Determination of the minimum improvement in pain, disability, and health state associated with cost-effectiveness: introduction of the concept of minimum cost-effective difference.

BACKGROUND: Minimum clinical important difference (MCID) has been adopted as the smallest improvement in patient-reported outcome needed to achieve a level of improvement thought to be meaningful to patients.

OBJECTIVE: To use a common MCID calculation method with a cost-utility threshold anchor to introduce the concept of minimum cost-effective difference (MCED).

METHODS: Forty-five patients undergoing transforaminal lumbar interbody fusion for degenerative spondylolisthesis were included. Outcome questionnaires were administered before and 2 years after surgery. Total cost per quality-adjusted life-year (QALY) gained was calculated for each patient. MCED was determined from receiver-operating characteristic curve analysis with a cost-effective anchor of < $50,000/QALY and < $75,000/QALY. MCID was determined with the health transition item as the anchor.

RESULTS: Significant improvement was observed 2 years after transforaminal lumbar interbody fusion for all outcome measures. Mean total cost per QALY gained at 2 years was $42,854. MCED was greater than MCID for each outcome measure, meaning that a greater improvement was required to represent cost-effectiveness than a clinically meaningful improvement to patients. The area under the receiver-operating characteristic curve was consistently ≥ 0.70 with both cost-effective anchors, suggesting that outcome change scores were accurate predictors of cost-effectiveness. Mean cost per QALY gained was significantly lower for patients achieving compared with those not achieving an MCED in visual analog scale for leg pain ($43,560 vs. $112,087), visual analog scale for back pain ($41,280 vs. $129,440), Oswestry disability index ($30,954 vs. $121,750), and EuroQol 5D ($35,800 vs. $189,412).

CONCLUSION: MCED serves as the smallest improvement in an outcome instrument that is associated with a cost-effective response to surgery. With the use of cost-effective anchor of <  $50,000/QALY, MCED after transforaminal lumbar interbody fusion was 4 points for visual analog scale for low back pain, 3 points for visual analog scale for leg pain, 22 points for Oswestry disability index, and 0.31 QALYs for EuroQol 5D.