Scientific basis of screening in early detection

The basis of screening for cancer is related to the natural history of the disease, to the screening test, to the treatment of preinvasive lesions, and to the application of a screening program at a population level. It is not only the test but the total program that should be valid, ie, detect both those having a preinvasive abnormality and those being healthy. The only reliable indicator on the efficacy of a screening program is the reduction in the mortality from the cancer subjected to screening. The reduction in mortality should be assessed by a randomized preventive trial. There are several examples on inconclusive evidence stemming from observational studies and based on indicators other than mortality.