• To determine the daily variability of spirometry, we performed spirometry on five consecutive days on 15 normal subjects and on 24 patients with chronic bronchitis. The patients with chronic bronchitis were divided into the following two groups: 13 with chronic airflow obstruction (forced expiratory volume in 1 s [FEV1] 80%, FEV,/forced vital capacity [FVC] ratio < 75%) and 11 without airflow obstruction (FEV1 and FVC > 80%). We attempted to control most of the factors known to affect spirometric variability. Our results indicate that the day-to-day FEV, has to change by 17% and the FVC by 15% in patients with airflow obstruction before the change should be considered significant. In patients with nonobstructive chronic bronchitis or in normal subjects, a greater than 5% change should occur in FEV, and FVC before considering the change to be significant.
(Arch Intern Med 1982;142:1287-1291)