The Minimal Important Difference in Physical Activity in Patients with COPD

Background
Changes in physical activity (PA) are difficult to interpret because no framework of minimal important difference (MID) exists. We aimed to determine the minimal important difference (MID) in physical activity (PA) in patients with Chronic Obstructive Pulmonary Disease and to clinically validate this MID by evaluating its impact on time to first COPD-related hospitalization.
Methods
PA was objectively measured for one week in 74 patients before and after three months of rehabilitation (rehabilitation sample). In addition the intraclass correlation coefficient was measured in 30 patients (test-retest sample), by measuring PA for two consecutive weeks. Daily number of steps was chosen as outcome measurement. Different distribution and anchor based methods were chosen to calculate the MID. Time to first hospitalization due to an exacerbation was compared between patients exceeding the MID and those who did not.
Results
Calculation of the MID resulted in 599 (Standard Error of Measurement), 1029 (empirical rule effect size), 1072 (Cohen's effect size) and 1131 (0.5SD) steps.day-1. An anchor based estimation could not be obtained because of the lack of a sufficiently related anchor. The time to the first hospital admission was significantly different between patients exceeding the MID and patients who did not, using the Standard Error of Measurement as cutoff.
Conclusions
The MID after pulmonary rehabilitation lies between 600 and 1100 steps.day-1. The clinical importance of this change is supported by a reduced risk for hospital admission in those patients with more than 600 steps improvement.

研究背景
由于尚未建立最小重要差异（minimal important difference, MID）的分析框架，身体活动（physical activity, PA）的变化幅度难以被精准解读。本研究旨在明确慢性阻塞性肺疾病（Chronic Obstructive Pulmonary Disease, COPD）患者身体活动的最小重要差异，并通过评估其对首次COPD相关住院时间的影响，对该MID进行临床验证。

研究方法
对74例慢性阻塞性肺疾病患者在接受3个月肺康复治疗前后的身体活动水平进行了为期1周的客观测量，构建康复样本组。此外，纳入30例患者构建重测样本组，连续两周测量其身体活动水平以计算组内相关系数（intraclass correlation coefficient）。本研究选择每日步数作为主要结局指标，采用多种基于分布法与锚定法计算MID，并比较达到MID阈值以上与未达阈值患者的首次急性加重相关住院时间。

研究结果
计算得到的MID分别为：599步·日⁻¹（测量标准误，Standard Error of Measurement）、1029步·日⁻¹（经验法则效应量，empirical rule effect size）、1072步·日⁻¹（科恩效应量，Cohen's effect size）以及1131步·日⁻¹（0.5倍标准差，0.5SD）。由于缺乏足够相关的锚定指标，未能获得基于锚定法的MID估计值。以测量标准误作为截断值时，达到MID阈值以上与未达阈值患者的首次住院时间存在显著差异。

研究结论
慢性阻塞性肺疾病患者接受肺康复（pulmonary rehabilitation）后，身体活动的最小重要差异范围为600~1100步·日⁻¹。步数提升超过600步的患者住院风险显著降低，这一结果佐证了该身体活动变化的临床意义。