BACKGROUND: Cystic fibrosis (CF) spans a wide spectrum. Therefore, benchmarking between registries implies comparing similar cohorts.

OBJECTIVE AND METHODS: Explore patient characteristics in Belgian (B), French (F), German (G) and Dutch (NL) registries (total N=13,122) and determine whether they fulfill predefined diagnostic criteria.

RESULTS: Using as case definition sweat chloride >60mmol/L or 2 CFTR mutations identified, CF diagnosis was not documented in 2.8, 5.7, 6.5 and 21.6% of subjects in the F, B, NL, and G registries. Restricting CFTR mutation interpretation to 124 CF causing mutations in CFTR2, these numbers rose to 10.5, 10.4, 14.5 and 24.3% respectively. Excluding these subjects impacted on outcomes. The impact differed between countries; the largest changes seen were a decrease in % adults from 51.9 to 47.8% in G, a decrease in % pancreas sufficiency from 17.0 to 13.0 in F, an increase in % homozygous for F508del from 55.3 to 63.7 in NL and a decrease of % with sweat chloride ≤60mmol/L from 8.4 to 1.1 in B.

CONCLUSION: CF diagnosis is not documented in 10 to 24% of patients included in CF registries. Excluding these patients for analyses leads to significant changes in outcomes.