A pulmonary function test checks how well the lungs are working, states MedlinePlus. It checks for overall airflow and how well the lungs are converting air from the atmosphere into the body.
The test includes use of spirometry, which measures airflow, and is accomplished by breathing into a mouthpiece, which measures the amount of air exhaled and the rate of that air, states MedlinePlus. The test is performed while sitting, and it is important to remain in the same position to ensure that airflow is measured correctly. Testing checks lung function and can determine the presence of lung disease and other lung conditions.