The Repeatability of Forced Expiratory Volume Measurements in Adults With Cystic Fibrosis

Study objective To determine the repeatability of measurements of FEV 1 in adults with lung disease due to cystic fibrosis (CF). Design Single cohort study nested within a randomized controlled trial. Setting Adult CF of a university teaching hospital. Subjects were participants in a randomized trial of an experimental mucolytic drug. Patients Twenty-one adults (mean age, 27.5 ± 9.2 years [± SD]) with CF and mild-to-moderate airflow obstruction (FEV 1 70 ± 15% predicted). Patients were in clinically stable condition prior to and during the study. Interventions Repeated FEV 1 measurements were obtained at specific times of the day for 9 consecutive days, for a total of 31 measurements from each subject. Statistical measures of repeatability were calculated. Variation over the course of 1 day and variation from 1 day to the next were examined separately. Measurements and results For day-to-day FEV 1 measurements, the within-subject SD was 0.145 L (4.5% of predicted), indicating greater variation compared to values previously established in normal subjects. The coefficient of repeatability indicated that day-to-day measurements could differ by as much as 13% of predicted in the absence of clinical change. For measurements within a single day, variation was not observed to be greater than normal. Conclusions In adults with CF, day-to-day variation in FEV 1 measurements is greater than normal and similar to that seen in other obstructive lung diseases. Changes in FEV 1 over time in adults with CF can likely be interpreted using the same criteria that apply to asthma or COPD.