Affiliations: Dr. Ferreira is affiliated with Hospital Universitario Principe de Asturias. Ms. Garvey is affiliated with Seton Medical Center. Dr. Collard is affiliated with the University of California San Francisco.
Correspondence to: Harold R. Collard, MD, FCCP, University of California San Francisco, Department of Medicine, 505 Parnassus Ave, M1093, Campus Box 0111, San Francisco, CA 94143; e-mail: email@example.com
Reproduction of this article is prohibited without written permission from the American College of Chest Physicians (www.chestjournal.org/site/misc/reprints.xhtml).
© 2009 American College of Chest Physicians
We agree completely with Drs. Al-Ghimlas and Todd that caution is needed when interpreting the results of studies such as ours1 that identify the predictors of response to an intervention. Their concern that patients with low baseline walk distances are more likely to get better and that patients with high baseline walk distances are more likely to get worse is reasonable, although not supported by the limited evidence available for patients with pulmonary fibrosis.2 A more careful look at the predictive value of baseline walk distance in our study and the potential influence of extreme values is warranted.
Table 1 shows the relationship of baseline walk distance to change in walk distance after pulmonary rehabilitation for the entire study group and three subgroups of increasingly “average” baseline walk distances. As these data demonstrate, removing outliers does not significantly change the predictive value of baseline walk distance; the relationship is preserved even when the bottom 10% and the top 10% of subjects are removed. Statistical significance is lost only with exclusion of 50% of subjects, at which point the analysis is substantially less powerful (ie, at high risk for a type II error).
P5 = 5th percentile; P95 = 95th percentile; P10 = 10th percentile; P90 = 90th percentile; P25 = 25th percentile; P75 = 75th percentile.
Additional hypothesis-driven studies like ours (baseline walk distance was identified a priori as a potential predictor of response) are certainly needed to validate the observed relationship of baseline walk distance to change in walk distance after pulmonary rehabilitation. Indeed, we were careful in our discussion to avoid recommending pulmonary rehabilitation based on baseline walk distance. It is our hope that future studies will address this important question.
Financial/nonfinancial disclosures: The authors have reported to the ACCP that no significant conflicts of interest exist with any companies/organizations whose products or services may be discussed in this article.
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