Severe asthma and cystic fibrosis: Overlapping phenotypes?

RATIONALE: Cystic fibrosis (CF) is known to mimic severe asthma. It is not known whether some severe asthma patients have a CF transmembrane conductance regulator-related disorder (CFTR-RD) or whether CFTR mutations contribute to asthma severity. OBJECTIVES: To compare the clinical features of CFTR-RD and the prevalence of CFTR mutations in those with severe asthma and non-severe asthma. METHOD: This retrospective observational study investigated individuals with suspected or confirmed asthma as identified with a diagnostic code of asthma between January 2010 and December 2012. Confirmed asthma and severe asthma were classified based on the Canadian Thoracic Society and American Thoracic Society/European Respiratory Society guidelines, respectively. Patient characteristics and investigations were abstracted. RESULTS: Of 314 charts abstracted, 87 patients (28%) had confirmed asthma, of whom 64 (74%) had severe asthma, 16 (18%) had non-severe asthma and 7 (8%) had unknown asthma severity. Rhinosinusitis, nasal polyposis and bronchiectasis were more prevalent in severe versus non-severe asthma (53% vs. 19%, p = 0.023; 20% vs. 0%, p = 0.061; and 20% vs. 0%, respectively, p = 0.061). Sweat chloride values >= 30 mmol/L were detected in 5 (6%) patients with confirmed asthma, all of whom had severe asthma, and 5 (2%) patients with non-confirmed asthma. Of the 24 (8%) patients who underwent CFTR genotyping, 20 had mutations, 7 of whom had severe asthma. Ten patients met criteria for CF or CFTR-RD. CONCLUSIONS: Features of CFTR-RD were seen frequently in severe asthma. Cases of CF, CFTR-RD and non-disease-causing mutations were detected in those with sweat chloride values in and below the borderline range. The clinical relevance of non-disease-causing mutations in severe asthma patients merits further research.