Slope of FEV1 by age at enzyme replacement therapy (ERT) initiation FEV1 z-score slope (y-axis) by age categorised in three groups (red, ERT initiation <35 years; green, ERT initiation between 35 and 45 years; blue, ERT initiation above 46 years). Slope of FEV1 by age at enzyme replacement therapy (ERT) initiation FEV1 z-score slope (y-axis) by age categorised in three groups (red, ERT initiation <35 years; green, ERT initiation between 35 and 45 years; blue, ERT initiation above 46 years). ERT initiation <35 years resulted in improvement of FEV1 over time compared with later initiation. Median (IQR) spirometric follow-up time was 7.7 (3.7, 10.1) years. Daniel Franzen et al. BMJ Open Resp Res 2018;5:e000277 ©2018 by British Thoracic Society