Pulmonary function tests (PFT's) are breathing tests to find out how well you move air in and out of your lungs and how well oxygen enters your body. The most common PFT's are spirometry (spy-RAH-me-tree), diffusion studies and body plethysmography (ple-thiz-MA-gra-fee). Sometimes only one test is done, other times all tests will be scheduled, often on the same day.