Cost Effectiveness Ratio

To the Editor: Tang et al. [1,2] have published two excellent and informative articles on the cost of preventing postoperative nausea and vomiting (PONV). However, their calculation of the cost-effectiveness ratio is inexact. Although it does not substantially affect the validity of their conclusions, the economic considerations may be different. First, the average cost-effectiveness ratio [3], which they calculated, should be Equation 1 Second, if a new drug or technology is compared with an older standard and the new drug is expected to provide additional benefits, but at an increased cost, then an incremental cost-effectiveness analysis should be performed, based on the incremental health benefit gained from an additional cost [3,4]: Equation 2 Tang et al. [2] calculated a number they referred to as the cost-effectiveness ratio Table 2of their study). They do not use either of the standard formula above. Their methodology understates the relative difference in treatment costs among the therapies studied. Assuming 100 patients and using their data to compare ondansetron 4 mg and droperidol 0.625 mg, the cost of ondansetron would be $1797, and the cost of droperidol $379. Sixty-five patients would have had no PONV with ondansetron, and 54 would have had no PONV with droperidol. Substituting those values into the above equation, the incremental cost-benefit ratio is $128.90 for ondansetron treatment per added patient with no PONV. That is a markedly different result than that Tang et al. suggest-a cost-effectiveness ratio of only $27.65 for ondansetron. Furthermore, the authors ignore the fact that when the ratio is negative (i.e., Treatment A is less expensive than Treatment B or Treatment B is more successful), that therapy is considered dominant (therapy with similar or greater efficacy at a lower cost) [3]. This is the case when comparing the two droperidol doses against placebo, which would both give negative ratios. In summary, Tang et al.'s article [2] should conclude that droperidol 0.625 mg is the dominant therapy compared with placebo or droperidol 1.25 mg. When comparing droperidol 0.625 mg with ondansetron, the incremental cost-effectiveness of ondansetron is $128.90 per patient with no PONV. although our suggestion does not alter their conclusions, it does alter the degree of the differences found ($128.90 vs $27.65). We believe that it is important that the standard formula for a cost-effectiveness ratio be used. Our specialty is constantly being asked to make judgments about which drugs to use and how to economically justify that use. Only correct calculations will give us the facts that we need to make considered judgements on behalf of our patients. T. J. Gan, MD D. A. Lubarsky, M.D.