The Apple Watch's blood oxygen sensor is less accurate than you think

As always, it's best to consult with a doctor before trusting Apple's medical advice.

Apple spent much of its live-streamed event yesterday speaking to the ways in which the Apple Watch can help users take better control of their health. The new Apple Watch Series 6 even comes complete with a new sensor set to monitor blood oxygen levels.

But the Series 6’s blood oxygen monitor is not medically accurate. As The Verge points out, the blood oxygen monitor’s fancy sensors can only measure blood oxygen levels through your wrist — but almost all medically approved pulse oximetry technology does so through your fingertips, the spot where oxygen levels can be measured most accurately.

The Apple Watch’s new tech is of the same sort being used by fitness-tracking rivals Fitbit and Garmin, both of whom have released similar features in recent months.

Apple is very careful in the language it uses to describe the new blood oxygen monitor; the company says it “offers users even more insight into their overall wellness.” But is it enough to keep users from thinking this is medical equipment?

How does it work? — Measuring blood oxygen saturation is tricky — and the methodology is so strange it almost sounds fake. Pulse oximeters send infrared and red light through your fingertips. Oxygenated hemoglobin absorbs more infrared light; deoxygenated hemoglobin absorbs more red light.

The key difference here is that, as a wrist-facing device, the Apple Watch can only measure blood oxygen levels through your wrist. Blood doesn’t gather quite as densely or close to the surface in your wrist as it does in your fingertips — which means from the get-go the Watch can never be as accurate as a true pulse oximeter.

The Watch Series 6 also measures red and infrared lights’ refraction, rather than measuring the light itself. This methodology makes the Apple Watch even less accurate. Multiple studies have found that outside light can easily skew the results of blood saturation tests using refracted light.

Check with your doctor — Someday Apple’s blood oxygen monitoring could be accurate enough to actually detect medical conditions. But right now it’s more of a gimmick than anything else. Sure, it might be fun to monitor your blood oxygen levels throughout the day, but those readings are rendered medically useless by the technology’s penchant for inaccuracy.

Apple knows this: it doesn’t make any claims of medical accuracy. But the company also doesn’t go out of its way to warn users of the technology’s shortcomings. There will surely be customers who purchase the new Apple Watch thinking it can help them detect serious illness — especially during a global pandemic, when consumers are jumping at every opportunity to monitor their health at home.

The Apple Watch Series 6 looks incredible. It's just not a medical device. In the end, the new Apple Watch is a great reminder that — like all at-home diagnoses — it’s best to consult with a doctor before jumping to conclusions. We hope Apple encourages its customers to do the same.