Our aim was to determine the minimal important difference for 6-minute walk distance and maximal cycle exercise capacity in patients with severe COPD.
1218 patients enrolled in the National Emphysema Treatment Trial completed exercise tests before and after 4–6 weeks of pre-trial rehabilitation, and 6 months after randomization to surgery or medical care. The St Georges Respiratory Questionnaire (domain and total scores) and UCSD Shortness of Breath Questionnaire (total score) served as anchors for anchor-based minimal important difference estimates. To calculate distribution-based estimates we used the standard error of measurement, Cohen's effect size and the empirical rule effect size.
Anchor-based estimates for the 6-minute walk distance were 18.9 meters (95% CI 18.1–20.1), 24.2 meters (23.4–25.4), 24.6 meters (23.4–25.7) and 26.4 meters (25.4–27.4), which were similar to distribution-based minimal important difference estimates of 25.7, 26.8 and 30.6 meters. For maximal cycle exercise capacity, anchor-based estimates for the minimal important difference were 2.2 Watts (2.0–2.4), 3.2 Watts (3.0–3.4), 3.2 Watts (3.0–3.4) and 3.3 (3.0–3.5) Watts while distribution-based estimates were 5.3 and 5.5 Watts
We suggest a minimal important difference of 26±2 meters for 6-minute walk distance and 4±1 Watts for maximal cycle exercise capacity for patients with severe COPD.