Selective screening - When should screening be limited to high-risk individuals?

Issues related to selective screening are discussed. The distinction between test accuracy and program accuracy is presented in the context of impact on cost/true case detection, which in turn reflects the gain in specificity and loss in sensitivity for the total target population. When two or more risk factors are combined to define high-risk subjects, a gain in program accuracy and a relative reduction in cost/true case found ensue if there is additive interaction between these risk factors. The author also discusses periodicity of screening and emphasizes the inappropriateness of using the notion of risk for disease occurrence as a criterion to define periodicity.