From asthma to chronic obstructive pulmonary disease (COPD), there are a variety of conditions that can affect the lungs. Wheezing or general shortness of breath may be signs that the lungs aren’t functioning exactly as they should be. If you exhibit signs of lung problems, your doctor may order tests to assess lung function.

One of these tests is a lung diffusion test. A lung diffusion test is used to examine how your lungs are processing air. Along with other tests, it can help your doctor determine whether your respiratory system is working properly and efficiently. It may also be known as a diffusing capacity of the lung for carbon monoxide (DLCO) test.

Lung diffusion testing is designed to test how well your lungs allow oxygen and carbon dioxide to pass in and out of your blood. This process is called diffusion.

When you breathe, you inhale air containing oxygen through your nose and mouth. This air travels down your trachea, or windpipe, and into your lungs. Once in the lungs, the air travels through a series of increasingly smaller structures called bronchioles. It eventually reaches tiny sacs called alveoli.

From the alveoli, the oxygen from the air you breathe enters your blood in nearby blood vessels. This is a process called oxygen diffusion. Once your blood is oxygenated, it carries oxygen throughout your body.

Another form of diffusion occurs when blood containing carbon dioxide travels back to your lungs. The carbon dioxide moves from your blood to your alveoli. It’s then expelled through exhalation. This is a process called carbon dioxide diffusion.

Lung diffusion testing can be used to analyze both oxygen and carbon dioxide diffusion.

Doctors typically use lung diffusion testing to assess people who have lung disease or to help diagnose such diseases. Proper assessment and diagnosis is essential for providing the optimal treatment.

If you show symptoms of lung disease, lung diffusion testing may be used to analyze how your lungs are functioning. Also, if you’re undergoing treatment for lung disease, your doctor may order this test from time to time to monitor the disease’s progress and how well your treatment is working.

Before the test, your doctor may ask you to take certain steps to prepare for a lung diffusion test. You may be asked to:

  • avoid using a bronchodilator or other inhaled medications prior to testing
  • avoid eating a large amount of food before the test
  • avoid smoking for several hours before the test

In most cases, a lung diffusion test involves the following steps:

  1. A mouthpiece will be placed around your mouth. It will fit snugly. Your doctor will place clips on your nose to prevent you from breathing through your nostrils.
  2. You’ll take a breath of air. This air will contain a tiny, and safe, amount of carbon monoxide.
  3. You’ll hold this air for a count of 10 or so.
  4. You’ll quickly exhale the air you’re holding in your lungs.
  5. This air will be collected and analyzed.

Lung diffusion testing is a very safe and straightforward procedure. A lung diffusion test doesn’t involve any serious risk. It’s a quick procedure and shouldn’t cause most people any significant pain or discomfort.

Most likely, you won’t experience any negative side effects after the test is complete.

This test looks at how much of a certain gas you inhale and how much is present in the air you exhale. Usually, the lab will use carbon monoxide, or another “tracer” gas, to determine your lungs’ ability to diffuse gases.

The lab will consider two things when determining the results of the test: The amount of carbon monoxide that you originally inhaled and the amount that you exhaled.

If there’s far less carbon monoxide in the exhaled sample, it indicates that a large amount of the gas was diffused from your lungs into your blood. This is a sign of robust lung function. If the amount in the two samples is similar, the diffusing capacity of your lungs is limited.

Test results are variable, and what’s considered “normal” will vary from person to person. Your doctor will need to consider a number of factors to decide whether your test results suggest problems with lung function, including:

  • whether or not you have emphysema
  • whether you’re a man or a woman
  • your age
  • your race
  • your height
  • the amount of hemoglobin in your blood

Generally speaking, your doctor will compare how much carbon monoxide they expect you to exhale to the amount of carbon monoxide that you actually exhale.

If you exhale anywhere from 75 to 140 percent of the amount that they predicted you would, your test results may be considered normal. If you exhale between 60 to 79 percent of the amount predicted, your lung function may be considered mildly reduced. A test result below 40 percent is a sign of severely reduced lung function, with a result below 30 percent making you eligible for Social Security disability benefits.

If your doctor determines that your lungs aren’t diffusing gas at the level they should be, there may be a number of causes. The following conditions may lead to abnormal results:

If your doctor suspects your lungs aren’t working properly, they may order several tests in addition to the lung diffusion test. One such test is spirometry. This measures the amount of air you take in and how fast you can exhale it. Another test, lung volume measurement, determines your lung size and capacity. It’s also called a lung plethysmography test.

The combined results of these tests can help your doctor figure out what’s wrong and what steps can be taken to relieve your symptoms.