What you can expect

To do this test, you'll be seated. Your doctor will have you put in a mouthpiece that's attached to a tube that leads to an electronic measurement device. Next, you'll breathe in slowly and deeply until your lungs are filled with air. Your doctor will then have you exhale slowly and steadily so that the air flows out of your lungs at a steady, even rate.

Your doctor may have you watch a computer monitor that registers how much you're breathing out so that you can maintain a steady exhalation. You'll need to repeat the test a few times to confirm your results. The entire test generally takes five minutes or less.