Screening is a test applied either to an individual or to groups who, although apparently healthy, are known to be at special risk of developing a particular disease. It is used to detect treatable diseases while they are still in the early stages. For example, middle-aged women may be offered mammography to rule out breast cancer, and, in most developed countries, newborns are routinely screened for phenylketonuria (PKU), an inherited metabolic defect which can lead to severe mental retardation. Screening, therefore, is an important form of preventive health care.