Digital Biomarkers: How Wearables Are Becoming Medical Devices

Some wearable features can help detect or monitor a health problem. Others are useful mainly for personal tracking. The difference is not whether the number came from a smartwatch. It is what that particular feature measures, how well it was tested, and what the maker says it is for.

A Digital Biomarker Is More Than a Number on a Screen

A digital biomarker is a measurable signal collected through a digital tool and used to indicate a biological process or response. Heart rate, movement, and glucose are possible inputs. A branded readiness score is an interpretation built from inputs. Before treating either as a health finding, ask whether the measurement and its claimed use have been checked against a suitable reference.

For everyday choices about sleep and activity, see Wearables for Longevity Tracking. This page focuses on the stronger claim: when a digital feature is intended to support a medical decision.

The Apple Heart Study Shows Both the Value and the Limit

In the 2019 Apple Heart Study, 419,297 participants were enrolled and 0.52% received an irregular-pulse notification. Among 450 notified people who later returned a usable electrocardiogram patch, 34% had atrial fibrillation recorded on that patch. The patch was applied about 13 days after the alert on average. No rhythm recorded later does not prove that the earlier alert was false, because an irregular rhythm can come and go.

The lesson is practical: a notification can be a reason for clinical follow-up. It is not a diagnosis by itself. The study tested one algorithm and one follow-up process, not every watch feature or every person wearing one.

Clearance Belongs to a Feature and Its Intended Use

A device can offer both general wellness features and functions reviewed as medical devices. The FDA's general wellness guidance draws a line around low-risk products intended to encourage a healthy lifestyle rather than diagnose or treat disease. A cleared feature has been reviewed for its stated use. Clearance does not validate every other number in the same app, and it does not mean the device can replace a clinician.

• Irregular-rhythm feature: check the exact feature's clearance and intended population. An alert calls for clinical confirmation, not self-diagnosis.
• Blood oxygen estimate: check whether the specific product is marketed for wellness or medical use. The FDA notes that even medical pulse oximeters have limits affected by factors including circulation and skin pigmentation.
• Continuous glucose monitor: over-the-counter does not mean unregulated. The FDA cleared the Stelo system in 2024 for adults who do not use insulin and do not have problematic low blood sugar. Its labeling does not make it a replacement for a different device or care plan.
• Readiness or recovery score: find out which inputs and outcomes were tested. A proprietary score can be useful for habits without being a validated diagnosis or a medical device.

What the Software Adds

Sensors collect imperfect signals. Software filters motion and other noise, combines measurements, and decides when to show a result. That interpretation is part of the product being evaluated. Two devices that measure the same thing can give different answers, and an update to the algorithm can change performance. More data and a more elaborate model do not remove the need to test the finished feature in the people and situations where it will be used.

Four Questions Before Trusting a Health Feature

1. What does the feature actually measure, and is the displayed value measured directly or estimated?
2. What was it compared with, and in which people was it tested?
3. What is its stated purpose: general wellness, screening, monitoring, or diagnosis?
4. If it flags a problem, what confirmation or follow-up does the maker recommend?

These questions also help separate a useful trend from a sales claim. The signal may be worth discussing with a healthcare professional even when the app cannot tell you what caused it.