Blood Oxygen showing low on Apple Watch Ultra 4

Do the Apple watches have the same watch band?

Normally the Apple watches are fairly accurate for blood oxygen measurements -- there have been some scholarly papers written on this, but those papers did note outliers sometimes occur, due to individual variation in how the watches were used and also in other variables like the watch band and how it fit the users' wrists. The blood oxygen level is sensitive to the fit of the watch and how it is worn (location on the arm/wrist).

My Series 11 watch is fairly consistent showing 94%-96% on average but that watch measurements range from 91% to 100%, obviously more variable since every time I have had it measured with a finger sensor in a medical clinic (which is not often, maybe once or twice in a year) it was always 98%-100%. So I infer from this that watch measurements can sometimes not be precise.

If one watch is systematically different (lower or higher) than the other, I would look carefully at the fit and the bands.

Apple does say its watches are not certified as medical devices, but are instead intended to provide general indicators of health. I would suggest using it more to identify trends versus looking for absolute calibration accuracy.

Per: https://www.nature.com/articles/s41746-025-02238-1

“limits of agreement approximating ±5% SpO2 were reported in multiple studies, indicating variability in measurements” — yours seems to be within that range, too.

There’s an old joke about a person with one watch knowing the time, while a person with two watches is unsure.

Yes, this joke doesn’t work as well in an era with watches that have an external time source, but you get the joke…

German Apple Support already confirmed (case number ***) that o2 and temp is a known problem and they are working on it! But they can’t say when the fix will be published :(!

My whole training data is a mess since I got the new ultra 4 (coming from 2). I use Athlytic training app and I can see it very clearly that since I got watch 4 the blood oxygen and temp giving false measurements.

[Edited by Moderator]

RunGo_guy wrote:

Yes they have identical bands, and I make sure the fit and tightness was the same, my previous watches Blood O2 all seemed accurate and in line with a medical finger sensor.

You might read this paper on this subject:

https://www.mcpdigitalhealth.org/article/S2949-7612(24)00010-5/fulltext

"Although smartwatches are able to provide SpO2 readings, their overall accuracy may not be sufficient to replace the standard photoplethysmography technology in detecting hypoxia in patients ..."

Also this https://www.apple.com/healthcare/docs/site/Blood_Oxygen_app_on_Apple_Watch_October_2022.pdf

clearly shows variability of 5% in laboratory tests for spO2. This is consistent with the Nature paper link provided by MrHoffman.

Yeah I am getting significantly lower spo2 numbers. I live at higher altitude and my Ultra 3 always showed overnight SpO2 averaging around 94-95 and overnight when switching to Ultra 4 I am now showing 88-91 lol. Not accurate at all since I compare with a finger oximeter that shows normal while watch shows “go to hospital” low numbers. Pretty disappointing since I upgraded from ultra 3 to 4 and now have a way less accurate device tracking my health metrics. I really hope it is a software update fix. It is very frustrating in the meantime though. Same bands as ultra 3 and same wearing routine. Just wanted to say that before the Apple warriors start jumping out of their closets to defend Apple lol.

Wonder if there is a calibration period seems like my readings are improving and are more credible. Also I am on a version of the OS that is not allowed to be mentioned in here as of yesterday so perhaps there is a fix on the horizon.

The Apple watch does not measure the same temperature as a medical thermometer. The watch measures skin temperature at the wrist, which is not the same as the core temperature a medical thermometer measures. Also, my understanding is that the watch measures changes in surface skin temperature at the wrist, which may be different from changes measured at the forehead.

https://support.apple.com/en-us/102674

Yes they have identical bands, and I make sure the fit and tightness was the same, my previous watches Blood O2 all seemed accurate and in line with a medical finger sensor.

Also getting much higher wrist temp numbers too. Also proven inaccurate by forehead temp check I do every morning for basal temp reading upon waking. So disappointing

Yes, myself included. After switching the from the Ultra 3, I’ve noticed a lot more frequent, lower readings

Recently purchased an AWU4 and have experienced a 5% on average drop in spO2 levels and low temp readings,

comparing it to my outgoing AW8 and Helio Strap.