Intrasubject Variability of Forced Expiratory Flow Rates in Children with Chronic,Stable Asthma

Spontaneous variability is an important consideration in defining "significant" change in forced expiratory flow rates. The currently used definitions are based on values from studies of normal populations and do not address the issue of individual variability. This study assesses intrasubject variability in a group of children with stable, chronic asthma. It was found that changes of greater than 6% in FVC (forced vital capacity), greater than 11.9% in PEFR (peak expiratory flow rate), greater than 7.2% in FEV₁ (forced expiratory volume in 1 sec), and greater than 17% in FEF_25-75% (forced expiratory flow rate over 25 to 75% of FVC) were significantly greater than the ranges of spontaneous variation. These values are less than the percent changes currently used.