Smartwatch Blood Oxygen (SpO2) Accuracy Against Arterial Blood (SaO2)
Question assessedDo consumer smartwatches and rings measure blood oxygen saturation (SpO2) closely enough to arterial blood oxygen (SaO2) to be meaningful?
At normal and moderately low oxygen levels, some tested watches land close to a medical reference, but the evidence splits by device and by how low oxygen falls. Two Garmin models failed their tests outright, and an Apple Watch Series 7 lost accuracy once oxygen fell well below normal, which is where the reading matters most.
Why this is rated Uncertain
SpO2 is the estimate a pulse oximeter makes from light passing through skin; SaO2 is the oxygen saturation of arterial blood measured directly by a blood gas analyzer. The question is how close the first comes to the second when the sensor is on a wrist or finger ring rather than a clinical finger clip.
Several direct tests affirm close agreement. Under deliberately induced low oxygen, an Apple Watch 8, a Samsung Galaxy Watch 5 and a Withings ScanWatch all stayed within the 4 percent accuracy limit that medical oximeters must meet, and in hospital patients with a COPD flare an Apple Watch 6 differed from arterial blood by less than half a percentage point on average.
Other tests contradict it. A Garmin fenix 6 failed its pre-set accuracy criteria for blood oxygen under every condition tested, and a Garmin Forerunner 245 read too high more than half the time at simulated altitude. An Apple Watch Series 7 study found the error grew once saturation fell below 88 percent. Because the evidence contains both clean affirmations and core-hitting contradictions, the rule treats it as genuinely disputed and rates it Uncertain.
- Anchor-grade evidence affirms the core relationship.
- Multiple supporting studies are consistent with the core relationship.
- An anchor study is present but its own reported finding does not cleanly affirm this claim, so it does not count toward the Established threshold.
- A clean anchor supports this claim, but an unresolved conflicting study contradicts it on the core relationship, so the evidence is treated as in genuine dispute.
- Some evidence conflicts on secondary points that do not change the core direction.
Key results
- Apple Watch 8, Samsung Galaxy Watch 5 and Withings ScanWatch all met the 4 percent root-mean-square limit of ISO 80601-2-61 against a Masimo Radical-7, at normal oxygen and below 90 percent saturation, in 18 healthy adults; Apple Watch 8 had the lowest bias. (Walzel et al., Sensors, 2023)
- In 167 patients with an acute COPD flare, Apple Watch 6 differed from arterial blood by a mean of 0.458 percent (SD 2.78, limits of agreement -5.9 to 5.0) and correlated with a medical pulse oximeter at r = 0.89. Patients whose watch needed more than 2 minutes or eight attempts were excluded. (Arslan et al., Irish Journal of Medical Science, 2024)
- Below 88 percent saturation, Apple Watch Series 7 had a root-mean-square error of 5.82 percent against 3.52 percent for a clinical finger oximeter, and landed within 2 percent of arterial blood 32.14 percent of the time against 49.03 percent. Both devices overestimated oxygen. (Jiang et al., JMIR Formative Research, 2026)
- Garmin fenix 6 failed to measure blood oxygen accurately under any condition, with a concordance correlation of 0.10 against a medical-grade oximeter, below the study's pre-set threshold of 0.7. (Carrier et al., Sensors, 2025)
- Garmin Forerunner 245 read blood oxygen higher than the reference with an error rate above 50 percent overall and above 80 percent at simulated altitudes over 4,800 m, in 10 healthy adults. (Hermand et al., International Journal of Sports Medicine, 2021)
What this finding covers
Applies to: Wrist-worn and ring-worn consumer pulse oximeters compared with arterial blood gas or a clinical reference oximeter, in healthy adults under induced low oxygen and in hospital patients. Device-level results apply only to the models named.
Does not cover: Apple Watch Series 9 and later, Ultra models and the US version that calculates on the paired iPhone, none of which has a published independent test; diagnosis of any condition; children; finger-clip oximeters except as references.
Limitations
- Most tests are small, from 10 to 18 healthy adults under induced low oxygen; the largest clinical sample is 167 patients with one condition.
- Accuracy depends on how low oxygen goes, and several studies did not test below 88 to 90 percent, where clinical decisions are made.
- One of the main reference oximeters is made by Masimo, a company in patent litigation with Apple over the blood oxygen feature.
- Skin pigmentation was not tested in the arterial blood studies cited here. One cited Apple Watch study tested it against a fingertip oximeter in 49 patients and found no significant difference in error across skin tone groups (Jiang et al., PLOS Digital Health, 2023); see the related finding on skin tone.
Evidence base
29 studies scored · 6 anchor · 4 conflicting · 19 supporting · 2 letters or comments not counted · Evidence last evaluated Oct 25, 2026
Anchor studies
The studies the rating rests on.
- Performance of Wearable Pulse Oximetry During Controlled Hypoxia Induction: Instrument Validation Study.
