What is the best pulmonary diagnostic approach for wheezing patients with normal spirometry?

Asthma is characterized by airway inflammation, airway hyper-responsiveness (AHR) and variable air flow obstruction. The diagnosis of asthma, however, is often based upon nonspecific clinical symptoms of cough, wheeze, and shortness of breath. Furthermore, the physical examination and measurements of pulmonary function are often unremarkable in patients with asthma, thereby complicating the diagnosis of the disease. The following discussion will review approaches to the diagnosis of asthma when lung functions are normal, and will largely focus on the use of bronchial provocation tests to detect underlying AHR.