Breathing and lung function tests

Gas transfer (or TLco) test

What is a gas transfer test?

A gas transfer test measures how your lungs take up oxygen from the air you breathe. The result of the test is called the transfer factor, or sometimes the diffusing capacity.

A gas transfer test is sometimes known as a TLco test. TLco refers to the transfer capacity of the lung, for the uptake of carbon monoxide (CO).

What’s it used for?

A gas transfer test is used to help diagnose and monitor lung conditions including COPD and pulmonary fibrosis. It may also be used to assess your lungs before surgery, or to see how a person’s lungs react when having chemotherapy.

What happens during a gas transfer test?

You breathe in air containing tiny amounts of helium and carbon monoxide (CO) 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, then breathe out steadily into the machine.

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

The results can be affected by smoking, so if you are a smoker, don’t smoke for 24 hours before your test.

What will the results look like?

The gas transfer test tells your doctor how well your lungs can exchange oxygen from the lungs into the blood. The results will depend on your age, height, sex and ethnicity as well as the level of haemoglobin in your blood. Haemoglobin is the protein in red blood cells that carries oxygen.

Typically, a gas transfer test will give 3 results:

  • TLco. This shows how efficiently your lungs are working to take oxygen from the air you breathe. It is calculated from 2 other measurements: Kco and VA.
  • Kco. This measures how well the airways are performing.
  • VA (alveolar volume). This estimates the lung surface area available for gas exchange.

Low lung efficiency is when your measured results are less than 80% of the normal predicted values.

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Last medically reviewed: January 2020. Due for review: January 2023

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.