Rapid Access Diagnostics for Asthma (RADicA): protocol for a prospective cohort study to determine the optimum series of investigations to diagnose asthma using conventional and novel tests.

Introduction: The diagnosis of asthma is often based on characteristic patterns of symptoms in the absence of an alternative explanation, resulting in over and under diagnosis. Therefore, diagnostic guidelines usually recommend including confirmation of variable airflow obstruction. Some recommend using a sequence of objective tests; however the tests used, the specific cut-off values and the specified order are yet to be validated. We aimed to determine the optimal cut-off values and series of investigations to diagnose asthma. We also explore the potential for novel tests of small airways function and biomarkers, which could be incorporated into future diagnostic pathways.

Methods And Analysis: The Rapid Access Diagnostics for Asthma study is an observational study of 300 symptomatic patients with 'clinician-suspected asthma' and healthy controls (aged ≥3 to <70 years), recruited from primary and secondary care in Greater Manchester, UK. Symptomatic participants will undergo four core visits and one optional visit. Participants will complete two baseline visits and undergo a series of established (spirometry, bronchodilator reversibility, exhaled nitric oxide, home peak flow monitoring and bronchial challenge testing) and novel tests. Following visit 2, participants will receive monitored medium-dose inhaled corticosteroid therapy for 6-8 weeks, after which they will return for repeat testing. Patients will be diagnosed with asthma by 'expert panel' opinion (minimum two respiratory specialists) on review of all data (excluding novel tests) pre and post treatment. Healthy controls will attend two visits to establish reference intervals and calculate repeatability coefficients for novel tests where there is a lack of evidence on what threshold constitutes a 'normal' set of values. The primary end point is to determine the optimum diagnostic pathway for diagnosing asthma.

Ethics And Dissemination: The study was approved by Greater Manchester East Research Ethics Committee (18/NW/0777). All participants or parents/guardians are required to provide written informed consent and children to provide written assent. The results will be published in peer-review journals and disseminated widely at conferences and with the help of Asthma and Lung UK (www.asthmaandlung.org.uk).

Trial Registration Number: ISRCTN11676160.