Interpretation of treatment changes in six-minute walk distance in patients with COPD

¹ Horten Centre, University of Zurich, Switzerland
² Dept. of Medicine, University at Buffalo, NY, USA
³ West Park Healthcare Centre, University of Toronto, Toronto, Ontario, Canada
⁴ Dept of Clinical Epidemiology and Biostatistics, McMaster University, Hamilton, Ontario, Canada
⁵ Dept of Clinical Epidemiology and Biostatistics, McMaster University, Hamilton, Ontario, Canada; and Dept of Epidemiology, Italian National Cancer Institute Regina Elena, Rome, Italy

^* To whom correspondence should be addressed. E-mail: milo.puhan{at}usz.ch.

	Abstract

There is uncertainty about the interpretation of changes in the Six-minute walk distance (SMWD) in COPD patients and whether the minimal important difference (MID) for this useful outcome measure exists.

We used data from nine trials enrolling a wide spectrum of COPD patients with SMWD at baseline and follow-up and determined threshold values for important changes in SMWD using three distribution based methods. We also evaluated anchor-based methods to determine a MID.

We included data of 460 COPD patients with a mean FEV₁ of 39.2% predicted (SD 14.1) and a mean SMWD of 361 meters (112) at baseline. Threshold values for important effects in SMWD were between 29 and 42 meters, respectively, using the Empirical Rule Effect Size and the Standardized Response Mean. The threshold value was 35 meters (95% 30–42) based on the Standard Error of Measurement. Correlations of SMWD with patient reported anchors were too low to provide meaningful MID estimates.

SMWD should change by around 35 meters for patients with moderate to severe COPD to represent an important effect. This corresponds to about a 10% change of baseline SMWD. The low correlations of SMWD with patient reported anchors question whether a MID exists for the SMWD.

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