Let's say a person has allergies (such as penicillin). He is in good health, currently. However, if he were unconscious, EMT technicians would want to know about such info as this would affect the type of treatment he receives both during transport as well as while in E.R. until someone who knew him could supply necessary medical history.
Could he wear a med. emergency bracelet that contains this info on Shabbat? My understanding of the rules of tachshit (jewelry) is that it must be something that is used to "adorn" someone such as a gold bracelet or earings. This bracelet is not for "adornment", so I'm uncertain if this is permitted. Also, the person doesn't have even a safek piku'ach nefesh - at least not currently. He is in good health.
For that matter, I would apply this question to anyone, as the bracelet has emergency contact info, even if it has no personal med. info. This would be useful for an EMT as the relative can supply vital medical history, quickly.
In all this, I am assuming that there is no eruv.