I have a dogtag on my keyring with allergy and emergency contact phone numbers.  It has my allergy, wife's allergy and 2 phone numbers.  Bright red, medic alert symbol/caduceus on back. She carries same. 
That may go unnoticed on your keyring.  Normally, your keyring isn't something that often gets looked at in an emergency by medical folks.  A LEO or tow truck operator might notice it, or you could get lucky if the distinctive color or symbol catches someone's eye.  
But if you don't have a major medical problem to worry about, it might come in handy.  Do you sometimes leave your wallet but always carry your keys?  I do that myself occasionally.