Introduction Treatment decisions for children with asthma are often made in response to changes in lung function. Reproducibility limits may help to distinguish a clinically significant change from ?noise? introduced by age or time. A task force has recently highlighted the merits of FEV1 change score.

Aim Derive a change score for FEV1 in children with asthma.

Methods Spirometry data from five studies in asthma that measured FEV1 at ~quarterly intervals over one year were collated. A change score (aka conditional z-score for change, Zc) was calculated using standard methodology, taking into account the correlation (r) between paired measurements and both the initial age and time interval (Stanojevic S et.al, Thorax 2020). Exploratory analyses determined how r changed according to demographic and clinical factors (sex, height, weight, level of asthma control, and treatments). We further evaluated whether Zc was associated with between-visit changes in level of asthma control.

Results There were 5,211 FEV1 measurements from 1,264 individuals aged 4.7-19.0y (mean=12.2y). The asthma-defined Zc was similar to the Zc published in health, (mean Zc: asthma=-0.12; health=-0.05), and it was independent of sex, height, weight, level of asthma control, and treatments. Of 4,339 pairwise measurements where asthma was controlled on both occasions, FEV1 was within the Zc reproducibility limits (< 1.96) for 86% of cases; in comparison 74% of cases had <10% change in FEV1% predicted.

Conclusions. We have derived a change score for children with asthma. Zc was comparable between healthy children and children with controlled asthma. The relationship between Zc and future asthma outcomes needs further investigation.