Pulmonary Function Test

What are pulmonary function tests?

Pulmonary function tests (PFTs) are diagnostic tests to evaluate 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.

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.

If your child is 5 years old or older, they should be capable of performing the breathing maneuvers necessary for meaningful pulmonary function testing.

What are the different types of PFTs?

PFTs may involve several different tests.


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 the largest possible breath
  • measures how fast the air comes out (the flow speed)

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.

Oxygen saturation

Another important test 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 measures the amount of oxygen in the blood without using a needle.

Arterial blood gas

For more detailed information, your child's blood needs to be tested. This test, called an arterial blood gas, involves drawing blood from an artery in the wrist to 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, your clinician may perform tests to:

  • measure respiratory muscle strength
  • determine responsiveness to inhaled medications
  • evaluate lung responsiveness to cold air and exercise

What happens before a pulmonary function 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 your child to be relaxed and breathe into a mask, so we will give them a mild sedative, which will let them sleep through the entire procedure. Once your child is asleep, the test takes about 50 minutes.

How are PFTs performed?

Your child is placed in a special Plexiglas bed and a mask is placed over the mouth and nose. A loose fitting jacket is placed around the chest and stomach. They are 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 they are 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 observe them 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.