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LETTERS TO THE EDITOR

Cost Effectiveness Ratio: An Often Misunderstood Term

Department of Anesthesiology, Duke University Medical Center Durham, NC 27710

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

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):

Tang et al. (2) calculated a number they referred to as the cost-effectiveness ratio (Table 2 of 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.

References

1. Tang J, Wang BG, White PF, et al. The effect of timing of ondansetron administration on its efficacy, cost-effectiveness, and cost-benefit as a prophylactic antiemetic in the ambulatory setting. Anesth Analg 1998;86:274–82.[Abstract]
2. Tang J, Watcha MF, White PF. A comparison of costs and efficacy of ondansetron and droperidol as prophylactic antiemetic therapy for elective outpatient gynecologic procedures. Anesth Analg 1996;83:304–13.[Abstract]
3. Udvarhelyi IS, Colditz GA, Rai A, Epstein AM. Cost-effectiveness and cost-benefit analyses in the medical literature : are the methods being used correctly? Intern Med 1992;116:238–44.
4. Detsky AS, Naglie IG. A clinician's guide to cost-effectiveness analysis. Ann Intern Med 1990;113:147–54.

Response

Department of Anesthesiology and Critical Care University of Pennsylvania Philadelphia, PA 19104-4399

Paul F. White, PhD, MD

Department of Anesthesiology & Pain Management University of Texas Southwestern Medical Center at Dallas Dallas, TX 75235-8894

In their letter, Gan and Lubarsky emphasize the use of incremental or marginal cost-effectiveness in comparing older drugs and technology. We disagree with their suggestion that there were fundamental errors in the calculations of the cost-effectiveness ratios for the antiemetic drugs we studied. In our article (1), we used a decision analysis to determine the total costs associated with strategies to prevent postoperative nausea and vomiting (PONV) and calculated the total costs to prevent PONV in one patient with each strategy. We used these values as a reflection of the cost-effectiveness of the regimen, rather than an incremental cost-effectiveness analysis, which has been used by other authors. Methods similar to the ours have been used in recent comparisons of cost-effectiveness in major journals, including:

1.the use of smoking cessation services, in which the average costs to the health plan per user who quit smoking were calculated (2)

2.shortened screening intervals for breast screening programs (3)

3.regimens for eradication of Helicobacter pylori (4)

4.routine immunization against varicella (5).

Although Gan and Lubarsky present arguments in favor of the use of incremental cost-effectiveness analysis to determine the additional costs for prevention of one additional case of PONV, this approach also has some significant limitations. For example, small changes in mean costs or efficacy can result in vast changes in the incremental cost-effectiveness ratios, even if there are no statistically significant differences between the study groups in either the costs or the efficacy. In addition, incremental cost-effectiveness ratios may not provide meaningful values in some clinical situations. Consider, for example, the cost-effectiveness of a prophylactic regimen of a hypothetical new, second-generation antiserotonin drug ("bettersetron") that prevents PONV in 85% of subjects at an overall cost of $2.25 per patient. The formula recommended by Gan and Lubarsky would result in a negative value for incremental cost-effectiveness that has no clinical meaning. These investigators would recommend the use of yet another term, "dominant therapy," for this particular situation. However, the approach used in our analyses is much simpler and is more meaningful from the clinical care perspective. Our analysis suggests that this new regimen would cost the healthcare institution $2.65 (2.25/0.85–2.65) to avoid one case of PONV, compared with $7.06 for droperidol 0.625 mg IV, and $27.65 for ondansetron 4 mg IV.

Regardless of the approach used, the message from our study remains the same: droperidol 0.625 mg IV provides antiemetic prophylaxis comparable to ondansetron 4 mg IV without increasing side effects or delaying discharge and is associated with decreased costs. Of interest, our basic findings have been confirmed in a subsequent multicenter study (6) in which Dr. Gan was one of the principal investigators.

References

1. Tang J, Watcha MF, White PF. A comparison of costs and efficacy of ondansetron and droperidol as prophylactic antiemetic therapy for outpatient gynecologic procedures. Anesth Analg 1996;83:304–13.
2. Curry SJ, Grothaus LC, McAfee T, Pabiniak C. Use and cost-effectiveness of smoking cessation services under four insurance plans in health maintenance organization. N Engl J Med 1998;339:673–9.[Abstract/Free Full Text]
3. Boer R, de Koning H, Trefall A, et al. Cost effectiveness of shortening screening interval or extending age range of NHS breast screening programme : computer stimulation study. BMJ 1998;317:376–9.[Abstract/Free Full Text]
4. Duggan AE, Tolley K, Hawkey CJ, Logan RFA. Varying efficacy of Helicobacter pylori eradication regimens : cost effectiveness study using a decision analysis model. BMJ 1998;316:1648–54.[Abstract/Free Full Text]
5. Tracy L, Cochi SL, Steven B, et al. Cost-effectiveness of a routine varicella vaccination program for US children. JAMA 1994;271:375–81.[Abstract/Free Full Text]
6. Fortney JT, Gan TJ, Graczyk S, et al. A comparison of the efficacy, safety, and patient satisfaction of ondansetron versus droperidol as antiemetics for elective outpatient surgical procedures. Analg 1998;68:731–8.

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