There is another important distinction between competence and decision making capacity. Generally speaking, competence is a more global concept. When a patient is declared incompetent in court, it is often for a variety of purposes (property, health care decisions etc.). Decision making capacity is specific to the decision at hand. A patient may therefore be capable of making some decisions, and simultaneously incapable of making others. It all depends upon how well the patient can understand and appreciate the situation, and the consequences of any decision. Furthermore, the patient’s capacity may fluctuate from day to day or hour to hour. Clearly this is a more flexible concept than competence, but more limited (particularly in those situations where a patient frankly refuses a treatment).
Finally, a corollary of the definition of capacity is its practical application and determination. Since it is specific to a particular decision, there is often no way to assess capacity without asking the question at hand. Unless a patient is comatose or otherwise non-communicative, an attempt must be made to address the specific question. Given the patchy nature of decision making capacity, a patient might well surprise you with a reasonable understanding of the decision to be made. By observing how a patient manipulates the information and attempts to apply personal values and attitudes will help make the determination. While one must not necessarily agree with the decision, observing the process will help guide the assessment. For example, a patient might refuse a procedure because of intense fear of a bad outcome (based upon stories from friends). Another patient might refuse a treatment because of delusional thinking or frank hallucinations. We would probably find the former patient capable, but not the latter. Generally speaking, a psychiatry evaluation is not necessary (or even helpful) unless there is a question about mental illness contributing the possible incapacity. Thus, when evaluating a patient with an apparent psychiatric disorder, a psych consult may clarify things. In such cases, a psychiatrist is being (appropriately) used for his or her medical expertise. Psychiatrists have no legal authority to make decisions about competence.