Variability of spirometry in chronic obstructive pulmonary disease: Results from two clinical trials

Objective: Our goal is to determine short-term intraindividual biologic and measurement variability in spirometry of patients with a wide range of stable chronic obstructive pulmonary disease severity, using datasets fromthe National Emphysema Treatment Trial (NETT) and the Lung Health Study (LHS). This may be applied to determine criteria that can be used to assess a clinically meaningful change in spirometry. Methods: A total of 5,886 participants from the LHS and 1,215 participants from the NETT performed prebronchodilator spirometry during two baseline sessions. We analyzed varying criteria for absolute and percent change of FEV₁ and FVC to determine which criterion was met by 90% of the participants. Results: The mean ± SD FEV₁ for the initial session was 2.64 ± 0.60 L (75.1 ± 8.8% predicted) for the LHS and 0.68 ± 0.22 L (23.7 ± 6.5% predicted) for the NETT. The mean ± SD number of days between test sessions was 24.9 ± 17.1 for the LHS and 85.7 ± 21.7 for the NETT. As the degree of obstruction increased, the intersession percent difference of FEV₁ increased. However, the absolute difference between tests remained relatively constant despite the severity of obstruction (0.106 ± 0.10 L). Over 90% of participants had an intersession FEV₁ difference of less than 225 ml irrespective of the severity of obstruction. Conclusions: Absolute changes in FEV₁ rather than percent change should be used to determine whether patients with chronic obstructive pulmonary disease have improved or worsened between test sessions.