Pulmonary Function Test Pulmonary function tests are in a wide range, among all of tests which are used to measure how well the lungs take air in and exhale air out and how efficiently they will transfer oxygen into the blood. Spirometry is the most common of the pulmonary function tests it measures how well the lungs will exhale air.The information which is gathered during this test is useful in diagnosing certain types of lung disorders, but it is most useful when assessing for obstructive lung diseases. How the test is performed? In a spirometry test, a person breathes into mouthpiece which is connected to an instrument called a spirometer. The spirometer records the amount and the rate of air which is breathed in and out over a specified time. Some of the test measurements are obtained by normal way, quiet breathing, and also other tests generally require forced inhalation or exhalation after a deep breath.