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?

Lung function in children with cystic fibrosis in the USA and UK: a comparative longitudinal analysis of national registry data

Rationale

A previous analysis found significantly higher lung function in the US paediatric cystic fibrosis (CF) population compared with the UK with this difference apparently decreasing in adolescence and adulthood. However, the cross-sectional nature of the study makes it hard to interpret these results.

Objectives

To compare longitudinal trajectories of lung function in children with CF between the USA and UK and to explore reasons for any differences.

Methods

We used mixed effects regression analysis to model lung function trajectories in the study populations. Using descriptive statistics, we compared early growth and nutrition (height, weight, body mass index), infections (Pseudomonas aeruginosa, Staphylococcus aureus) and treatments (rhDnase, hypertonic saline, inhaled antibiotics).

Results

We included 9463 children from the USA and 3055 children from the UK with homozygous F508del genotype. Lung function was higher in the USA than in the UK when first measured at age six and remained higher throughout childhood. We did not find important differences in early growth and nutrition, or P.aeruginosa infection. Prescription of rhDNase and hypertonic saline was more common in the USA. Inhaled antibiotics were prescribed at similar levels in both countries, but Tobramycin was prescribed more in the USA and colistin in the UK. S. aureus infection was more common in the USA than the UK.

Conclusions

Children with CF and homozygous F508del genotype in the USA had better lung function than UK children. These differences do not appear to be explained by early growth or nutrition, but differences in the use of early treatments need further investigation.

Understanding FEV1 for the purpose of cystic fibrosis registry comparisons: Does bias in annual review FEV1 affect between-centre comparison within the UK? An analysis of registry data

RATIONALE, AIMS, AND OBJECTIVE

We previously demonstrated that annual review %FEV₁ underestimates lung health of adults with CF compared with %FEV₁ captured during periods of clinical stability. This has implications in the comparisons against registries with encounter-based FEV₁ , 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 %FEV₁ according to centre size; hence, we investigated whether paired differences between annual review and best %FEV₁ also vary according to centre size.

METHODS

This registry analysis included 18 adult CF centres in the United Kingdom with ≥80% completeness for best FEV₁ data in 2014. Mean discrepancy between annual review and best %FEV₁ is a surrogate for the extent by which annual review %FEV₁ 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 %FEV₁ and centre size.

RESULTS

There is an inverted U-shaped relationship between mean discrepancies in %FEV₁ and centre size. A regression of the paired mean difference in %FEV₁ against centre size showed a significant improvement in the goodness of fit for a quadratic model (R²  = 23.8% for a quadratic model compared with 0.4% for a linear one; P = 0.048 for the quadratic term).

CONCLUSIONS

Annual review %FEV₁ 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 %FEV₁ against centre size (eg, funnel plot comparison) would be affected by systematic bias in annual review %FEV₁ . Therefore, annual review %FEV₁ is an unreliable metric to compare health outcomes of adult CF centres within the United Kingdom.

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.