Why no phone app can measure your blood pressure
Search the Play Store for blood pressure and you will find apps whose screenshots show a fingertip on the camera lens and a number like 128/82 appearing a few seconds later. The number is not a measurement. It is either a guess derived from something else, or a value you typed in earlier being shown back to you.
What a phone camera can actually read
Put a fingertip over the lens with the flash on and the camera sees light that has passed through your skin. With each heartbeat the volume of blood in the tissue changes, so the brightness pulses. Count those pulses and you get a heart rate. This is photoplethysmography, and it works reasonably well for pulse.
Blood pressure is a different quantity. It is the force the blood exerts against the artery wall, in millimetres of mercury, and reading it requires either a cuff that occludes the artery and detects when flow returns, or an arterial line. Brightness through a fingertip does not contain that information. A camera can see when your heart beats; it cannot see how hard it is pushing.
The same is true of blood glucose, and more so. Glucose concentration in blood cannot be inferred from skin colour with a consumer phone camera. There is no consumer phone on the market that measures it non-invasively.
So what are those apps doing?
Broadly three things, sometimes all at once:
- Measuring pulse and implying the rest. The camera reads heart rate honestly, the screen shows a blood pressure field next to it, and the marketing lets you draw the conclusion. Read the store listing to the bottom and you often find a line admitting the app does not measure blood pressure — placed after the eight screenshots that suggest it does.
- Estimating from what you told it. Age, height, weight, sex, and a pulse reading go into a formula that produces a plausible-looking number. It is not your blood pressure. It is a number shaped like one.
- Showing your own entry back. Some apps ask you to type readings in during setup, then display them in a screen that looks like a measurement result.
How to tell before you install
- Read the one-star reviews first, not the five-star ones. In this category the five-star reviews are often written by people who believe the app measured them. The one-star reviews are written by people who found out it did not. The word to search for is "doesn't measure".
- Check the permission list for the camera. A pure log needs no camera permission at all. If a blood pressure app asks for the camera, it intends to point it at your finger.
- Look for a cuff in the screenshots. A photograph of a real upper-arm monitor in the store listing, in an app that has no way to talk to that monitor, is there to transfer the credibility of the device to the app.
- Find the disclaimer. Most of these apps do carry one, because they have to. If the disclaimer contradicts the screenshots, believe the disclaimer.
What to do instead
Take the reading with a validated upper-arm monitor, and use the phone for what the phone is good at: keeping the numbers, doing the arithmetic, and producing something a clinician can read. That division of labour is boring and it is also the only one that produces trustworthy data.
A few habits make home readings worth recording at all: sit for five minutes first, feet flat, arm supported at heart height, no coffee or cigarette in the previous half hour, and take two readings a minute apart at the same times each day. A log full of readings taken standing up in a hurry is worse than no log, because it looks like evidence.
Vitajot
Vitajot is the logging half and nothing else. It has no camera permission, does not claim to measure anything, and says so on its third onboarding screen before you have entered a single number. You type in what your own monitor showed; it keeps the readings with tags and notes, shows the average, lowest and highest as plain numbers, and builds a summary image plus a CSV you can hand your doctor. Everything stays on the phone, and a backup file moves it to your next one.