The within session repeatability of body plethysmography parameters in patients with chronic pulmonary diseases

To define significant changes in pulmonary function parameters over time it is necessary to know the natural variability and repeatability of the measurement. Currently, there are no generally accepted values for clinically significant changes in total lung capacity (TLC), residual value (RV) and other body plethysmography parameters. Methods: 102 patients with chronic pulmonary diseases (63 male; aged 50.3±14.2; forced expiratory volume in one second 76.5±25.8% of predicted) performed pulmonary function testing. We evaluated Intraclass Correlation Coefficients (ICC), the coefficient of repeatability [CR] (defined as the difference between the biggest and lowest values) in three maneuvers in one test where functional residual capacity (FRC) and vital capacity were within 5% of mean value and 200 ml, respectively. In order to compare the repeatability we evaluated Cohen`s effect size [ES] (the difference divided by SD of mean value). Results: ICC was greater than 0.98 for all the parameters of body plethysmography. CR (mean+1.96SD) and ES for FRC (in L) were 0.09±0.06 (0.21) and 0.08; for TLC - 0.16±0.11 (0.37) and 0.1; for RV - 0.14±0.1 (0.34) and 0.17; for expiratory reserve volume - 1.27±0.63 (2.5) and 2.02; for inspiratory capacity (IC) - 0.14±0.10 (0.35), respectively. Conclusion: FRC and TLC had best repeatability and CR was within 0.21 L and 0.37 L, respectively. CR for RV and IC was within 0.34L and 0.35 L, respectively. The changes of the parameters exceeding these limits are unlikely to occur as a result of natural variation.