General Principles of a Screening Test

Screening tests are used to identify asymptomatic individuals with early stage, potentially curable disease. The ultimate goal of screening is to alter the prognosis of a given condition by identifying patients early and instituting effective therapy. For a screening program to be worthwhile, the disease of interest must fulfill a number of criteria including, but not limited to, the following: the disease must be common; it must be accompanied by significant morbidity and mortality if not treated; therapy must alter its natural history; and there must be some benefit in terms of outcome or associated morbidity when the disease is treated in the presymptomatic versus the symptomatic stage.3 Based on these criteria, prostate cancer screening is unquestionably appropriate,3 but the question of whether or not current screening programs have been successful in altering the natural history of this disease or improving outcomes for patients remains controversial.4,5

Two factors confounding interpretation regarding results of screening programs are "lead-time bias" and "length-time bias." Lead-time bias may occur if early diagnosis results in patients living longer with a disease without ultimately affecting mortality.6 With lead-time bias, the apparent improvement in survival occurs only because of a shift in the date of diagnosis, and intervention produces no real prolongation of life. Length-bias sampling refers to the tendency of screening programs to preferentially detect more slow growing or benign cancers.3 This occurs because tumors that grow rapidly and are more aggressive typically produce symptoms and are primarily identified by routine diagnostic procedures rather than screening tests. Length-time bias occurs when there is an impression of improved survival due to screening programs, based solely on the preferential detection of slowly progressive disease.6 These biases must be taken into account when assessing the value of any screening program.3

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