I don't. I think most people don't, and I think it's something that should be on a driver's license or birth certificate, or something.
If you really need blood transfusion, I'm pretty sure bloody type is quickly and easily determined when you are brought to the hospital. No experience there, but I have to assume.....
Ah, I wonder if I will find the seller where they keep the blood for my morning vampire meals Oh yes, I should have known. Thank you for that information.