Validation of a consensus-based minimal clinically important difference (MCID) threshold using an objective functional external anchor

Background context: The minimal clinically important difference (MCID) is defined as the smallest change in an outcome that a patient would perceive as meaningful. The Initiative on Methods, Measurement and Assessment in Clinical Trials (IMMPACT) group proposed defining the MCID as a 30% improvement in self-reported pain or function. However, this MCID threshold has not been validated against an objective physical measure. Purpose: To test the validity of the IMMPACT-based MCID threshold, using an objective physical measure as an external anchor. Study design/setting: Prospective study of chronic disabling occupational lumbar disorder (CDOLD) patients completing a functional restoration program. Patient sample: A consecutive cohort of 743 CDOLD patients. Outcome measures: Self-report measures of pain-related function were compared with an objective lifting measure, the progressive isoinertial lifting evaluation (PILE), obtained after treatment. Methods: The association between reporting 30% or greater improvement (the IMMPACT's MCID key criterion) and the PILE score after treatment was assessed. Results: A 30% or greater improvement on the self-report measures was significantly associated with improvement in physical function on the PILE task. Conclusions: Despite extensive use of the MCID to evaluate effects of treatment in spinal disorders, this is the first empirical documentation of the validity of the IMMPACT's 30% change criterion compared with an objective physical anchor.