Duration and Frequency of Spirometry Needed to Accurately Reflect Annualized Change of FEV1 in COPD.

Abstract

The slope of FEV1 decline is commonly used to reflect the rate of disease progression for descriptive studies and therapeutic trials in COPD. Frequency and duration of spirometric testing needed to report the true slope is unknown. 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. We performed a post-hoc analysis of the annualized rate of FEV1 change among 4412 subjects previously enrolled in the four-year UPLIFT trial of tiotropium versus placebo. Slope estimates were modeled for different iterations of semiannual or annual testing over a variable duration up to four years. All models were compared to a reference of semiannual spirometry for four years. The overall rate of post-bronchodilator FEV1 decline measured semi-annually for four years (44.6 ml; 95% 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 two years (44.3 ml; 95% CI:41.1-47.5). Agreement was consistent for two follow-up values as far as 24 months apart (43.3ml; 95% CI:39.9-46.8). Models based on less than two follow-up values or duration less than 18 months were characterized by relative underestimation of the slope. 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 follow-up measurements over 18 months compared to semiannual testing over four years.