With carrier charges being what they are, you might need one of these.

The Blood Drop phone strap won't infuse you with blood, but providing you buy the right one, should inform doctors what your correct blood type is, should anything happen to you. 

Meant for the Japanese Market (of course), you can still get a hold of your own online for $5.82 here.

But seriously, make sure you order the one with the right blood type! It might seem like a gag gift, but if your ever actually in an accident it could become the difference between life and death.