Objective: To develop a comprehensive disease-specific COPD severity instrument for survey-based epidemiologic research.
Study design and setting: Using a population-based sample of 383 US adults with self-reported physician-diagnosed COPD, we developed a disease-specific COPD severity instrument. The severity score was based on structured telephone interview responses and included five overall aspects of COPD severity: respiratory symptoms, systemic corticosteroid use, other COPD medication use, previous hospitalization or intubation, and home oxygen use. We evaluated concurrent validity by examining the association between the COPD severity score and three health status domains: pulmonary function, physical health-related quality of life (HRQL), and physical disability. Pulmonary function was available for a subgroup of the sample (FEV1, n = 49; peak expiratory flow rate [PEFR], n = 93).
Results: The COPD severity score had high internal consistency reliability (Cronbach alpha = 0.80). Among the 49 subjects with FEV1 data, higher COPD severity scores were associated with poorer percentage of predicted FEV1 (r = - 0.40, p = 0.005). In the 93 subjects with available PEFR measurements, greater COPD severity was also related to worse percentage of predicted PEFR (r = - 0.35, p < 0.001). Higher COPD severity scores were strongly associated with poorer physical HRQL (r = - 0.58, p < 0.0001) and greater restricted activity attributed to a respiratory condition (r = 0.59, p < 0.0001). Higher COPD severity scores were also associated with a greater risk of difficulty with activities of daily living (odds ratio [OR], 2.3; 95% confidence interval [CI], 1.8 to 3.0) and inability to work (OR, 4.2; 95% CI, 3.0 to 5.8).
Conclusion: The COPD severity score is a reliable and valid measure of disease severity, making it a useful research tool. The severity score, which does not require pulmonary function measurement, can be used as a study outcome or to adjust for disease severity.