LAUSR

STUDY DESIGN Prospective cohort study. OBJECTIVE The aim of this study was to determine the minimum clinically important difference (MCID) of the 6-minute walking test (6WT) after surgery for lumbar degenerative disc disease (DDD). SUMMARY OF BACKGROUND DATA The smartphone-based 6WT is a valid and reliable tool to quantify objective functional impairment in patients with lumbar DDD. To date, the MCID of the 6WT has not be described in patients with DDD. METHODS We assessed patients pre- and 6-weeks postoperatively, analysing both raw 6-minute walking distances (6WD; in meters) and standardized 6WT z-scores. Three methods were applied to compute MCID values using established patient-reported outcomes measures (PROMs) as anchors (VAS back/leg pain, Zurich Claudication Questionnaire (ZCQ), Core Outcome Measures Index (COMI)): (1) average change, (2) minimum detectable change, and (3) the change difference approach. RESULTS We studied 49 patients (59% male) with a mean age of 55.5 ± 15.8 years. The computation methods revealed MCID values ranging from 81m (z-score of 0.9) based on the VAS back pain to 99m (z-score of 1.0) based on the ZCQ physical function scale. The average MCID of the 6WT was 92m (z-score of 1.0). Based on the average MCID of raw 6WD values or standardized z-scores, 53% or 49% of patients classified as 6-week responders to surgery for lumbar DDD, respectively. CONCLUSION The MCID for the 6WT in lumbar DDD patients is variable, depending on the calculation technique. We propose a MCID of 92m (z-score of 1.0), based on the average of all three methods. Using a z-score as MCID allows for the standardization of clinically meaningful change and attenuates age- and sex-related differences. LEVEL OF EVIDENCE 3.