Adjusting for Inflation and Higher Surgery Costs in a Cost-Effectiveness Study of Spinal Surgery

IN RESPONSE:

We thank Drs. Shusterman and Erder for their comments, which highlight both the challenges inherent in using any single threshold as a benchmark for ascribing value to health care interventions and the pitfalls in adjusting for inflation and converting between currencies. If one first adjusts for the inflation of Canadian dollars from mid-1990 to mid-2004 (an adjustment of 1.319 [1]), then the “costly” $100 000 threshold (2) becomes $131 900. Converting between 2004 Canadian and U.S. dollars (1.322 Canadian dollars per U.S. dollar as of 1 July 2004 [3]), one achieves a value of $131 900/1.322 = $99 773, or approximately $100 000 in 2004 U.S. dollars.

To overcome relying on a single benchmark, we showed results for the most important sensitivity analyses by using a cost-effectiveness acceptability curve, which graphed the proportion of times the cost per quality-adjusted life-year gained achieved willingness-to-pay thresholds ranging from $20 000 to $300 000. Such graphical presentations should be helpful to those who are skeptical about the use of any particular cost-effectiveness benchmark. This methodology was also useful for demonstrating the marked impact that higher surgery costs have on the value of surgery—an issue raised by Dr. Balestra's comment. Our costing approach used national standardized Medicare payment amounts as a proxy for actual resource use. Thus, the surgery costs in our analysis were lower than what many private insurers pay. When the cost of surgery was based on 70% of the amount hospitals billed to Medicare (a higher amount that what Medicare actually pays), the cost per quality-adjusted life-year gained for surgery fell below $100 000 in only a small minority of samples.

The role that our costing approach had on comparisons between instrumented and noninstrumented fusion surgery warrants comment. Our report of no difference in cost between fusion types is largely an artifact of our costing approach, because Medicare payment amounts are minimally different for these surgical alternatives. Although no statistically significant differences in health outcome were observed between fusion types over 2 years, further follow-up of SPORT (Spine Patient Outcomes Research Trial) participants by using several approaches to costing will be important for understanding the long-term value of surgery.

Although the SPORT protocol specified a minimum symptom duration of 12 weeks, we are aware of no referral restrictions at participating centers. Previous treatments received by SPORT observational cohort participants have been described (4) and may be indicative of varied referral pathways. Our study was not designed to address the optimal symptom duration before surgery.