PREVENTION AND SCREENING - THE DECISIONS TO BE TAKEN

the definition of the various types of prevention, of screening and of case-finding are recalled, and the particular requirements of screening within an apparently normal population are analyzed. The characteristics of a screening test are reviewed, with emphasis on the prevalence of the disease, which, together with the specificity of the test, conditions the positive predictive value. The consequences of the false positive and false negative are briefly detailed and the benefit/cost ratio is defined. Interpretation of the results is rendered difficult by the existence of certain biases including the lead-time bias and the length-time bias. These complex elements should be taken into consideration in a physician's regular case-finding in his patients.