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. 
 

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.
   

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.
 

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