Pulmonary Function Test In-Depth

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Contact the Pulmonary Function Test Lab

  • 617-355-7510
  • Fax Number: 617-730-0373
  • International: 01-617-355-5209

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
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Boston Children's Hospital
300 Longwood Avenue, Boston, MA 02115
For Patients: 617-355-6000
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