ON OPTIMAL SCREENING AGES

Several chronic diseases are characterized by an initial asymptomatic stage during which, if detected by screening, they can be cured in a m ore effective way. This article considers two statistical design probl ems in screening for chronic disease: the choice of examination ages a nd the choice of the part of the population to be screened. One main g oal is capturing the trade-off between the costs of examination and th e losses due to late detection, while accounting for the effects of ag e on the incidence of the disease, on mortality, and on the relative a dvantages of early detection. The problem is posed in a decision theor etic way. The model adopted considers a single individual, whose histo ry relative to the disease is represented by a discrete-valued stochas tic process. The transition structure is general, but known. The decis ion space includes all sequences of examination times, as well as no e xamination. The optimality criterion accounts for the cost of examinat ions and, in a general way, for the goals of screening in terms of mor tality and morbidity. So the optimality criterion may depend on surviv al, quality-adjusted life years, cost of care, and so on, as well as o n combinations of these factors. A general solution and computational algorithms are derived by extending to this context methodologies deve loped in reliability theory. The case in which the test used for scree ning has high sensitivity is studied in detail; then the determination of the optimal schedule and stopping rule is reduced to a one-dimensi onal optimization problem by recursive dynamic methods. Moreover, suff icient conditions for screening to be increasingly worthwhile with age are derived. Under these conditions, the optimal number of planned ex aminations is either 0 or infinity, and there is a simple check to est ablish whether or not to screen without having to compute the optimal schedule. Under slightly stronger conditions, the times between examin ations decrease and the optimal solution is unique and easy to compute . The conditions mentioned relate increasing times between checks to p roperties of the failure rate of the time to onset of the disease and of the relative incidence of the disease. Applications of the results include developing guidelines for screening for breast and cervical ca ncers-currently a controversial issue.