BackgroundIncreasing awareness of milder presentations of cystic fibrosis (CF) and greater interest in non-CF bronchiectasis is likely to lead to more CF screening by respiratory clinicians. As a result, adults who may not strictly fulfil CF diagnostic criteria, yet display evidence of abnormal cystic fibrosis transmembrane conductance regulator (CFTR) function are being identified. The degree of agreement on diagnosis and care needs in these cases between CF-clinicians remains unknown, and has implications for patient care, including access to CFTR-modulator therapies.MethodsWe surveyed adult-CF physicians in Canada, the USA, the UK, and Ireland, and presented them with anonymised vignettes of adult patients referred for assessment of possible CF. Diagnostic inter-rater agreement over diagnosis, ease of classifying cases and appropriate follow-up was assessed using Krippendorf's alpha statistic.ResultsAgreement over diagnosis (alpha=0.282), ease of classification (alpha=-0.01) and recommended follow-up (alpha=0.054) was weak. Clinician experience (>10 years and 5–10 years versus <5 years) and location (UK/Ireland versus Canada) were associated with higher odds of recommending further testing compared to selecting a formal diagnosis (OR 2.87, p=0.022, OR 3.74; p=0.013; and OR 3.16, p=0.007, respectively). A modified standard of care was recommended in 28.7% of cases labelled as CF. 70% of respondents agreed with the statement that “distinction between CF and CFTR-RD has become significantly more pertinent with the advent of highly effective CFTR-modulators”.InterpretationOur results demonstrate low diagnostic concordance among CF specialists assessing cases of possible adult CF and highlights an area in need of improvement.