A patient is presumed to have decision-making capacity unless proved otherwise. In medical settings it is the physician's responsibility to determine it.
Capacity may change depending on the patient's condition and the complexity of the decision. He/she needs to be able to:
- understand relevant information and the implications of treatment choices,
- reflect in accordance with personal values and draw conclusions,
- make and communicate a choice to health care professionals.

In clinical settings, physicians should first assess the patient's mental status and then, if necessary, look for treatable physical conditions that may be interfering with his/her ability to process information and draw conclusions.

Decision-making capacity:
- can be temporarily compromised by delirium, dementia, metabolic imbalances, severe pain, and life-threatening infections;
- may fluctuate hour by hour because of drugs to control symptoms;
- shifts in levels of consciousness caused by advancing disease and psychological factors, such as the patient's denial of serious illness.

Jaffe E and Knight CF. Unipac six: ethical and legal dimensions of treating life-limiting illness. Hospice and palliative care training for physicians: a self-study program. American Academy of Hospice and Palliative Medicine. 3rd ed. 2008. p. 15.