The Minimal Clinically Important Difference for the Oswestry Disability Index Substantially Varies Based on Calculation Method

Study Design. Retrospective study. Objective. Evaluate differences in the minimal clinically important difference (MCID) threshold value based on various acceptable statistical methods, and how these differences may influence the interpretation of surgical benefit following elective 1 and 2 level lumbar fusion. Summary of Background Data. The MCID is a statistically determined threshold value to evaluate if a patient has received benefit from a medical procedure. In the era of value based medicine, the MCID has become increasingly important. However, there is substantial ambiguity surrounding the interpretation of this value given that it can be influenced by both demographic and methodological factors. Methods. 371 patients that underwent one or two level elective lumbar fusion between June 2021 and June 2023 were included in this study. All patients completed both their preoperative and 6 month post-operative Oswestry Disability Index (ODI), as well as two anchor questions concerning satisfaction with their surgical procedure. The MCID values were calculated using 16 accepted statistical methods, and the resulting MCID values were applied to the cohort to determine how many patients reached MCID by 6 months post fusion. Results. Results demonstrated significant variability in the MCID value. The average MCID value for all 16 methods was 10.5±7.0 points range (0.8 to 25). Distribution methods provided lower threshold MCID values but had greater variability while the Anchor methods resulted in higher threshold values but had lower variability. Depending on the method used 30 to 83% of the cohort reached MCID by 6 months. Conclusion. The statistical method used to calculate the MCID resulted in significantly different threshold values, and greatly affected the number of patients meeting MCID. The results demonstrates the complexity surrounding the interpretation of MCID values, and calls into question the utility of a single statistically determined value to assess surgical success.