Incremental cost-effectiveness analysis: the optimal strategy depends on the strategy set.

Evaluating the incremental cost-effectiveness of a technology is critical to understanding the impact of its adoption. The purpose of this study was to evaluate, using a particular example, how the specific alternatives selected for a cost-effectiveness analysis may influence the results of the analysis. In this example, we analyzed the incremental cost-effectiveness of estriol screening for Down syndrome. Model assumptions of expected costs and effectiveness were based on previously published work involving four clinical strategies, including a "do nothing" (no screening) strategy. When the analysis started with all four strategies, two of the strategies could not be considered cost-effective because of extended dominance. However, when we eliminated the "do nothing" from the strategy set because of its clinical irrelevance, all three remaining strategies might be considered cost-effective from a policy perspective. We concluded that the incremental cost-effectiveness of clinical strategies could be strongly affected by the starting point for the analysis.