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This department is currently not accepting appointment requests online. Please call us at: 617-355-6000. International +1-617-355-6000.

This department is currently not accepting appointment requests online. Please call us at: 617-355-6000. International +1-617-355-6000.

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Pulmonary Function Test

  • Pulmonary function tests are diagnostic tests that can provide your child's caregiver with information about:

    • how well his lungs are working
    • how well the medicine is working (if your child takes medicine to help him breathe)

    Here at Boston Children’s Hospital, we provide pulmonary function tests for infants, older children and adults.

    Contact Us
    Boston Children's Hospital
    300 Longwood Avenue
    Farley 4
    Boston MA 02115 

    fax: 617-730-0373

    Contact us text

  • Infant pulmonary function tests: an introduction

    Since infants aren’t capable of breathing on demand for a test, special equipment has been created to measure their lung function. There is more preparation involved for infant PFTs than for tests in older children. The procedure is reviewed for you below, along with answers to commonly asked questions:

    PFTs in older children and adults

    If your child is 5 years old or older, he should be capable of performing the breathing maneuvers necessary for meaningful pulmonary function testing. These tests are performed in our Pulmonary laboratory on Farley 5.


    Pulmonary function tests always include spirometry.  This is a test that:

    • measures the volume of air that can be exhaled from the lung after your child takes his largest possible breath
    • measures how fast the air comes out (the flow speed)

    Here’s how a spirometry is performed:

    1. First, your child is first asked to place his mouth around a cardboard mouthpiece on a long tube attached to our computers. He will also put on a nose-clip to prevent air from leaking out of his nose during the test.
    2. Your child will begin by breathing quietly through the mouthpiece.
    3. He will then be instructed to take in the biggest breath possible, and then blow it out as hard, as fast and as long as possible.
    4. The computer measures the amount of air breathed out over time.

    Total Lung Capacity (TLCC)

    Another important test is the Total Lung Capacity (TLC), which measures total amount of air your child’s lungs can hold.

    This measurement is performed in a body plethysmograph, or "the box"— a clear Plexiglas booth, similar to a telephone booth. Once inside the booth, this test is similar to the spirometry: your child breathes into a tube, and again the tube attached to a computer. The computer then calculates his lung volume.

    Oxygen saturation

    Another important test we perform measures the amount of oxygen and carbon dioxide in the blood. The simplest test is the oxygen saturation. This is a routine test in which a small red light is placed on the bed of your child’s fingernail; the light lets us measure the amount of oxygen in the blood without using a needle.

    Arterial blood gas

    However, to get more detailed information, we need to test his blood. This test, called an arterial blood gas, involves drawing blood from an artery in the wrist. We then measure the amount of oxygen and carbon dioxide in the arterial blood to determine how well the lungs are working.

    These are the most common tests performed in this lab; however, we can also perform tests to:

    • measure respiratory muscle strength
    • determine responsiveness to inhaled medications
    • evaluate lung responsiveness to cold air and exercise
  • When are these tests done?

    Your child's doctor may request tests in order to evaluate baseline your child’s lung function or to measure any change in your child's condition. Pulmonary function testing is often used to assess lung function if your child has a chronic pulmonary disease, such as cystic fibrosis or bronchopulmonary dysplasia.

    What happens before the test?

    We ask that you not feed your child any solid food for six hours before the test and any liquids for four hours before.

    These tests cause no discomfort. They do require him to be relaxed and breathe into a mask, so we will give him a mild sedative which will let him sleep through the entire procedure. The medicine used for sedation is called chloral hydrate and is administered by mouth. Once your child is asleep, the test takes about 50 minutes.

    How are these tests done?

    Your child is placed in a special Plexiglas bed and a mask is placed over his mouth and nose. A loose fitting jacket is placed around his chest and stomach. He is given a few deep breaths from the mask and then the bag is inflated so it gently squeezes the chest causing him to blow out. This is similar to asking the older children to take a huge breath and blow out as hard as they can.

    For the last part of the test, we measure the size of your child’s lungs. The air coming in and out of your child's lungs is stopped for a few seconds, which causes a pressure change in the box and enables the computer to calculate lung size.

    What happens after the test?

    We will wake your child up immediately following the test. Most children wake up easily and are eager for something to drink. Once he is drinking well, you will be free to leave the lab. Your child may be a bit floppy and uncoordinated the better part of the day and it is important to watch him carefully to prevent any injuries from falls. Most children nap off and on the rest of the day and return to their normal sleep pattern that night.

    You will be given special discharge instructions as well as a number to call in case you have any questions after the test. Your child's provider will have the results of the test within a week.

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- Sandra L. Fenwick, President and CEO