In that chase scene before the title card when Max is first captured he is shown getting a tattoo done on his back that has his blood type on it (O negative) and a notice saying that he is eligible for organ donation. The camera pans on it very briefly. That's when he tries to escape
ETA: found a screenshot
http://i.kinja-img.com/gawker-media/image/upload/s--u5Jd7_2D--/c_fit,fl_progressive,q_80,w_636/d2topj9n8bu5psqqszg1.png