guidance on using the sensor for fitness tracking—Fitbit only measures SpO2 during sleep, while Apple Watch frequently reports errors when the user is active. Without certification and with uncertain accuracy, these sensors are not suitable for medical purposes.

Accurately monitoring one’s health in real-time is an exciting prospect. Unfortunately, current SpO2 sensors in consumer smartwatches fall short of reliable accuracy. For those needing precise blood oxygen data, dedicated medical devices remain the recommended choice.

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