Background:  Most people with PCD have impaired lung function, which deteriorates over time. Longitudinal research of lung function is scarce and there is a lack of guidance on intra-individual variability during routine clinical surveillance.

Aims: In this prospective study, we aimed to assess the variability of forced expiratory volume in the first second (FEV1) at routine appointments whilst stable over 18 months, across BEAT-PCD centres.

Methods: We analysed 1297 measurements from 302 individuals, ages 5 to 70 years (median = 37.5) from 12 countries in 2017 and 2018. We used Global Lung Function Initiative 2012 references to calculate FEV1 z-scores. To model the variability of lung function over time, we fitted linear mixed-effects models with random effects at country and patient level, with relevant adjustments.

Results:  The coefficient of variation, which indicates the intra-individual variability of repeated FEV1 measurements, was 10%.  Whilst stable, the 95% upper limit of normal for relative changes in FEV1 between visits was 26%. There was minimal change of average FEV1 z-scores over a year (-0.008; 95%CI -0.016 to -0.0003), although there was inter-individual variability in this rate of change.

Conclusions: Between visits when stable, 95% of relative changes were <26%.  This suggests large intra-individual variability of FEV1 when stable and monitoring lung function must take this into account.