Short answer: Smartwatches are good at some measurements and only estimate others. Resting heart rate and step counts are usually close enough for trends; heart rate during intense exercise, blood oxygen (SpO2) and sleep stages are less reliable; a single-lead ECG can detect some rhythm problems such as atrial fibrillation on devices cleared for it, but it is not a 12-lead ECG. Treat watch data as a trend to show your doctor, not as a diagnosis.
At a glance
| Usually close enough for trends | Resting heart rate, step counts, overall sleep duration |
|---|---|
| Less reliable | Heart rate during intense exercise, blood oxygen (SpO2), sleep stages, calories |
| Watch ECG | A single-lead strip that can flag some rhythm problems on cleared devices; not a 12-lead ECG |
| What affects accuracy | Fit of the strap, skin tone and tattoos, movement, cold hands, the sensor and its algorithm |
| How doctors use it | As a trend over weeks and months, next to measurements taken in the clinic |
| What it is not | A diagnosis or a replacement for a medical test |
What affects accuracy
- Optical heart rate (light through the skin) loses accuracy with movement, a loose strap, cold skin, tattoos and some skin tones.
- SpO2 on a wrist is affected by fit, movement and perfusion; medical pulse oximeters on the finger are the reference, and even they have limits.
- Sleep stages are estimated from movement and heart rate; total sleep time is closer than the split into light, deep and REM.
- Calories are estimates from formulas.
- ECG features cleared by regulators detect specific rhythms in specific conditions; a normal watch ECG does not rule out heart problems.
Using it with your doctor
Wear the watch the same way, note the device and app version, and look at weeks, not single nights. Bring trends (resting heart rate over months, irregular rhythm notifications with dates) rather than screenshots of one reading, and mention symptoms separately. Our guide on Apple Health and Health Connect explains where the data lives.
In Anpheros
Anpheros imports heart rate, sleep, steps and other data from Apple Health, Health Connect and Samsung Health into your record with its source, and keeps heart rate as 4-hour summaries so trends stay readable. See Apple Health and Health Connect.
Related pages
- ECG (electrocardiogram) report explained
- Is a health app a medical device? EU MDR Rule 11 and intended purpose, explained
- How to measure blood pressure at home and keep readings your doctor can use
Sources
Frequently asked questions
Is smartwatch heart rate accurate?
At rest and for trends, usually close enough; during intense or irregular exercise, less so.
Can a smartwatch measure blood oxygen accurately?
Only approximately. Fit, movement and skin perfusion affect it; finger pulse oximeters are more reliable.
Is smartwatch sleep tracking accurate?
Total sleep time is reasonably close; sleep stages are estimates.
Can a smartwatch ECG replace a doctor's ECG?
No. It can detect some rhythm problems; a 12-lead ECG shows much more. See our page on the ECG report.
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