Aims 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 (FEV1) in children with symptomatically well-controlled asthma by applying three different methods of expressing change in FEV1 over 3-month intervals.

Methods 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.

Results 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.

Conclusion 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.