Breathing and lung function tests

Gas transfer (or TLco) test

What is it?

It’s to see how well your lungs are able to take up oxygen from the air you breathe. The result of the test is called the transfer factor or sometimes the diffusing capacity.

What’s it used for?

It’s used to help diagnose and monitor certain lung conditions including COPD and pulmonary fibrosis. It may also be used to assess your lungs before surgery or to assess how your lungs react when undergoing chemotherapy.

What happens during the test?

You breathe in air containing very small amounts of helium and carbon monoxide gases. These are completely harmless at the very low levels used. You will be asked to take in a big breath through a mouthpiece while wearing a nose clip. You then hold your breath for a minimum of 8 seconds and then breathe out steadily into the machine.

You will need to do this a few times with a pause in between. The whole test should take no more than 10 minutes.

What will the results look like?

The test tells your doctor how efficient your lungs are at exchanging oxygen from the lungs into the blood. The results will depend on your age, height, gender and ethnicity as well as the level of haemoglobin in your blood. Haemoglobin is the protein in red blood cells that carries oxygen.

The results can be affected by smoking, so if you are a smoker, try not to smoke for 24 hours before your test.

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Last medically reviewed: January 2017. Due for review: January 2020

This information uses the best available medical evidence and was produced with the support of people living with lung conditions. Find out how we produce our information. If you’d like to see our references get in touch.