How to define severe asthma and an overview of disease classification.


Severe asthma is the presence of genuine asthma, resistant to standard asthma therapies, resulting in adverse outcomes. It was defined by the National Review of Asthma Deaths (NRAD) as present in those patients receiving treatment at British Thoracic Society (BTS) steps 4 and 5 and/or the additional criteria of hospital admissions and asthma attacks recorded in the previous year.1

The exact prevalence of severe asthma is not known, but it is thought to affect 5%–10% of all asthma patients.2

Although many patients have asthma that can be controlled in primary care, there are a significant number who would benefit from referral to a specialist asthma clinic for further investigation and consideration of alternative treatment options.1

Types of asthma

Asthma is a heterogeneous disease, which can be categorised into different endotypes and phenotypes. The type 2 (also known as Th2 or T2) endotype is characterised by Th2 cell-driven inflammation, while the non-type 2 endotype can be classified according to clinical characteristics such as obesity, smoking and age.2

Endotypes and phenotypes of asthma2–4

Table showing summarising the endotypes and phenotypes of asthma and their features


IgE, immunoglobulin E; Th2, T-helper cell type 2; SAA, severe allergic asthma.


  1. Royal College of Physicians. National Review of Asthma Deaths: Why asthma still kills. 2015. Available at [Accessed April 2020]. 
  2. Kuruvilla ME et al. Clin Rev Allergy Immunol 2019;56(2):219–233.
  3. Severe Asthma Toolkit. Asthma phenotypes. 2019. Available at: [Accessed April 2020].
  4. Esteban-Gorgojo I et al. J Asthma Allergy 2018;11:267–281.
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XSA20-C018 July 2020.

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