INTRODUCTION: The minimal clinically important difference (MCID) for self-walking test (SPWT) used with nonsurgical treatment for patients with lumbar spinal stenosis (LSS) has been reported, but multiple anchor- and distribution-based approaches using a validated disorder-specific questionnaire such as the Zurich Claudication Questionnaire (ZCQ) as the anchor for estimating the MCID have not been reported. In addition, the MCID for physical activity, such as the number of daily steps, in the treatment of people with LSS has not been reported. The purpose of this study was to use disorder-specific anchor and, multiple anchor-, and distribution-based approaches to determine the MCIDs for walking capacity and physical activity in patients with LSS receiving nonsurgical treatment.
METHODS: This study was a secondary analysis of a randomized controlled trial (RCT), in which patients with LSS received either supervised physical therapy or home exercise programs. All patients completed the ZCQ, SPWT, and pedometry at the baseline and after 6 weeks. Correlation coefficients between the changes in outcomes (SPWT and daily steps) and changes in anchor measures from the baseline to the 6-week follow-up (ZCQ symptom severity and physical function) or ZCQ satisfaction at 6 weeks were examined to confirm the usefulness of the anchors. For the anchor-based approach, ZCQ symptom severity, physical function, and satisfaction subscales were used as the external anchors. Using the receiver-operating characteristic (ROC) curve, the MCIDs were determined based on the optimal cutoff points for changes in the SPWT or daily steps. For the distribution-based approach, the MCIDs were estimated from the standard deviations (SDs) of the baseline scores of the SPWT and daily steps.
RESULTS: Sixty-nine (33 men and 36 women, average age 71.8 years) of 86 patients from the original RCT were included in this secondary analysis after excluding patients who were lost to follow-up (n = 2) or had missing data in the ZCQ or objective physical measurements (n = 15). Among each outcome, the highest although weak, correlations were detected between the SPWT and ZCQ satisfaction subscale (rs = 0.29), and between daily steps and ZCQ symptom severity subscale (rs = 0.40) (>0.30 is considered a good anchor). In the anchor-based approach, only the ZCQ satisfaction subscale for the SPWT (0.73), and ZCQ symptom severity subscale for daily steps (0.71) exceeded the area under the ROC curve value of 0.7, which is considered acceptable. When using these subscales as anchors, the ROC curves and optimal cutoff points indicated MCIDs of 151 m for the SPWT and 1149 steps for daily steps. The distribution-based approach estimated the MCIDs as 280 m for the SPWT and 1274 steps for daily steps, and had a moderate effect size (0.5 SD).
DISCUSSION: The anchor-based approach had limited external responsiveness when the ZCQ was used as the anchor. However, this information may be helpful for interpreting walking capacity and physical activity in patients with LSS receiving nonsurgical treatment and for estimating power and sample size when planning new trials.