What Counts as a Wearable
A wearable is more than a fitness band counting steps — the category runs from consumer gadgets to medical-grade devices a doctor prescribes. Where a device sits on that spectrum changes what its numbers are actually worth.
Say “wearable” and most people picture a fitness band counting steps. That is the least consequential end of the category. The same word covers a device a doctor actually prescribes to catch a heart condition — and the two ends of that spectrum are worth telling apart before you trust either one.
Further than a fitness band on your wrist
A smartwatch counting your steps is a wearable. So is a continuous glucose monitor taped to a diabetic patient's arm, a heart-rhythm patch a cardiologist has you wear for two weeks, and a smart ring tracking your sleep overnight. What ties them together is not the shape of the hardware — it is that a sensor is on or in your body, continuously, generating a stream of readings nobody had access to before you put it on.
Two questions actually separate one wearable from another, and neither is “how expensive is it.” The first is what it measures — a step count and a heart rhythm are different kinds of signal entirely. The second is what a regulator has said about the number it produces — a distinction that decides whether a doctor can act on that number at all.
Consumer-grade versus medical-grade
The two ends of that spectrum are not just different in quality — they are built, tested, and regulated completely differently, and knowing which one you are holding changes what its number is actually worth.
Consumer-grade
Optimised for comfort and battery life. Accuracy is good enough for trends, not for a diagnosis. No clinical trial required before it ships.
Example: a fitness band's heart-rate estimate during a workout.
Medical-grade
Validated against a clinical gold-standard measurement, cleared by a regulator, and built to a specific accuracy tolerance a doctor can rely on for an actual decision.
Example: a hospital-grade pulse oximeter used during surgery.
A wellness claim versus a cleared medical claim
“Medical-grade” is a useful shorthand, but the regulatory reality underneath it runs through a specific pathway. In the United States, most fitness features — step counts, general activity, calories estimated from movement — ship under the FDA's general wellness category, which requires no premarket review at all as long as the claim stays low-risk and non-diagnostic.
- 1
A general wellness claim needs no clearance
"Tracks your daily activity" or "helps you build healthier habits" stays inside a category the FDA does not review before it ships.
- 2
A diagnostic claim needs evidence, submitted to the FDA
"Detects atrial fibrillation" is a medical claim. The manufacturer has to submit clinical validation data through a 510(k) clearance or De Novo pathway before making it.
- 3
The regulator clears the claim, not the whole device
Clearance attaches to one specific feature and its specific evidence — not to every other function the same piece of hardware happens to perform.
The different signals a wearable can actually measure
“Wearable” also hides real variety in what physical signal a device is actually reading, before any regulatory question even applies:
- •Movement — an accelerometer and gyroscope infer steps, activity type, and fall detection from acceleration patterns.
- •Optical — a light sensor infers heart rate and blood-oxygen trend from how much light reflects off blood near the skin.
- •Electrical — a single-lead ECG reads the heart's actual electrical activity, closer to what a hospital ECG captures, though with far fewer leads.
- •Electrochemical — a continuous glucose monitor reads glucose from interstitial fluid just under the skin, not from a blood draw.
- •Thermal — a temperature sensor tracks skin temperature, used as an indirect signal for things like ovulation timing or early illness.
Each of those signals has its own error profile and its own regulatory history — which is exactly why the next chapter deals with what a wearable is measuring instead of treating “wearable data” as one undifferentiated thing.
What a wearable is actually measuring
Almost nothing on your wrist measures what you think it measures directly. A smartwatch does not have a direct line to your heart — it shines light through your skin and infers your pulse from how much of that light bounces back, a technique called photoplethysmography. A step counter does not count steps; it infers them from acceleration patterns that look like a stride. Every reading you see is the output of a model guessing at the real number from an indirect signal, which is exactly the subject of the next two chapters.
The number on the screen is an estimate, not a direct readout — true of both ends of the spectrum, though medical-grade devices are held to a far tighter standard for how close that estimate has to be, and are required to prove it before they can make the claim.
Key takeaways
- A wearable is any sensor on or in your body generating a continuous stream of readings — the category runs from a fitness band to a prescribed heart-rhythm patch.
- Consumer-grade devices optimise for comfort and trends; medical-grade devices are validated against a clinical gold standard before a doctor can act on them.
- A general wellness claim needs no premarket review, but a diagnostic claim like arrhythmia detection needs submitted clinical evidence and formal FDA clearance.
- Regulatory clearance attaches to one specific feature and its specific evidence, not to every other function running on the same piece of hardware.
- Neither kind of wearable measures your body directly — a wrist-worn heart-rate sensor infers your pulse from reflected light, not a direct electrical reading.
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