The Apple Watch blood-oxygen sensor was never a medical device — and briefly, not there

The blood-oxygen reading on an Apple Watch is a wellness feature, not a cleared medical instrument, and the distinction is the whole point. Unlike the watch's ECG and its atrial-fibrillation, hypertension and sleep-apnea notifications — each of which cleared the FDA — the Blood Oxygen app was released for general fitness and wellness use, without the regulatory review that would let it claim clinical accuracy. That framing is easy to forget when the watch displays a number that looks exactly like the one on a hospital pulse oximeter.

The accuracy question

Wrist-worn reflectance oximetry is a harder measurement than the fingertip clip used in clinics, and even the clinical devices have a well-documented problem: they read high on darker skin. In a study of paired oximeter and arterial blood readings, pulse oximeters overestimated oxygen saturation more often in Black patients than in White patients, meaning true low-oxygen states — occult hypoxaemia — were missed more frequently in patients with darker skin [s1]. That bias is inherited from the optics of the method, not specific to any brand, and it has since been reproduced in NHS devices at scale, where the misses cluster in exactly the same group.

An Apple Watch faces every difficulty a clinical oximeter faces, plus motion, a looser fit, and the optical variability that already limits wrist photoplethysmography across skin tones. Reflectance oximetry read off the back of the wrist, through more tissue and with the sensor pressed against a moving arm, is a harder measurement than the transmission oximetry of a fingertip clip, and it is more sensitive to fit, cold hands and poor perfusion. Apple's own guidance has consistently steered users away from treating the figure as diagnostic. For a healthy person curious about their oxygen saturation, that is a reasonable feature. For anyone who might actually be hypoxaemic, a reassuring wrist reading is not something to rely on.

The stakes are clearest when oximetry is used to triage — to decide who needs escalation of care — because that is exactly the setting in which a device that reads high on darker skin can leave a genuinely hypoxaemic person falsely reassured, the failure mode the NHS home-monitoring data documented at scale [s1]. A watch marketed for wellness was never meant to fill that role. But the number it shows looks identical to the one a clinician would act on, and users do not carry the caveats in their heads.

The feature that vanished

The more visible story is that US customers lost the feature entirely. Masimo, a medical-device maker, accused Apple of infringing its pulse-oximetry patents, and a US International Trade Commission exclusion order took effect in December 2023. Rather than stop selling the watch, Apple disabled blood-oxygen sensing in the United States in January 2024 and continued shipping the hardware with the function switched off in software [s2].

The feature returned in August 2025 through a redesign that changed where the computation happens: the watch's sensor still captures the signal, but the oxygen-saturation value is calculated on a paired iPhone and displayed there rather than on the watch [s2]. Masimo challenged the workaround, and in March 2026 an ITC judge ruled that Apple's revised implementation did not infringe the patents at issue [s3]. For roughly 19 months, whether a US owner could see a blood-oxygen number depended not on physiology but on litigation.

What a wearer should take from it

Two lessons survive the legal noise. The first is that a feature's presence on the watch says nothing about its validation. The blood-oxygen reading was pulled and restored without any change in what it can or cannot measure clinically, because its status was never medical to begin with. The second is that the skin-tone bias is real, documented, and not fixed by moving the maths to a phone. A wearable oximeter can be interesting; it should not be the thing a person leans on to decide whether their oxygen is dangerously low.

The contrast with the watch's cleared features is instructive. Apple submitted the ECG and the atrial-fibrillation, hypertension and sleep-apnea notifications to the FDA and published validation data for each; the Blood Oxygen app went to market as wellness, under the regulatory carve-out that lets devices report physiologic signals without medical claims. The number on the screen looks the same either way. The evidence behind it does not.