Gas transfer test

Find out how gas transfer tests are done, what they are used for, and what your results might mean.

What is a gas transfer test?

A gas transfer test is a type of breathing test to see how well oxygen passes from your lungs to your blood. The test is usually for people over five years old.  

A gas transfer test is used to help diagnose and monitor lung conditions including COPD, pulmonary fibrosis and asthma.  

It may also be used to check how well a person’s lungs are working. This could be to see if you are fit enough for surgery or to see how you react to treatments such as chemotherapy. 

How do I prepare for a gas transfer test?

Your appointment letter will tell you what you need to do to prepare. You may be asked to:  

  • take all your usual medicines as normal, unless your letter tells you not to
  • wear loose fitting clothing 
  • not smoke 24 hours before the test (if you smoke) 
  • not drink alcohol 24 hours before the test (if you drink alcohol)
  • not have caffeine (tea, coffee) 24 hours before the test
  • not eat a large meal 24 hours before the test
  • not do any intense exercise 30 minutes before the test.

How is a gas transfer test done?

During the test, you breathe in air from a mouthpiece that has tiny amounts of helium and carbon monoxide (CO) gases added. These very low levels of gas are completely safe.  

You will be asked to take in a deep breath through a mouthpiece while wearing a nose clip. You then hold your breath for at least 10 seconds, then breathe out slowly into the machine.

You may need to do this a few times, with a pause of a few minutes in between. Don’t worry if it takes a few attempts to get a reading.

The whole test takes around 10-30 minutes.

Side effects

You might feel dizzy or cough more than normal after your gas transfer test. Don’t worry, these are common side effects. Tell your healthcare professional if you need to rest after the test.

What will the results look like?

The results depend on your age, height, sex, and ethnicity.

Higher than average results can be seen in people with asthma or people who are overweight (obese).   Lower than average results can tell your doctor if you have a restrictive or obstructive lung condition

Get support

Call our Helpline for support with your condition. Get advice on your medicines, symptoms or travelling with a lung condition, or just call us to say hello.

Did you find this information useful?

We use your comments to improve our information. We cannot reply to comments left on this form. If you have health concerns or need clinical advice, call our helpline on 0300 222 5800 between 9am and 5pm on a weekday or email them.

Page last reviewed:
Next review due: