How can a doctor assess the decision–making capacity of patient?

Assess the patient’s ability to reason and deliberate about treatments and their consequences. The patient should be able to express their goals and values and explain their choices by reference to these values and goals. Sometimes a patient is unable to give reasoning for his or her decision; this makes it difficult to consider the patient fully capacitated. In addition, the patient’s behavior should be consistent with his or her general character i.e. the decision should not be widely "out–of–character". There may be good reasons for a complete "change of heart" but at least preliminarily consistency with past preferences is a reasonable test.

If a patient is not able to fulfill these requirements, then his or her capacity to make health care decisions is questionable.

(Ref: Veatch RM, ed. Medical Ethics, 2nd ed, Jones and Bartlett, Boston 1997)