A spirometry test, also called forced lung capacity in clinical practice, is the amount of air that can be forcibly exhaled from the lungs after inhaling as deeply as possible. The exhaled air is collected by a device that tests lung function, and the value is recorded. The test value of spirometry, which depends on other factors such as height, weight and atmospheric pressure, can help diagnose lung disease, determine its severity and progression, and can be used to assess a person’s ability to tolerate surgery. In addition spirometry values can help distinguish obstructive diseases, such as asthma and chronic obstructive pulmonary disease, from restrictive lung diseases, such as pulmonary fibrosis and nodular disease. The spirometry test is a pulmonary function test used to determine exertional lung capacity. Generally, no special preparation is required, but on the day of the test one should choose comfortable clothing, such as loose fitting clothes, as the person being tested will be asked to take as many deep breaths as possible.