Asthma is diagnosed with a combination of clinical history, physical examination, imaging studies, and laboratory tests. In straightforward cases, clinical history and pulmonary function testing is enough to make a diagnosis. Pulmonary function testing includes spirometry, which measures inhalation and exhalation airflow, response to bronchodilator medications such as albuterol, lung volumes and diffusion capacity. Asthmatics tend to have normal to mild airflow obstruction on spirometry with significant bronchodilator response. In patients with advanced asthma, testing for a blood eosinophil levels, IGE levels and aero allergens can be helpful. These tests distinguish between eosinophilic T2 high asthma and non-eosinophilic T2 low asthma. T2 high asthma is linked to allergies, while non-eosinophilic T2 low asthma is not as well defined and associated with smoking, occupational exposures, environmental pollutions, acid reflux, frequent viral infections or obesity. Treatment of these two types of asthma is similar in patients with mild disease, but differs with severe disease as we will later discuss.