Very scifi idea that is equally very plausible in the real world. I can think of more possibilities for what the tattoos could be made to do, like animate in time with heartbeat or pulse making it easier for medical personnel to count that (maybe even include numbers sef) -- which would also be useful for fitness people checking heart rate versus exertion.
Another trick could be it acting as a reminder for medications (like blood pressure or blood sugar) ... 🤔 ... although by then people will probably have internal drug dispensing implants that auto- release the medicine once the BP goes too high 🤔