Assessing patient report of function: content validity of the Functional Performance Inventory-Short Form (FPI-SF) in patients with chronic obstructive pulmonary disease (COPD)

Nancy Kline Leidy,¹ Alan Hamilton,² Karin Becker<sup>3

1</sup>Outcomes Research, United BioSource Corporation, Bethesda, MD, USA; ²Medical Department, Boehringer Ingelheim (Canada) Ltd, Burlington, ON, Canada; ³Global Health Economics and Outcomes Research, Boehringer Ingelheim GmbH, Ingelheim am Rhein, Germany

Purpose: The performance of daily activities is a major challenge for people with chronic obstructive pulmonary disease (COPD). The Functional Performance Inventory (FPI) was developed based on an analytical framework of functional status and qualitative interviews with COPD patients describing these difficulties. The 65-item FPI was reduced to a 32-item short form (SF) through a systematic process of qualitative and quantitative item reduction and formatted for greater clarity and ease of use. This study examined the content validity of the reduced, reformatted form of the instrument, the FPI-SF.
Patients and methods: Qualitative cognitive interviews were conducted with COPD patients recruited from three geographically diverse pulmonary clinics in the United States. Interviews were designed to assess respondent interpretation of the instrument, evaluate clarity and ease of completion, and identify any new activities participants found important and difficult to perform that were not represented by the existing items.
Results: Twenty subjects comprised the sample; 12 (60%) were male, 14 (70%) were Caucasian, the mean age was 63.0 ± 11.3 years, 12 (60%) were retired, the mean forced expiratory volume in 1 second (FEV₁) was 1.5 ± 0.5 L, and the mean percent predicted FEV₁ was 48.4% ± 13.1%. Participants understood the FPI-SF as intended, including instructions, items, and response options. Two minor formatting changes were suggested to improve clarity of presentation. Participants found the content of the FPI-SF to be comprehensive, with items covering activities they felt were important and often difficult to perform.
Conclusion: These results, together with its development history and previously tested quantitative properties, suggest that the FPI-SF is content valid for use in clinical studies of COPD.

Keywords: activities of daily living, chronic pulmonary disease, COPD, functional status, health outcomes, health-related quality of life, patient-reported outcomes, physical activity