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

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

METHODS

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

RESULTS

A total of 881 children had 3338 FEV measurements on occasions when asthma was controlled; 5184 pairs of FEV measurements made at 3-month intervals were available. Each unit change in FEV z score was equivalent to a Zc 1.45 and an absolute change in FEV% of 11.6%. The limits of agreement for change in FEV% were -20 and +21, absolute change in FEV 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 FEV% and FEVz comparisons, but not Zc.

CONCLUSION

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