What Is a Diffusion Test (DLCO)? 

A common test in the COPD community is a pulmonary function test (PFT). A PFT includes a series of breathing tests to help determine how your lungs are functioning. One part of a PFT your doctor may order is a lung diffusion test. What is this? Why would your doctor order it? Here’s what to know.

What is a diffusion test?

A diffusion test is a neat test for determining how good your lungs are at getting oxygen (O2) from the air you inhale to your blood system. When you inhale, oxygen molecules travel down your airways. At the end of these airways are collections of grape-like structures called alveoli. These alveoli are tiny air exchange units. Oxygen collects inside these alveoli.1-2

Alveoli come into contact with a series of blood vessels called capillary blood vessels. The walls of alveoli and capillaries are permeable to oxygen which allows it to travel across alveolar walls and into a capillary.1-2 This is how oxygen gets from the air you inhale into your blood system.

Your blood consists of hemoglobin molecules. These are specialized molecules that allow oxygen to be transported through your blood. Oxygen has a high affinity for hemoglobin. So, once in your blood system, it binds with a hemoglobin molecule. In this way, hemoglobin takes oxygen to all the cells of your body. Then they return to your lungs to collect more oxygen molecules.1-3

In short, a diffusion test determines how good your lungs are at allowing oxygen to travel from your alveoli to hemoglobin.

How is this test useful?

A diffusion test is helpful for both diagnosing and monitoring the progression of emphysema. As we know, emphysema is a disease that causes the breakdown and destruction of the alveoli and capillary system. So, it makes it so there is less surface area for gas exchange to occur.3-4 With less surface area, less oxygen will travel from alveoli to hemoglobin molecules.

For the test, a gas called carbon monoxide (CO) is used, which is why the test is sometimes referred to as a DLCO. This stands for diffusion capacity through the lungs of carbon monoxide.3 CO is used because, like oxygen, it has a high affinity for hemoglobin. Also like oxygen, it has the ability to pass through alveolar and capillary walls. So, once you inhale it into your alveoli, it will cross the alveolar-capillary membrane into capillary blood. From there it will bind with hemoglobin.3-4

There are three things that may decrease the diffusion capacity of oxygen:4

  1. Loss of alveoli (less surface area) caused by emphysema.
  2. Loss of blood, which would cause your hemoglobin levels to decline.
  3. Both of the above.

When you have less blood than normal it is called anemia. If your blood volume is low, your hemoglobin level will be decreased. If you have less hemoglobin, then your DLCO may be decreased even if you have normal lung function. For this reason, the test results need to be adjusted for your hemoglobin level, which is why you need to have your blood drawn before you have the test.4

After that, you enter your PFT appointment. A clinician may perform other PFT tests, the most common of which is spirometry. The next test to be performed is your DLCO.

How is the test performed?

For this test, you sit straight up in a chair. You then put your mouth securely around a mouthpiece. Then you will be asked to breathe normally. When you are ready, you exhale hard, followed by a full deep inhalation. Take in as deep a breath as you can. It is at this time you will be inhaling a gas that contains CO. Then you are asked to hold your breath for ten seconds.4

This pause is important. It gives the CO time to cross from alveoli to your blood system and onto hemoglobin. You are then asked to exhale completely. At this time, the exhaled gas is collected and measured. Using a formula, your DLCO is determined.3 This test is done three times. The results of the best test will be recorded and sent to your doctor for his or her interpretation.

What do test results show?

The following is a useful guide for physicians. It can help determine if emphysema is present and the severity of it.3

  • Normal: 75% of predicted or greater
  • Mild: 60% of predicted
  • Moderate: 40-60% of predicted
  • Severe: 40% or less of predicted

The test is not helpful for diseases that affect your airways. Examples here are asthma and chronic bronchitis. These diseases obstruct airways, but they do not impact the ability of oxygen to diffuse from your lungs to your blood. So, with asthma and chronic bronchitis, your DLCO should be normal.

However, the test is very effective for helping physicians both diagnose and monitor the progression of emphysema. As emphysema progresses, your DLCO decreases. Your DLCO may also be decreased if you have diseases such as interstitial lung disease or pulmonary fibrosis. These diseases, like emphysema, do affect the diffusion of oxygen from your alveoli to your blood system.

An decreased DLCO with normal spirometry may also indicate pulmonary vasculature diseases. These are diseases that decrease the amount of blood flowing through the lungs and include diseases such as pulmonary embolism (PE) and pulmonary hypertension.

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