A health app that keeps no server-side profile: what on-device means in practice, what one photo does leave the phone, and why the choice was structural.
Most health apps know more about you than you'd like. Not because anyone set out to spy — because the default architecture of a connected app is a server that holds your profile, and every feature grows around that server until removing it becomes unthinkable.
Delta was built the other way around. There is no account system. There is no profile database. The decisions that matter were made structurally, before any feature existed.
Concretely, not as a marketing phrase:
Photographing a blood panel is a hard computer-vision problem, and doing it well currently requires more compute than a phone. So: the photo of your panel is sent to Delta's analyzer, read, and discarded. The analyzer keeps nothing — no image, no extracted values, no identifier linking back to you.
That is the full list of what leaves the device, it is disclosed on the screen where it happens, and the on-device fallback (Apple Vision on iOS, ML Kit on Android) runs whenever the analyzer is unreachable, so the no-network path is never a dead end.
Everything after that first read — scoring, storage, curves, history — never leaves the phone.
A privacy policy is a promise about behavior; an architecture is a fact. A company with a profile database can promise anything today and get acquired, breached, or subpoenaed tomorrow. A product that never collected the data cannot leak it, cannot sell it, and cannot be compelled to hand over what it does not hold.
Which is also why there's no account. An account exists to attach a profile to; kill the profile and the login is pure friction — a password standing guard over nothing.
Going server-free costs something. We chose it with eyes open: no cross-device sync today, no web dashboard, and losing your phone means losing your history unless you backed it up yourself. Those are real trade-offs, and if sync matters enough we would rather build it encrypted-and-user-held than quietly grow a database of everyone's bloodwork.
Measurement tools earn trust by being checkable. Deterministic arithmetic, disclosed data flows, and an architecture that makes the promise physical — that is the whole privacy story. No fine print exists, for the simple reason that there's no server to print it on.
