Physicians should assess the mental capacity of a patient whenever the patient is required to make a decision, states the American Family Physician. However, a patient's mental capacity should also be carefully assessed when the patient has a change in mental status, if the patient refuses recommended treatment without good reason, or when a patient has hastily consented to an invasive or risky treatment.
Laws in the United States, Canada and the United Kingdom protect the patient's right to make his own decisions unless they are proved to legally incompetent, according to the Journal of the Royal Society of Medicine. Legal competence requires the ability to understand information and the circumstances in the situation, to reason and deliberate, and to communicate a decision.
While a change in mental capacity, refusal of recommended treatment, or hasty approval of radical or risky treatment are reasons to assess mental capacity, it does not necessarily mean that the patient is not competent enough to make his own decisions, asserts the American Family Physician. Some changes in mental capacity are temporary, and hasty consent or refusal of treatment is not always indicative of diminished mental capacity. Some patients are able to understand and consent to some treatment options, but not others.