What Lung Diffusion Tests Tell Your Doctor About Your Health

Lung Diffusion Tests Can Help Your Doctor Diagnose a Variety of Conditions

Lung diffusion tests measure how well your lungs perform their job. When you breathe your lungs take in oxygen, which then passes into your bloodstream and is expelled by your lungs as carbon dioxide. Lung diffusion tests compare the amount of carbon monoxide in the air you inhale versus the amount in the air you then exhale.

Lung function test
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These tests can help your healthcare provider determine how well oxygen passes from your alveoli, small air sacs in your lungs, into your bloodstream. 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.

Doctors use lung diffusion testing to help diagnose conditions that affect the lungs, including COPD, asthma, emphysema, pulmonary hypertension, and sarcoidosis. Lung diffusion testing may be ordered to diagnose a medical condition or to monitor how you're doing after you've been diagnosed.

How Are Lung Diffusion Tests Performed?

Your test will likely be performed in your doctor's office by an experienced technician. You'll be sitting down and wearing a mouthpiece that fits tightly around the outside of your lips. You will also 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 your breath.

Preparing for Your Lung Diffusion Test

If your doctor has scheduled you for a lung diffusion test, you will need to abstain from smoking at least four to six hours before the test. You should also avoid eating any heavy meals prior to the test.

If you use bronchodilators or other inhalers, be sure to check with your healthcare provider to see if you can use them on the day of your test. In most cases, your doctor will ask you not to use them, since that will make the test results more accurate. The test is simple to perform and carries no significant risks. 

Understanding Your Results

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

If you see the acronym "DLCO" on your paperwork or results, it is just shorthand for diffusing capacity of the lungs for carbon monoxide, which is what lung diffusion tests measure. 

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  • U.S. National Library of Medicine. Lung Diffusion Testing fact sheet. 2016.