Minimally important change values of a measurement instrument depend more on baseline values than on the type of intervention

Objectives Multi-item questionnaires are frequently used to measure outcomes in randomized controlled trials (RCTs) in patients with sciatica. Knowing the minimaly important change (MIC) values for these instruments will facilitate interpretation of change scores. MIC values have been shown to be dependent on baseline values. The question is whether they also depend on the type of intervention. To estimate the MIC of the Roland Morris Disability Questionnaire (modified 23 item version) (RMDQ) and of intensity of leg pain measured by a Visual Analogue Scale (VAS) in patients with sciatica and to assess to what extent MIC values depend on type of intervention and on baseline values. Study Design and Setting This is a secondary analysis of RCT data of the effects of early surgery vs. prolonged conservative treatment in patients with sciatica. Baseline and 8-week data were used to assess MIC of the RMDQ-23 and VAS leg pain. We used the receiver operator characteristic (ROC) method to assess the MIC. Global Perceived Recovery (rated 8 weeks after baseline) was used as anchor. Subgroups were created based on type of treatment and baseline severity. Results The MIC value of the RMDQ-23 for the total group of sciatica patients was 7.5. The values were 8.1 and 6.9 for surgery and conservative treatment, respectively. For high and low baseline values, the MICs were 9.0 and 4.9, respectively, irrespective of treatment received. The MIC values of the VAS leg pain were 34.4 for the total group. For surgery and conservative treatment, the MIC values were 38.5 and 30.4, respectively, whereas for groups with high and low baseline values, MIC values of 53.5 and 17.2 were found. Conclusion The MIC values of the RMDQ-23 and VAS leg pain were found to be highly dependent on their baseline values, although the type of intervention appeared to influence the MIC value only slightly