This is a randomised, controlled, cross-over study to determine the effect of indirect bronchial challenge testing upon volatile organic compounds (VOCs) in the exhaled breath of adults with well-controlled, mild-to-moderate asthma.

Participants will undergo baseline evaluation including clinical history, spirometry, exhaled nitric oxide measurement (FeNO), phlebotomy (blood eosinophil count), and sampling of exhaled breath VOCs. Participants will be randomised to receive either an indirect bronchial challenge test using mannitol dry powder (MDP) or a sham bronchial challenge, followed by further sampling of breath VOCs. The study will consist of a consent visit and two assessment visits with participants allocated to receive one of the two challenges (MDP or sham) at each assessment. Data will be analysed to determine the effect of bronchial challenge upon exhaled breath VOC profiles.

Inclusion Criteria:

• Male or female
• Aged 18 or over
• Able to provide informed consent
• Self-report of asthma diagnosis from health professional
• Diagnostic confirmation meeting British Thoracic Society guidelines will be sought from primary care
• Non-smokers; or ex-smokers of at least two years duration with less than a ten pack year history
• Asthma Control Questionnaire score < 1.0
• Asthma treated according to level-1 to level-2 of British Thoracic Society treatment guidelines

Exclusion Criteria:

• Respiratory tract infection, asthma exacerbation or change in treatment step within the previous four weeks
• Major chronic cardiorespiratory disease other than asthma
• Significant comorbid condition
• Condition that may be compromised by repeated spirometry manoeuvres or induced bronchospasm
• Asthma Control Questionnaire score >1.0
• Asthma treated at level 3 of the treatment guidelines or higher
• Pregnant or nursing mothers
• Current smokers or 'vapers'
• Ex-smokers of < 2 years duration or > 10 pack years.
• Participating in a clinical trial of an investigational medicinal product (CTIMP).
• Unable to speak English.
• Low baseline lung function (FEV1 <1.50 litres or < 70% predicted value)
• Known hypersensitivity to mannitol, gelatin or strong anaphylactic response in the past.

Adults with well-controlled, mild asthma, managed in primary care and treated at level 1 to 2 of the British Thoracic Society treatment guidelines.