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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