The importance of data issues when comparing cystic fibrosis registry outcomes between countries : are annual review FEV1 in the UK only collected when subjects are well?

Rationale, aims and objective Cross‐country comparisons of cystic fibrosis (CF) outcomes can potentially identify variation in care but are dependent on data quality. An important assumption is that the UK annual review FEV1 is only collected during periods of clinical stability. If this assumption does not hold, results of FEV1 comparisons may be biased in favour of registries with encounter‐based FEV1. We aimed to test the assumption that CF annual reviews in the UK are only performed during periods of clinical stability. Method Prospective encounter‐based data collected in Sheffield (n = 174) was used to establish whether annual review FEV1 were always collected during periods of clinical stability and to determine the group‐level discrepancy between annual review vs best FEV1. We then went on to quantify the group‐level discrepancy between annual review and best annual FEV1 readings within the UK registry (n = 2995) to determine if the differences observed in Sheffield also apply to the wider UK data. Results Sheffield results showed a group‐level discrepancy between best and annual review FEV1 of −2.5% (95% CI −3.95% to −1.2%) for annual reviews performed during periods of clinical stability (n = 50). The group‐level discrepancy is larger at −8.0% (95% CI −11.2% to −4.9%) among annual reviews performed during periods of clinical instability (n = 13). Therefore, the magnitude of this group‐level discrepancy is a surrogate for the proportion of clinically stable annual reviews—smaller discrepancy indicates a higher proportion of clinically stable annual reviews and vice versa. The overall group‐level discrepancy in the UK registry (−5.6%, 95% CI −5.9 to −5.4%) was similar to Sheffield (−6.1%, 95% CI −7.1 to −5.1%). Around 20% of the clinician reviewed, annual reviews in Sheffield were performed during periods of clinically instability. Conclusions Annual review FEV1 underestimates lung health of adults with CF in the UK and may bias cross‐country comparisons