Daily spirometric variability: normal subjects and subjects with chronic bronchitis with and without airflow obstruction.

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%, FEV1/forced vital capacity [FVC] ratio less than 75%) and 11 without airflow obstruction (FEV1 and FVC greater than 80%0. We attempted to control most of the factors known to affect spirometric variability. Our results indicate that the day-to-day FEV1 has to change by 17% and the FCV 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 FEV1 and FVC before considering the change to be significant.