The diagnosis of asthma is made based on symptoms (wheezing, chest tightness, difficulty breathing and cough). Patients with asthma will also show reversibility (airways respond to a reliever inhaler) and or bronchial hyperresponsiveness (twitchiness in airways).

Spirometry is a test for assessing lung function and diagnosing asthma. If a patient is given a short-acting reliever (i.e.salbutamol) and further spirometry is performed with good improvement, then reversibility is demonstrated and the diagnosis of asthma is suggested.

In the case of normal spirometry, a challenge test using an inhalation of a bronchoconstriction agent (i.e. Methacholine or Histamine) is useful to show a higher level of bronchial responsiveness. If there is evidence of it, the diagnosis of asthma is likely. Other tests that may help the doctor diagnose asthma are non-invasive tests for assessing bronchial inflammation (i.e. exhaled nitric oxide measurement or induced sputum analysis).