Testing minimal clinically important difference: consensus or conundrum?

Background Context: Various methodologies have been used in attempting to elucidate a standard method for calculating minimal clinically important difference (MCID). A consensus-based decision (Initiative on Methods, Measurement, and Pain Assessment in Clinical Trials [IMMPACT] group) suggested a 30% reduction from baseline as a means to define the MCID of self-report back pain measures. Additionally, important psychometric issues need to be addressed regarding use of an independent measure of the same construct as an external criterion, instead of simply using another self-report measure, when using an anchor-based approach to MCID. Purpose: The purpose was to test the validity of recently published guidelines regarding MCID using self-report back pain measures and objective socioeconomic outcomes. Study Design/Setting: This is a prospective study assessing change scores on commonly used spinal pain assessment measures in patients with chronic disabling occupational spinal disorders (CDOSDs) treated in a regional referral rehabilitation center performing interdisciplinary functional restoration. Patient Sample: The study consisted of consecutive cohort of patients (N=1,180) with CDOSDs completing a functional restoration program. Outcomes Measures: Self-report measures including the Oswestry Disability Index (ODI) and the physical component summary (PCS) and mental component summary (MCS) of the Short Form-36 (SF-36) obtained before and after treatment, were compared with objective socioeconomically relevant outcomes obtained 1 year after treatment (ie, work status and additional health-care utilization), that were the external criteria for evaluating MCID. Methods: Pre- to posttreatment improvement was calculated separately for each measure, and subjects were divided into two groups based on the change in scores relative to baseline: 30% or greater versus less than 30% improvement. One-year posttreatment objective socioeconomic outcomes were used as independent external criteria relevant to the CDOSD population. This population is often studied as the most costly and problematic cohort in spine care. Results: The ODI and SF-36 MCS were not associated with any of the objective 1-year outcomes used as external criteria. Reduced post-rehabilitation health-care utilization (based on the percentage of patients pursuing health care from a new provider) was weakly associated with 30% or greater improvement on the SF-36 PCS, relative to patients whose scores changed by less than 30% relative to baseline (17.0% vs. 21.1%). The same was true for the ODI and return-to-work. Conclusions: When objective and independent criteria are used (socioeconomic outcomes) in a CDOSD cohort, the 30% improvement in the ODI and SF-36 may not be a valid MCID index. This replicates similar conclusions made by an independent research group using a distribution-based approach to MCID. The validity of the MCID concept rests on future research using objective external criteria. Moreover, there remains a question whether the term "important" in MCID can be unequivocally and operationally defined as a reliable construct.