How accurate is a spirometry test in diagnosing asthma?

Very but.. Asthma is an intermittent disease, thus a relatively normal lung function test on one occasion does not rule out asthma. However it should be abnormal when you have symptoms and the numbers (fev-1) often improves after bronchodilator . As with all tests, a doctor can only make a diagnosis after having obtained your history, performed a physical exam, and then interpret the lung function results.