Patients have the right to refuse medical care if they possess what's called "decision-making capacity." In order for a patient to possess decision-making capacity, she must possess four subcapacities. She must be able to understand the relevant information, appreciate the medical consequences of the situation, reason about treatment options, and communicate a choice.
Understanding the relevant information means grasping the fundamental meaning of information communicated by the medical team.
Appreciating the medical consequences of the situation means acknowledging the medical condition and likely consequences of treatment options (including the consequences of non-treatment).
Reasoning about treatment options means engaging in a rational process of manipulating the relevant information. This includes being able to weigh the risks and benefits of treatment options rationally.
Communicating a choice means clearly indicating the preferred treatment option
Importantly, a patient must possess all four subcapacities to possess decision-making capacity.
If a doctor deems you to lack decision-making capacity, then medical decisions are to be made in conjunction with your surrogate decision-maker, which is a person who makes medical decisions on your behalf. It's usually a family member.
The first responsibility of surrogate decision-makers, including health care agents, is to use what's called "substituted judgment," meaning they're supposed to tell the doctors what you would want.
If they don't know what you would want, then they're supposed to make decisions according to "the best interest standard," meaning that they're supposed to decide what's good for you.
Since different people have different conceptions of well-being, there's no consistent standard that's applied in all situations. Hence, what a surrogate decision-maker thinks is in your best interest may not be what you would think is in your best interest. This is why it's important for you to communicate your values and wishes to whomever your surrogate decision-maker is.
To read about some complicated cases where having a health care agent would've helped, click here.