Pulmonary function tests, or PFTs, measure how well your lungs work. They include tests that measure lung size and air flow, such as spirometry and lung volume tests. Other tests measure how well gases such as oxygen get in and out of your blood. These tests include pulse oximetry and arterial blood gas tests. Another pulmonary function test, called fractional exhaled nitric oxide (FeNO), measures nitric oxide, which is a marker for inflammation in the lungs. You may have one or more of these tests to diagnose lung and airway diseases, compare your lung function to expected levels of function, monitor if your disease is stable or worsening, and see if your treatment is working.