Abstract
Objective: To examine the epidemiologic theory of screening as it applies to low prevalence disorders, such as schizophrenia, in order to identify the tasks required for primary and secondary prevention.
Method: Review of principles of screening, computation of prevented fraction for varying sensitivities, specificities and prevalences of disease, and review of prevalence of schizophrenia in Australian general practice.
Results: There is no currently available efficient method of screening for schizophrenia or for prodromal symptoms. From the genesis of disease to eventual outcome, the milestones that are passed in the case of schizophrenia are uncertain in their nature and the intervening time periods are of uncertain and possibly varying duration. The extent of false positives and negatives in low prevalence disorders is high unless the specificity is very high.
Conclusion: It may be feasible to screen for behaviours that are precursors to schizophrenia; however, screening depends upon the existence of a reliable screening instrument that can be shown to discriminate accurately between diseased and disease-free individuals. Development of a method for screening requires comparison against formal clinical assessment of both screen positives and screen negatives. For low prevalence disorders the predictive values may be low unless specificity is high.