But, variations in patients' physiological and psychological adjustment, and in the quality of care they receive, make highly uncertain the outcomes of any treatment for any given patient.
Prognosis is especially difficult when the patient is elderly.
The inability to identify patients who will benefit from treatment arises because, contrary to popular belief, life-sustaining technologies are frequently ineffective. For acutely ill patients, aggressive treatment is associated with high death rate and serious complications. Patients receiving antibiotic therapy or nutritional support have a relatively high, but not necessarily predictable, chance of survival. Treatments whose goal is to control pain and suffering, even at the risk of hastening death, are regarded by many people as reasonable alternatives to aggressive life-sustaining medical treatment. Also there are legal and ethical uncertainties regarding when or how it may be appropriate to limit treatment.