Funny story: I'm a doctor, and sometimes I've been asked to operate on JWs without using any blood. Now, I think I'm pretty good and sometimes I think I can do some operations without losing enough blood to have to give you a transfusion, so I say okay. Here's the funny part. Often, when I meet in private right before the operation with the patient--who in front of his family and often his pastor said no way no how no tranfusions even if it kills me--he says to me, "Hey, Doc, if I really need it, give me blood."
See, principles can be, well, flexible sometimes.