How Do Early Weight Trajectories Explain Social Inequalities in Lung Function in Children With Cystic Fibrosis?: A Longitudinal Interventional Disparity Effects Analysis With Time-varying Mediators and Intermediate Confounders.

BACKGROUND: Children with cystic fibrosis (CF) from socioeconomically deprived areas have poorer growth, worse lung function, and shorter life expectancy than their less-deprived peers. While early growth is associated with lung function around age 6, it is unclear whether improving early growth in the most deprived children reduces inequalities in lung function. METHODS: We used data from the UK CF Registry, tracking children born 2000-2010 up to 2016. We extended the interventional disparity effects approach to the setting of a longitudinally measured mediator. Applying this approach, we estimated the association between socioeconomic deprivation (children in the least vs. most deprived population quintile; exposure) and lung function at first measurement (ages 6-8, outcome), and the role of early weight trajectories (ages 0-6) as mediators of this relationship. We adjusted for baseline confounding by sex, birthyear, and genotype and time-varying intermediate confounding by lung infection. RESULTS: The study included 853 children, with 165 children from the least and 172 from the most deprived quintiles. The average lung function difference between the least and most deprived quintiles was 4.5% of predicted forced expiratory volume in 1 second (95% confidence interval: 1.1-7.9). If the distribution of early weight trajectories in the most deprived children matched that in the least deprived children, this difference would reduce to 4% (95% confidence interval: 0.57- 7.4). CONCLUSION: Socioeconomic deprivation has a strong negative association with lung function for children with CF. We estimate that improving early weight trajectories in the most deprived children would only marginally reduce these inequalities.