In a Spirometry test, you breathe into a mouthpiece that is connected to an instrument called a Spirometer. The Spirometer records the amount and the rate of air that you breathe in and out over a period of time. A Spirometry test is used to test several outcomes including lung volume and diffusion capacity. For some of the test measurements, you can breathe normally and quietly.  By measuring how much air you exhale, and how quickly, spirometry can evaluate a broad range of lung diseases. Lung volume measures the amount of air in the lungs without forcibly blowing out. Some lung diseases (such as emphysema and chronic bronchitis) can make the lungs contain too much air. Other lung diseases (such as fibrosis of the lungs and asbestosis) make the lungs scarred and smaller so that they contain too little air. Testing the diffusion capacity (also called the DLCO) allows the doctor to estimate how well the lungs move oxygen from the air into the bloodstream.

Lung volume measurement can be done in two ways: 

The most accurate way is to sit in a sealed, clear box that looks almost like a telephone booth (called a body Plethysmograph) while breathing in and out into a mouthpiece. Changes in pressure inside the box help determine the lung volume.

Lung volume can also be measured when you breathe nitrogen or helium gas through a tube for a certain period of time. The concentration of the gas in a chamber attached to the tube is measured to estimate lung volume.

 To measure diffusion capacity:

You breathe a harmless gas for a very short time, often one breath. The concentration of the gas in the air you breathe out is measured. The difference in the amount of gas inhaled and exhaled measures how effectively gas travels from the lungs into the blood.

For some of the test measurements, you can breathe normally and quietly. Other tests require forced inhalation or exhalation after a deep breath. Since the test can involve some forced/rapid breathing, you may have some temporary shortness of breath or lightheadedness. You breathe through a tight-fitting mouthpiece, and you’ll have nose clips.