Lung function tests

Pulmonary function tests are a group of tests that measure how well the lungs are working. They measure how much air your lungs can hold and how well you can let the air out of your lungs. Pulmonary function tests are also called lung function tests.

Spirometry measures how much air you can breathe out from your lungs and how quickly you can do it.

Lung plethysmography measures how much air is in your lungs after you take a deep breath and how much air is left in your lungs after you breathe out as much as you can.

Lung diffusion testing measures how well oxygen moves from your lungs into your blood.

Why pulmonary function tests are done

Pulmonary function tests may be done to:

  • make sure that your lungs are healthy enough to have certain treatments
  • get a baseline to use during treatment before chemotherapy starts
  • measure how certain medicines or chemotherapy may be affecting the lungs
  • make sure that your other lung will work properly if you need to have a lung removed

How pulmonary function tests are done

Pulmonary function tests are usually done in a special department of a hospital by a trained respiratory therapist.

Your healthcare team will tell you how to prepare for the test. Tell your healthcare team about all prescription and non-prescription medicines you are taking, including ones for breathing problems. You are usually told not to smoke for 4 to 6 hours before you have a pulmonary function test.

Wear loose clothing so that you can breathe comfortably. Avoid eating a heavy meal before pulmonary function tests – it can make it harder for you to take deep breaths.

Each type of pulmonary function test is done differently.


During spirometry, you will be asked to breathe using a long tube with a cardboard mouthpiece. The long tube is attached to a computer that measures the amount of air breathed out over time. You may need to wear a nose clip to prevent air from leaking out of your nose.

You will first be asked to breathe gently through the mouthpiece. You will then be asked to take in the biggest breath you can and then blow it out as hard, fast and long as you can.

Lung plethysmography

During lung plethysmography, a machine measures the amount of air you breathe in and the force of air you breathe out. You sit in a very small space surrounded by clear walls inside the machine. You will be wearing nose clips and will need to blow into a mouthpiece that is connected to the machine.

During the test you can see the respiratory therapist and they can see you.

The test will be done several times to get an accurate reading.

Lung diffusion testing

During lung diffusion testing, you breathe in a small amount of gas through a mouthpiece on a tube. After holding your breath for about 10 seconds, you then blow out the gas.

This air is collected in the tube and examined.

Potential side effects

Pulmonary function tests are very safe procedures. Most people do not have any pain or discomfort.

What the results mean

Several different types of measurements are taken during pulmonary function testing. The results tell your healthcare team how well your lungs are working. Abnormal results could suggest that you have a lung problem or a lung disease such as:

  • asthma
  • chronic obstructive pulmonary disease (COPD)
  • scarring of the lung tissue (pulmonary fibrosis)
  • heart failure
  • loss of lung tissue

What happens if a change or abnormality is found

The results of your lung function tests may affect the type of treatment you are offered for cancer. The doctor will decide whether you need further tests, procedures, follow-up care or more treatment.

Special considerations for children

Being prepared for a test or procedure can reduce anxiety, increase cooperation and help the child develop coping skills. Parents and caregivers can help prepare children by explaining to them what will happen, including what they will see, feel, hear, taste or smell during the test.

Pulmonary function tests are done in a different way for babies and young children. Sometimes children need sedation to keep still or calm for the test. Your child may not be able to have solid food or liquids for a period of time before being sedated. Your child’s healthcare team will tell you how to prepare.


During spirometry, your child will wear a special loose-fitting jacket around their chest and abdomen and a mask over their mouth and nose. They’ll be easily seen in a bed that is made of clear plastic. The jacket inflates from your child’s breath. This gently squeezes the chest, which causes your child to breathe out.

Lung plethysmography

During lung plethysmography, your child will wear a mask over their mouth and nose. They’ll be easily seen in a bed that is made of clear plastic. The top of the bed is briefly closed around the child to form a box. The computer calculates lung capacity from pressure and air flow measurements.

Preparing a child for a pulmonary function test depends on the age and experience of the child. Find out more about helping your child cope with tests and treatments.

Expert review and references