Understanding FEV 1 for the purpose of cystic fibrosis registry comparisons: Does bias in annual review FEV 1 affect between-centre comparison within the UK? An analysis of registry data

We previously demonstrated that annual review %FEV underestimates lung health of adults with CF compared with %FEV captured during periods of clinical stability. This has implications in the comparisons against registries with encounter-based FEV , such as the United States. It is uncertain whether this bias affects between-centre comparison within the United Kingdom. Previous funnel plot analyses have identified variation in annual review %FEV according to centre size; hence, we investigated whether paired differences between annual review and best %FEV also vary according to centre size. This registry analysis included 18 adult CF centres in the United Kingdom with ≥80% completeness for best FEV data in 2014. Mean discrepancy between annual review and best %FEV is a surrogate for the extent by which annual review %FEV underestimates lung health, and was plotted against centre size. A local polynomial regression (LOESS) curve was used to explore the relationship between the two variables. An appropriate model is fitted based on the LOESS curve to determine the strength of relationship between discrepancies in %FEV and centre size. There is an inverted U-shaped relationship between mean discrepancies in %FEV and centre size. A regression of the paired mean difference in %FEV against centre size showed a significant improvement in the goodness of fit for a quadratic model (R = 23.8% for a quadratic model compared with 0.4% for a linear one; P = 0.048 for the quadratic term). Annual review %FEV underestimated lung health of adults from small and large centres in the United Kingdom to a greater extent compared with medium-sized centres. A plot of %FEV against centre size (eg, funnel plot comparison) would be affected by systematic bias in annual review %FEV . Therefore, annual review %FEV is an unreliable metric to compare health outcomes of adult CF centres within the United Kingdom.