Lung function test is used to see how well your lung work by looking at HOW MUCH air volume it can hold and HOW FAST you can move air out of your lung. Remember both of those indicators are indirectly represented if there are obstruction or the airway being squeezing in. Another indicator stated elsewhere is also important to see if the airway epithelium or the lumen of the airway is damaged inside. Spirometry is used to diagnose several types of lung diseases and its severity including asthma, COPD, fixed obstructive airway condition. It is also used to treat and to monitor how well the current treatment is so appropriate and timely treatment adjustment can be made. It takes only a few minutes to do the test. Sometimes, your physician may proceed to ask you do a second lung function test after giving you an albuterol or bronchodilatator inhaler to help diagnose and differentiate different types of lung diseases.