False. An individual gives up no authority or choice by doing an Advance Directive. As long as the person remains able to make decisions, his or her consent must be obtained for medical treatment. Health care providers cannot legally ignore the patient in favor of one's agent or written instruction. Indeed, in most states, health care advance directives are "springing." That is, they have no legal effect unless and until the patient lacks the capacity to make a health care decision. In a minority of states, immediately effective directives are permissible, but the maker always retains a right to override the proxy or revoke the directive.
There are situations in which a competent patient abdicates decision-making by saying, for example, "Do whatever my daughter thinks is best." However, this form of delegation of decision-making is effective only from moment to moment and needs to be rechecked at every significant decision point. Neither the proxy nor a written instruction can override one's currently expressed choice.