Table 1. Published definitions of severe asthma.

Guidelines	Definition		Required medications		Additional details
ATS (2000)	• Refractory asthma requires 1 or both major criteria and 2 minor criteria		• Major criteria		• Minor criteria; at least 2 of following categories
				1) Continuous or near-continuous (≥ 50% of the year) OCS		1) Requirement for daily treatment with additional controller
				2) High-dose ICS		2) Requirement for short-acting inhaled β2-agonist
						3) Persistent airway obstruction
						4) 1 or more urgent care visits for asthma per year
						5) 3 or more oral steroid “bursts” per year
						6) Prompt deterioration with ≤ 25% reduction in oral or ICS dose
						7) Near-fatal asthma event in the past
WHO (2009)	• Treatment-resistant severe asthma		• High-dose ICS or a high-dose ICS–LABA combination		• Level of control assessed based on following categories:
		1) “Control” is not achieved despite highest level treatment	• Frequent or chronic use of SCS			1) Daytime symptoms
		2) “Control” maintained only with highest level treatment				2) Limitations on activities
						3) Nocturnal symptoms/awakenings
						4) Need for short-acting inhaled β2-agonist
						5) Lung function
						6) Exacerbations
IMI (2011)	• Refractory asthma despite high-intensity treatment		• High-dose ICS with or without SCS		• Uncontrolled and/or frequent (≥ 2/year) exacerbations
ERS/ATS (2014)	• Asthma that requires high-intensity medication to prevent it becoming “uncontrolled” or that remains “uncontrolled” despite therapy		• Medications for GINA steps 4–5 asthma for the previous year or		• Uncontrolled asthma; at least 1 of the following:
	• Controlled asthma that worsens on tapering of these high doses of ICS or SCS (or additional biologics)		• SCS for ≥ 50% of the previous year			1) Poor symptom control
						2) Frequent severe exacerbations
						3) Serious exacerbations
						4) Airflow limitation
GINA (2019)	• Uncontrolled asthma despite adherence to maximal optimized therapy and treatment contributory factors		• Medications for GINA steps 4–5 asthma		• Uncontrolled asthma; at least 1 of the following categories:
	• Asthma worsens when high-dose treatment is decreased					1) Poor symptom control
						2) Frequent or serious exacerbations
					• Good adherence and inhaler technique

ATS, American Thoracic Society; WHO, World Health Organization; IMI, Innovative Medicine Initiative; ERS, European Respiratory Society; GINA, Global Initiative for Asthma; ICS, inhaled corticosteroids; LABA, long-acting β2-agonists; OCS, oral corticosteroids; SCS, systemic corticosteroids.