Variability in forced expiratory volume in 1 s in children with symptomatically well-controlled asthma

Spirometry is used by many clinicians to monitor asthma in children but relatively little is understood about its variability over time. The aim of this study was to determine the variability of forced expiratory volume in 1 s (FEV_1⁾ in children with symptomatically well-controlled asthma by applying three different methods of expressing change in FEV₁ over 3-month intervals.

Data from five longitudinal studies of children with asthma which measured FEV₁ at 3-month intervals over 6 or 12 months were used. We analysed paired FEV₁ measurements when asthma symptoms were controlled. The variability of FEV₁% predicted (FEV₁%), FEV₁ z-score (FEV₁z) and conditional z score for change (Zc) in FEV₁ was expressed as limits of agreement.

A total of 881 children had 3338 FEV₁ measurements on occasions when asthma was controlled; 5184 pairs of FEV₁ measurements made at 3-month intervals were available. Each unit change in FEV₁ z score was equivalent to a Zc 1.45 and an absolute change in FEV₁% of 11.6%. The limits of agreement for change in FEV₁% were -20 and +21, absolute change in FEV₁ z were -1.7 and +1.7 and Zc were -2.6 and +2.1. Regression to the mean and increased variability in younger children were present for change in FEV₁% and FEV₁z comparisons, but not Zc.

Given the wide limits of agreement of paired FEV₁ measurements in symptomatically well-controlled children, asthma treatment should primarily be guided by symptoms and not by a change in spirometry.