It gets a little more complicated unfortunately:

"At one time, type O negative blood was considered the universal blood donor type. This implied that anyone — regardless of blood type — could receive type O negative blood without risking a transfusion reaction. However, we now know even type O negative blood may have antibodies that cause serious reactions during a transfusion."

http://www.mayoclinic.org/tests-procedur...pe/faq-20058229

Medical technology is just about at the point where artificial blood substitutes that are easy to store, have a long shelf-life, don't need matching to patients, and perform most or all of the essential functions of real blood are becoming available.

The horrors of war inevitably contribute to advances in medical science. We've been more orless at war for a very long time now, and this has driven remarkable advances in fields like emergency care, telemedicine and prosthetics.

Like the OP, I carry it in my wallet. Emergency contact information (ID is my DL), perscription drugs, etc. But, the information on it is by no means proof of identity. It's just a hand written card, and there's not much that can be done with it. The rest of the cards in my wallet... Yeah, I might freak about those. The increase in risk of the carrying a medical or ICE card is so low compared to the existing risk from the rest of the contents of my wallet, it's just not worrying about.

Now, if it was a life threatening allergy or medical condition, I would be wearing a medical alert bracelet or pendant. I wouldn't risk a medical mistake during the delay between scraping me up and checking my wallet for ID and insurance.

I agree. I discussed this with paramedics and with trauma room staff. I'm told:
1)Checking through your wallet usually waits until they have stabilized you...by which time you may have already been administered some meds (which you may react to... but a medic alert bracelet or pendant WOULD probably have already been seen and checked).
2)The ER folks tell me they almost never give blood until THEIR lab has typed/matched... even if the patient has their blood type listed.
3)ICE on your phone is (if at all) checked later in the process
4)USB flashdrives and the like - will NEVER be consulted. Their ER computers are typically locked down for cyber security, and they just aren't going to put your drive in it.
BUT.. this is just FWIW. For me, an emergency is when I run out of Powers Irish Whiskey

Nothing in your wallet or around your neck/wrist is going to do much, if anything to change the initial care in an ER.

Drug allergies and implant details (heart valves etc) are the most useful.

Blood type is pretty much useless. I can't imagine giving someone type specific blood off of a card in their wallet.

Thanks! All of this is really helpful! I'm no longer concerned about security.