Clinical & Experimental Allergy

What is severe asthma?



Dr J. D. Blakey

Respiratory Medicine,

Clinical Sciences Building,

City Hospital Campus,

Nottingham NG5 1PB,




Asthma is common, and some individuals are severely affected by it. Learned institutions have sought to provide a definition of ‘severe asthma’ to facilitate research and clinical care. This is a challenging undertaking given the difficulty in defining asthma and the lack of supportive evidence for a distinct severe asthma phenotype. In this review, we discuss the rationale for a definition of severe asthma and the relative merits of the sequential attempts that have been made to produce such a definition. The difficulty in disentangling control and severity is highlighted, as is the heterogeneity of phenotype in severe asthma, and potential for misclassification. We conclude that the search for a singular definition of severe asthma is problematic, though likely to continue. We suggest the alternative strategy of using classifiers with a specific aim related to symptoms, pathophysiology or service provision.