Spirometry—You breathe into a mouthpiece that is connected to an instrument called a spirometer. The spirometer measures the volume of air that you can force out of your lungs in one second after having inhaled as much as you can. You will be asked to hold the tube of a spirometer in your mouth, inhale as much air as possible, then blow out as hard as you can into the spirometer for one second. The amount of air you can force out is called your forced expiratory volume, or FEV1.
Lung volume—You breathe nitrogen or helium gas through a tube for a certain amount of time. Then the concentration of the gas in a chamber attached to the tube is measured.
Diffusion capacity—You breathe carbon monoxide for a very short time (often one breath). The concentration of carbon monoxide in the air you exhale is then measured. The difference in the amounts of carbon monoxide inhaled and exhaled shows how quickly gas can travel from your lungs into the blood.
PFTs are painless, and you will have time to rest between the different breathing measurements. The measurements may be repeated two or more times.