What Are Lung Diffusion Tests and How Are They Used?

A lung diffusion test.
A lung diffusion test. Science Photo Library/Getty Images

Lung diffusion tests measure how well your lungs perform their job: taking in oxygen that then passes into your blood stream, and expelling carbon dioxide.

Specifically, the tests can help your health care provider determine how well oxygen passes from the alveoli, or air sacs, of your lungs into your bloodstream. To get even more technical, lung diffusion tests measure the "diffusing capacity of the lungs for carbon monoxide" or DLCO for short.

Doctors use lung diffusion testing in several different conditions that affect the lungs, including COPD, asthma, emphysema, pulmonary hypertension and sarcoidosis, a disease involving inflammation in the lungs and elsewhere in the body. Lung diffusion testing may be ordered both to diagnose your medical condition and to monitor how you're doing after you've been diagnosed.

How Is a Lung Diffusion Test Performed?

The test is performed by an experienced technician. You'll be sitting down, and you'll wear a mouthpiece that fits tightly around the outside of your lips. In addition, you'll need to wear clips on your nose to pinch it shut so you can't breathe through it.

For the actual test, you will inhale a small amount of carbon monoxide gas, hold your breath for 10 seconds, and then exhale as fast as possible. The exhaled gas is then analyzed to determine how much carbon monoxide was absorbed by your body during the breath.

How to Prepare for Your Lung Diffusion Test

Preparation for lung diffusion testing is simple:

  • Do not eat a heavy meal prior to the test
  • Refrain from smoking at least 4 to 6 hours before the test

If you use bronchodilators or other inhalers, be sure to check with your health care provider to see if you should use them prior to the test.

In most cases, your doctor will ask you not to use them, since that will make the test results more accurate.

The test isn't risky, and in fact most people find it's not problematic to perform.

Determining Results: Normal vs. Abnormal

Lung diffusion tests compare the amount of carbon monoxide in the air you inhale versus the amount in the air you then exhale.

In someone with strong, healthy lungs, far less carbon monoxide will be present in the exhaled air than was present in the inhaled air. This is because the lungs were able to diffuse, or process, most of that gas. In someone with diminished lung function, more carbon monoxide will be left in the exhaled air.

Normal results of this test are set to a standard for lung function based on a person's age, sex and height. Doctors use that standard, plus a patient's overall health status, to help diagnose lung problems. In order for your health care provider to make an accurate diagnosis, other pulmonary function tests may also be ordered in addition to lung diffusion tests.


U.S. National Library of Medicine. Lung Diffusion Testing fact sheet. Accessed Jan. 14, 2016.

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