Assessment of the Minimum Clinically Important Difference in the Smartphone-Based 6-Minute Walking Test after Surgery for Lumbar Degenerative Disc Disease.

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.