The typical natural history of diseases and conditions dictates that at some point the biological onset of the disease occurs and progresses at varying rates until they become clinically evident. These rates may be as short as instantaneous, as in acute trauma, or could be life-long, as in a genetic risk factor for Alzheimer's disease. Primary prevention attempts to intercept the conditions that lead to disease onset, while secondary prevention generally relates to the early and asymptomatic detection of disease; that is, disease screening, in the hope that the trajectory toward clinical illness can be stopped or mitigated in a helpful way. When overt clinical illness is present, tertiary prevention refers to rehabilitative and other factors that deter disease progression and help return the patient to a healthier state.
Disease screening usually takes two general forms: (a) screening for proven, biological, or behavioral risk factors for diseases that lead to interventions or treatments in themselves, such as abnormal blood cholesterol or blood pressure levels; or (b) screening directly for evidence of the disease itself, followed by provision of effective treatment to cure or to prevent the progression of pathophysiological processes that will cause overt clinical manifestations. This implies that screening may be done in stages, for instance by screening for general disease susceptibility first, such as for certain demographic or anatomic characteristics, or only if informed consent for the screening procedure is obtained. Disease screening may be applied to general populations irrespective of receipt of medical care (i.e., mass screening), or to clinical populations with various characteristics.
In general, disease screening is applied to populations with a relatively low risk of the condition of interest. Because of the great increase in types of screening that have been developed, the general definition of disease screening does not fit all situations. For example, the disease may be overt and the screening is to determine the cause, as in the detection of family violence, or the condition may be overt, but not clinically explored at a primary care visit, as in the case of cognitive impairment or depression.
THE ASSESSMENT OF SCREENING TESTS
There are several criteria that aid in selecting and applying an appropriate screening test.1 (a) The disease should be common enough to warrant a search for its risk factors or latent stages because screening for excessively rare diseases may result in unacceptable cost-benefit ratios; (b) The morbidity or mortality (i.e., burden of suffering) of the untreated target condition must be substantial; (c) An effective preventive intervention or therapy must exist and should not encumber a more beneficial outcome when applied to the presymptomatic rather than to the symptomatic stage; (d) The screening test should be acceptable to the population and suitable for general, routine application. Many other criteria for an effective screening test could be added, such as maintenance of test accuracy over time ...