Duration and Frequency of Spirometry Needed to Accurately Reflect Annualized Change of FEV₁ in Chronic Obstructive Pulmonary Disease

Rationale: The slope of decline in forced expiratory volume in 1 second (FEV₁) is commonly used to reflect the rate of disease progression for descriptive studies and therapeutic trials in chronic obstructive pulmonary disease (COPD). The frequency and duration of spirometric testing needed to report the true slope are unknown. Objectives: We sought to define the minimum frequency and follow-up duration needed to accurately describe the annualized rate of FEV₁ change among patients with moderate to very severe COPD. Methods: We performed a post hoc analysis of the annualized rate of FEV₁ change among 4,412 subjects previously enrolled in the 4-year Understanding Potential Long-Term Impacts on Function with Tiotropium-or, UPLIFT-trial of tiotropium versus placebo. Slope estimates were modeled for different iterations of semiannual or annual testing over a variable duration up to 42 months. All models were compared with a reference of semiannual spirometry for 42 months. Results: The overall annual rate of postbronchodilator FEV₁ decline measured semiannually for 42 months (44.6 ml; 95% confidence interval [CI] = 42.5-46.6) did not differ significantly from annual spirometry over the same period (43.7 ml; 95% CI = 41.3-46.1) or semiannual spirometry over the first 2 years (44.3 ml; 95% CI = 41.1-47.5). Agreement was consistent for two follow-up values as far as 24 months apart (43.3 ml; 95% CI = 39.9-46.8). Models that are based on less than two follow-up values or a duration less than 18 months were characterized by relative underestimation of the slope. Conclusions: In a large cohort of patients with moderate to very severe COPD, the annualized rate of change in FEV₁ was accurately represented by a minimum of two annual follow-up measurements over 18 months compared with semiannual testing over 42 months.