Diffusion capacity

What is diffusion capacity testing?

Diffusion capacity testing, also referred to as 'lung diffusion testing', a 'DLCO test' or a 'gas diffusion study' is a test to measure the diffusion capacity of the lungs using carbon monoxide. In other words the test uses a very small amount of carbon monoxide (CO) in a 'test gas' to measure the ability of the lungs to absorb oxygen from the air. The carbon monoxide present is referred to as the 'tracer gas'.

The test is commonly performed in conjunction with spirometry and plethysmography.

What is diffusion capacity testing used for?

Diffusion capacity testing helps in the process of diagnosing the following conditions among others.

  • Emphysema - a type of chronic bronchitis that often affects smokers
  • Pulmonary hypertension - high blood pressure in the arteries of the lungs
  • Pulmonary fibrosis - thickening/scarring of the airways
  • Pulmonary embolism - a blood clot blocking an artery
  • Blood loss (haemorrage) in the lung

It is also used to monitor existing lung conditions and check the effectiveness of any treatments.

What is involved in a diffusion capacity test?

During the test you inhale a test gas (which contains a small quantity of carbon monoxide) through a mouthpiece, hold it in your lungs for 10 seconds and then exhale. You will also have to have a clip over your nose to ensure you do not breathe through it. The exhaled gas is then analysed to measure its makeup.

How do I prepare for the test?

As with other lung function tests, you should let the doctor know if you are taking any medications to help with breathing difficulty and you may need to stop taking these before the test; you should avoid eating a large meal within two hours of the test; you should not smoke or drink alcohol in the 4 hour period before the test. Wear loose fitting clothing so you can breathe properly during the test.