The Limits of Consumer Health Data

The last chapter showed how much processing sits between a raw sensor reading and the number on your screen — noise removed, samples averaged, a pattern inferred rather than measured directly. A consumer wearable's “irregular rhythm detected” alert is the output of exactly that kind of estimate, tuned to flag anything unusual rather than to confirm a specific condition.

Flagging unusual and diagnosing a condition are not the same claim, even though a notification on your wrist can make them feel identical in the moment.

Wellness data is built to be directionally useful across millions of different bodies cheaply and continuously — good enough to notice a trend, not built to survive a courtroom or a treatment decision. Medical data is validated against a clinical gold-standard measurement, on a specific device, cleared for a specific claim. A hospital ECG and a watch's single-lead reading can describe the same heartbeat and still not carry the same evidentiary weight.

A single unusual reading is weak evidence — noise from the last two chapters can produce one on its own. A pattern that repeats across multiple days, especially paired with a symptom you can actually feel, is worth a real appointment where a clinician can order the medical-grade version of the same measurement. The wearable's real job is not to replace that appointment — it is to give you a reason to book it sooner than you otherwise would have.