- Accuracy of the Apple Watch in measuring oxygen saturation: comparison with pulse oximetry and ABG.
- Performance of a commercial smart watch compared to polysomnography reference for overnight continuous oximetry measurement and sleep apnea evaluation.
- Evaluation of Leading Smartwatches for the Detection of Hypoxemia: Comparison to Reference Oximeter.
- Validation of the Withings ScanWatch as a Wrist-Worn Reflective Pulse Oximeter: Prospective Interventional Clinical Study.
- Performance evaluation of a wrist-worn reflectance pulse oximeter during sleep.
Conflicting studies
Studies whose findings run against the claim.
- Validation of Aerobic Capacity (VO2max) and Pulse Oximetry in Wearable Technology.
- Accuracy of Smartwatch Pulse Oximetry Measurements in Hospitalized Patients With Coronavirus Disease 2019
- Accuracy and Reliability of Pulse O2 Saturation Measured by a Wrist-worn Oximeter.
- Reliability of Smartphone Pulse Oximetry in Subjects at Risk for Hypoxemia.
Supporting studies
Studies consistent with the claim that do not carry the rating on their own.
Show all 19 supporting studies
- An Adjustable Smart Ring to Monitor Pulse Rate and Peripheral Blood Oxygen Saturation.
- Reliability and Safety of Smartwatch Blood Pressure and Oxygen Saturation Measurements in Older Adults: Instrument Validation Study.
- Clinical evaluation of a smart wristband for monitoring oxygen saturation, pulse rate, and respiratory rate.
- Continuous SpO 2 Monitoring Using Reflectance Pulse Oximetry at the Wrist and Upper Arm During Overnight Sleep Apnea Recordings.
- Evaluation of Pulse Oximetry Accuracy in a Commercial Smartphone and Smartwatch Device During Human Hypoxia Laboratory Testing.
- The Accuracy of Readily Available Consumer-Grade Oxygen Saturation Monitors in Pediatric Patients.
- Validation of Polar Elixir™ Pulse Oximeter against Arterial Blood Gases during Stepwise Steady-State Inspired Hypoxia.
- Accuracy of the Apple Watch Oxygen Saturation Measurement in Adults: A Systematic Review
- Investigating the accuracy of blood oxygen saturation measurements in common consumer smartwatches
- Pulse Oximeter Performance, Racial Inequity, and the Work Ahead.
- Accuracy and Reliability of Commercial Wrist-Worn Pulse Oximeter During Normobaric Hypoxia Exposure Under Resting Conditions.
- A multicentre prospective observational study comparing arterial blood gas values to those obtained by pulse oximeters used in adult patients attending Australian and New Zealand hospitals.
- Four Types of Pulse Oximeters Accurately Detect Hypoxia during Low Perfusion and Motion.
- The Accuracy of 6 Inexpensive Pulse Oximeters Not Cleared by the Food and Drug Administration: The Possible Global Public Health Implications.
- "Motion-resistant" pulse oximetry: a comparison of new and old models.
- Meta-analysis of arterial oxygen saturation monitoring by pulse oximetry in adults.
- Pulse oximetry: principles and limitations.
- Accuracy of pulse oximetry in pigmented patients.
- Potential errors in pulse oximetry. I. Pulse oximeter evaluation.
Version history
| Version | Date | Rating | Change |
|---|---|---|---|
| 1.0 | October 24, 2026 | Uncertain | First published. |
| 1.1 | October 25, 2026 | Uncertain | Corrected a limitation: one cited Apple Watch study did test skin tone, against a fingertip oximeter rather than arterial blood. |
Rating history on the research desk: Established (July 21, 2026), then Emerging (September 4, 2026), then Uncertain (September 17, 2026).
Cite this finding
KnowYourPrime Evidence Registry, Finding KYP-ER-0001, version 1.1, reviewed 2026-10-25. https://knowyourprime.com/evidence-wearable-blood-oxygen-accuracy
Machine-readable: JSON · Markdown · All findings
Related
- Research desk: the blood oxygen research desk entry for this question, with every scored study and its role
- Related finding: Pulse Oximeter Accuracy and Skin Tone (KYP-ER-0002)
- On this site: How Accurate Is the Apple Watch's Blood Oxygen Reading?
- On this site: SaO2 vs SpO2: One Comes From a Blood Test, One Doesn’t
- On this site: Galaxy Watch 4 Oxygen Was About 2 Points Off in Sleep Tests
- On this site: Can You Trust Your Watch's Blood Oxygen at Altitude?
This is KnowYourPrime's own assessment of the published evidence, rated under the KnowYourPrime evidence methodology. The rating is set by a fixed rule from the roles of the studies above and moves only when the evidence or the rule changes. Every change to the rating or the text is published as a new dated version. To report an error, write to corrections@knowyourprime.com.