BOOTSTRAP ANALYSES OF COST-EFFECTIVENESS IN ANTIDEPRESSANT PHARMACOTHERAPY

In this study, we describe 'bootstrap' methodology for placing statist ical confidence limits around an incremental cost effectiveness ratio (ICER). This approach was applied to a retrospective study of annual c harges for patients undergoing pharmacotherapy for depression. We used MarketScan(SM) (service mark) data from 1990 to 1992, which includes medical and pharmacy claims for a privately insured group of employed individuals and their families in the US. Our primary effectiveness me asure was the proportion of patients who remained stable on their init ial antidepressant medication for at least 6 consecutive months. Our p rimary cost measure was the total annual charge incurred by patients t aking the selective serotonin reuptake inhibitor fluoxetine, a tricycl ic antidepressant or a heterocyclic antidepressant. On average, fluoxe tine pharmacotherapy tended to decrease annual charges by $US16.48 per patient for each percentage increase in depressed patients remaining stable on initial pharmacotherapy for 6 months, resulting in a negativ e ICER point-estimate. However, the upper ICER confidence limit is pos itive, which means that fluoxetine treatment may possibly increase ann ual per patient charges. With 95% confidence, any such increase was no more than $US130 per patient for each percentage increase in patients remaining stable on initial pharmacotherapy for at least 6 months. On e advantage of using a bootstrap approach to ICER analysis is that it does not require restrictive distributional assumptions about cost and outcome measures. Bootstrapping also yields a dramatic graphical disp lay of the variability in cost and effectiveness outcomes that result when a study is literally 'redone' hundreds of times. This graphic als o displays the ICER confidence interval as a 'wedge-shaped' region on the cost-effectiveness plane. In fact, bootstrapping is easier to expl ain and appreciate than the elaborate calculations and approximations otherwise involved in ICER estimation. Our discussion addresses key te chnical questions, such as the role of logarithmic transformation in s ymmetrising highly skewed cost distributions. We hope that our discuss ion contributes to a dialogue, leading ultimately to a consensus on an alysis of ICERs.