Withings ScanWatch Accuracy: FDA-Cleared AFib, Analog Face
An analog watch face with an FDA-cleared ECG and pulse oximeter underneath, the most medically-adjacent hybrid design compared on this site.
Withings's approach is clinical-grade sensing inside a watch that looks like a normal analog watch, no always-on display, a real hybrid design choice most competitors here don't make. Below is what independent research and Withings's own documentation say about each metric it tracks, and every core health feature works without a subscription.
How Does Withings ScanWatch Measure HRV?
Withings measures HRV in milliseconds during overnight rest, when the body is stable, treating it as a marker of stress resilience, cardiovascular health, and readiness to perform.
What Withings calls it
"Heart Rate Variability (HRV)." A free-living-activity validation study of Withings ScanWatch heart rate measurement exists in the peer-reviewed literature, though it addresses heart rate specifically rather than HRV in detail; a dedicated peer-reviewed HRV accuracy study for the ScanWatch specifically wasn't found during research for this page.
What independent research found
HRV itself untestedNo dedicated validation of ScanWatch's HRV output against a reference standard turned up in this search, so the underlying signal HRV is calculated from is what the evidence actually speaks to. A small 2022 free-living study of 7 adults found the ScanWatch measured heart rate with under 10 percent mean absolute percentage error across most daily activities against a Polar H10 chest strap, best during sedentary activities like desk work and eating (under 5 percent error) and considerably weaker during walking, where agreement with the reference dropped to a weak concordance correlation of 0.164.
A 2025 study built specifically to test accuracy during rapid heart-rate changes found the ScanWatch performed acceptably at steady state but, like every wrist-worn device in that study, lost accuracy during the transitions themselves. Both studies point the same direction: the overnight, stable-heart-rate conditions HRV is actually measured under are close to this device's best-case scenario, but neither study validates the HRV calculation itself.
Giggins et al., JMIR Formative Research, 2022 (doi:10.2196/34280) · Van Oost et al., Sensors, 2025 (doi:10.3390/s25206319)How Accurate Is Withings ScanWatch's ECG?
ScanWatch takes an on-demand, medical-grade ECG reading via a finger touch on the bezel, screening for atrial fibrillation. It received FDA clearance in October 2021 for both ECG and SpO2 measurements specifically for AFib detection.
What Withings calls it
"ECG," clinically validated for AFib detection as part of its FDA clearance, the same regulatory bar Apple's and Withings's competitors clear for their own on-demand ECG features.
What independent research found
Strong AFib, weaker QTcAFib detection and QT-interval measurement are two different questions, and the independent evidence splits cleanly between them. On AFib detection specifically, a 2022 clinical validation running 163 patients through five direct-to-consumer smartwatches against a physician-interpreted 12-lead ECG found the ScanWatch's sensitivity and specificity (91 percent and 95 percent) comparable to an Apple Watch 6 (90 percent and 95 percent) tested in the same cohort. A separate study of 176 adults with congenital heart disease found the ScanWatch's AFib classification 100 percent accurate among the tracings it was able to classify, with fewer inconclusive results (5 percent) than a 6-lead Kardia device (31 percent) tested alongside it.
QT-interval measurement is the weaker half. A 2022 validation of 317 cardiology patients found only fair-to-good agreement between the ScanWatch's automated QTc and a manually measured 12-lead reference (bias 6.6 ms, but limits of agreement spanning -59 to 72 ms), and the watch could only calculate a QT interval automatically in 56 percent of patients, concluding the algorithm "needs improvement to be useful in a clinical setting." The congenital-heart-disease study found the same pattern from a different angle: QTc accuracy was acceptable in only 51 percent of ScanWatch readings, worse than a 6-lead Kardia (74 percent) or a precordial-lead device (70 percent) in the same comparison, even though the ScanWatch led on AFib detection. A 2024 pediatric validation found the same split by parameter rather than by device: automated heart rate was excellent (intraclass correlation 0.97), but automated QTc was only moderate (0.53), improving to 0.84 only when measured manually rather than by the watch's own algorithm.
Mannhart et al., EP Europace, 2022 (doi:10.1093/europace/euac053.572) · Pengel et al., Cardiology in the Young, 2023 (doi:10.1017/s1047951122002244) · Mannhart et al., Frontiers in Cardiovascular Medicine, 2022 (doi:10.3389/fcvm.2022.906079) · Ernstsson et al., European Journal of Pediatrics, 2024 (doi:10.1007/s00431-024-05648-7)Does Withings ScanWatch Detect Sleep Apnea?
