The Minimal Important Difference in Physical Activity in Patients with COPD

Heleen Demeyer, Chris Burtin, Miek Hornikx, Carlos Augusto Camillo, Hans Van Remoortel, Daniel Langer, Wim Janssens, Thierry Troosters, Heleen Demeyer, Chris Burtin, Miek Hornikx, Carlos Augusto Camillo, Hans Van Remoortel, Daniel Langer, Wim Janssens, Thierry Troosters

Abstract

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.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Fig 1. Correlation between change in daily…
Fig 1. Correlation between change in daily step count and possible anchors.
a) 6MWD and b) CRDQdyspnea in the rehabilitation sample (n = 74); 6MWD after 3 months was missing in 3 patients, CRDQdyspnea scores were missing in 10 patients.
Fig 2. Time to first hospitalization.
Fig 2. Time to first hospitalization.
difference between patients exceeding the MID (dotted line) and patients not exceeding the MID (solid line), based on a) SEM cutoff, b) empirical rule effect size and c) cohen effect size and 0.5 SD.

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