Quote Originally Posted by Erik
Why does anyone really need to know though? Hospitals keep these records and will double check and triple check before any blood transfusion is ever done.
What if you are outside your country or out of reach of your hospital and you need a blood transfusion? Knowing it can save your life.

This makes me remember a fact of the WWII: The Waffen SS usually had a tattoo of their blood type oin the underside of their left arm. After the war the Allies managed to caught some of them because of that little fact.
I've heard that at some point this was a standard practice in special ops teams, though I'm not sure.