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

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Abstract
To determine the daily variability of spirometry, spirometry was performed on 5 consecutive days on 15 normal subjects and on 24 patients with chronic bronchitis. The patients with chronic bronchitis were divided into the following 2 groups: 13 with chronic airflow obstruction (forced expiratory volume in 1 s [FEV1] 80%, FEV1/forced vital capacity [FVC] ratio < 75%) and 11 without airflow obstruction (FEV1 and FVC > 80%). Most of the factors known to affect spirometric variability were controlled. The day-to-day FEV1 apparently 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 > 5% change should occur in FEV1 and FVC before considering the change to be significant.