ScanWatch tracks blood oxygen, heart rate, movement, and respiratory rate overnight to flag breathing disturbances that can be signs of sleep apnea, using an algorithm built specifically for that purpose.
What Withings calls it
"Breathing Quality Assessment," built from a motion sensor, blood oxygen sensor, and HR sensor together, distinct from the separately named Respiratory Scan feature that supplies overnight SpO2 data to it. This is a real, important distinction Withings itself is explicit about: the SpO2 feature is FDA-cleared for AFib detection, but sleep apnea detection itself has not received separate FDA clearance as of this writing. The watch can aid in noticing signs consistent with breathing disturbances; it isn't a cleared diagnostic for sleep apnea specifically.
What independent research found
Emerging, measurability gapsA 2024 multicenter study tested exactly the scenario this feature is meant to help with: detecting clinically significant hypoxia in 200 patients hospitalized with COVID-19, comparing the ScanWatch and an Apple Watch Series 7 against ward-based oximetry. The ScanWatch caught more true cases of hypoxia than the Apple Watch (68.5 percent sensitivity versus 34.8 percent), but with more false alarms (80.8 percent specificity versus 97.5 percent) and a weaker overall correlation with the reference (r=0.51 versus r=0.61). Overall accuracy was 78.5 percent for the ScanWatch against 84.9 percent for the Apple Watch, and the study's authors concluded neither device was accurate enough to replace standard oximetry for this use.
A separate 2023 study of 49 adults found the ScanWatch had the highest rate of failed SpO2 readings of the devices it tested, only 69.5 percent of attempted measurements succeeded, worse measurability than either an Apple Watch or a Garmin tested in the same study, though accuracy did not vary significantly by skin tone in that sample.
Rajakariar et al., Mayo Clinic Proceedings: Digital Health, 2024 (doi:10.1016/j.mcpdig.2024.02.001) · Jiang et al., PLOS Digital Health, 2023 (doi:10.1371/journal.pdig.0000296)What Does Withings's Readiness Indicator Measure?
The Readiness Indicator synthesizes three components into a daily Readiness Breakdown: Recovery (the previous night's sleep data), Effort (daily steps and activity), and Night Vitals (sleep temperature, SpO2, and respiratory rate where available), Withings's answer to Oura's Readiness Score or Whoop's Recovery Score. Readiness cannot be computed without Recovery and Effort data; Night Vitals is supplementary.
What Withings calls it
"Readiness Indicator," broken into Recovery, Effort, and Night Vitals sub-statuses. The exact internal weighting formula wasn't clearly documented in verification for this page.
Correction, 2026-09-19
This section previously described the Readiness Indicator as drawing on nightly HRV, temperature, sleep, and breathing data. Withings's own current documentation describes three components instead: Recovery (sleep), Effort (daily activity, which the prior description omitted entirely), and Night Vitals (temperature, SpO2, respiratory rate). HRV is not named as a direct Readiness input in Withings's own description. Corrected to match.
Worth knowing
A real peer-reviewed evaluation found composite recovery scores across brands are not methodologically equivalent to each other, despite similar-sounding names and overlapping inputs. No independent validation of the Readiness Indicator specifically was found during research for this page.
How Does Withings ScanWatch Track Temperature?
ScanWatch tracks skin temperature overnight, feeding it into the Readiness Indicator's Night Vitals component alongside SpO2 and respiratory rate.
What Withings calls it
"Sleep Temperature," part of Night Vitals rather than a standalone branded score.
What Withings ScanWatch Doesn't Do
Two real absences tied to the hybrid analog design.
No always-on color display
ScanWatch is a real analog watch with a small secondary display for metrics, a deliberate design trade-off, not a software gap, but a real difference from every other device compared on this site.
No FDA clearance for sleep apnea detection specifically
Covered above: the underlying sensors can flag signs consistent with breathing disturbances, but that specific claim hasn't cleared the same regulatory bar as the ECG and AFib-detection features.
If an analog watch face with genuine FDA-cleared ECG and SpO2 sensing underneath matters more than an always-on smartwatch display, ScanWatch is a real, distinctive fit. If a full always-on display or a broader third-party app ecosystem matters more, that's not what this specific design offers.