Spirometry is the most common of the pulmonary function tests (PFTs), measuring lung function, specifically the amount and/or speed of air that can be inhaled and exhaled. Spirometry measures airflow. By measuring how much air you exhale, and how quickly, spirometry can evaluate a broad range of lung diseases. In a spirometry test, while patients are sitting, the patient breathes into a mouthpiece that is connected to an instrument called a spirometer. The spirometer records the rate and amount of air that the patients breathe in and out over a period of time.