Pulmonary function test is performed to evaluate how well your lungs work. It measure how well the lungs take in and exhale air, it’s volume, capacity, rate of flow & how efficiently they transfer oxygen into the blood, The information gathered during this test is useful in diagnosing certain types of lung disorders, but is most useful when assessing for obstructive lung diseases (especially asthma and chronic obstructive pulmonary disease, COPD). Your doctor will order these tests to determine how your lungs are working. If you already have a condition that’s affecting your lungs, your doctor may order this test to see if the condition is progressing or how it’s responding to treatment.