Researchers measured the accuracy of the Apple Watch's oxygen sensor, and found it was as reliable as a medical-grade device: https://data.empirical.health/p/is-the-apple-watchs-blood-ox...

The FDA doesn't regulate "over the counter" oxygen sensors (including those you find at a CVS or Walgreens), but that study suggests that if they did, Apple could get an FDA clearance. They already have several FDA clearances for the Apple Watch's ECG, irregular rhythm notifications, and afib history features.

The oxygen sensor is helpful for detecting, among other things, sleep apnea, where 60-80% of people are undiagnosed.

I believe the Apple Watch is a class 2 medical device because it alerts you if it thinks you’re having a heart attack. They probably got a 510k for this and based it on some other predicate device

“Apple Watch cannot check for signs of a heart attack.”

— Apple Watch ECG App

You’re probably thinking of afib, which I believe the watch will alert for.

All medical-grade blood oxygen sensors that I have seen in hospitals are put on the tip of the finger. This arrangement allows them to beam light through the human flesh and measure how much of it is absorbed - the light source is on one side of the finger and the sensor on the other and there is no bone between them at the tip of the finger, just tissue with lots of blood in it. Smart watches are carried on the wrist so they can only measure light reflected back which gives much lower sensitivity and accuracy.

They can be decent but can be totally wrong too:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10039641/

I use my Oura ring for that - https://support.ouraring.com/hc/en-us/articles/7328398760851...

Like other consumer device manufacturers they have a disclaimer that it isn't a medical device, with concomitant implications.

I have not compared with medical equipment but am interested in doing so, or hear from others who have.

Not great.

I have no idea how accurate they are, but they’re very fiddly and don’t produce a reading at all unless conditions are ideal.