How Accurate Are Smartwatches for AFib Detection? | Limits

One number hides the whole argument. Samsung’s on-demand ECG correctly flagged 96.0% of AFib recordings in its validation testing, while the same company’s background irregular-rhythm alerts caught 68.0% of people whose AFib ran for an hour or more. Same sensor, two jobs, a wide gap.

Whether a smartwatch is accurate for AFib detection rests less on the brand than on which feature fires and whether the reading comes back readable. A quiet watch proves nothing at all. Here is the data worth trusting, the failures it hides, and what to do when your wrist buzzes.

Smartwatch AFib Detection: The Numbers Behind the Alerts

An on-demand ECG is the more accurate of the two features; the background irregular-rhythm alert is the one likelier to miss an episode. Samsung’s Health Monitor ECG App, version 5.1, runs both, and its published validation keeps them separate.

The ECG you start yourself hit 96.0% sensitivity — 435 of 453 AFib recordings correctly classified — and 98.7% specificity, with 682 of 691 sinus-rhythm recordings classified correctly. The Irregular Heart Rhythm Notification feature, which samples your pulse periodically while you sit still, scored 68.0% sensitivity and 98.8% specificity. Samsung’s ECG App instructions for use lay out both sets of numbers.

Samsung is blunt about the passive side: it checks while the wearer is still, needs roughly an hour of irregular rhythm before a notification fires, and is not built to flag every episode. No buzz does not mean no AFib.

Compatibility narrows the field. The ECG app needs a Galaxy Watch4 or later running Wear OS 4.0 or later, and Samsung ties availability to watch model, paired phone, software, and country. No separate subscription is involved. If you are shopping for this one feature, our tested AFib monitoring watch roundup shows which models actually enable it.

Why Inconclusive Readings Matter More Than the Headline Number

A recording the algorithm cannot classify gives you nothing, and those recordings are common enough to reshape what the feature is worth. That is the gap between a validation table and a Tuesday morning on your wrist.

Most performance measures cleared 90% once readings were classifiable, and Apple, Samsung, and Huawei watches came out broadly comparable while other brands trailed. The authors concluded that smartwatches hold potential for screening and timely detection, not for definitive diagnosis.

The percentages describe recordings, not people. A 96.0% sensitivity figure means the algorithm labeled 435 of 453 known AFib strips correctly; it says nothing about whether your watch sampled during the episode that mattered.

Four things drive that gap:

• Movement. The passive system samples when you are still, so a brisk walk or a fidgety hand produces nothing usable.
• Contact. A loose band or dry skin weakens the optical signal — tighten the strap before you record again.
• Rhythm noise. Ectopic beats and other arrhythmias can look irregular without being AFib.
• Timing. A 20-minute episode may never get sampled, so no alert ever fires.

What Do You Do After an Irregular Rhythm Alert?

Record an ECG immediately, then give the report to a clinician. The notification is a reason to look at your rhythm, not a finding you can act on alone.

1. Turn on the Irregular Heart Rhythm Notification inside Samsung Health Monitor and allow the app to send alerts.
2. When the watch alerts you, open Samsung Health Monitor and start an ECG recording. Sit still with your forearm on a table and follow the on-screen finger placement on the watch’s side buttons.
3. Let the recording finish. The strip syncs to your paired Galaxy phone, and the app builds a PDF report you can save or email.
4. Send the PDF to your doctor, along with the date, time, and what you felt when the alert fired.

You will know the recording worked when the app names a rhythm and offers the PDF export; a failed one simply asks you to try again.

A log in your phone notes — alerts, symptoms, times — gives a cardiologist weeks of pattern instead of one snapshot.

Chest pain, fainting, severe breathlessness, or stroke signs are emergencies; call 911. Samsung states the ECG App cannot diagnose all cardiac conditions or detect a heart attack, and no watch is built to sort those symptoms.