WITHIN-SUBJECT VARIABILITY AND PERCENT CHANGE FOR SIGNIFICANCE OF SPIROMETRY IN NORMAL SUBJECTS AND IN PATIENTS WITH CYSTIC-FIBROSIS

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Abstract
To determine the within-subject variability of spirometry, moment analysis and slope ratios, and to calculate the percent change for significance, 15 normal subjects and 15 patients with cystic fibrosis (CF) were studied. Common sources of variability, such as patient and technician training, equipment errors, lung volume history, posture and circadian variation, were controlled. Flow-time curves were analyzed with a digital computer. The coefficient of variations of spirometry were smallest for forced vital capacity (FVC), forced expiratory volume in 1 s (FEV1) and peak flow (PF), and were largest for root moments (M) and slope ratios (SR) in both normal subjects and patients with CF. The coefficient of variations of all tests, except PF, were larger in patients with CF than in normal subjects. Significant change was calculated from the sample variance, mean and the number of trials using a modified equation for sample size. This calculation estimated the degree of change for significance for each participant that was required in subsequent testing.