Your doctor’s office will often ask you to perform a pulmonary lung function test. In another article, we talked about spirometry, the FEV, and the FVC measurements. Sometimes, your pulmonologist will include a test referred to as a diffusion test, transfer test, diffusion capacity of carbon monoxide (DLCO), or transfer capacity test (TLCO). Most commonly called the DLCO, this test indicates how well your lungs transfer oxygen — from the air you breathe — into your bloodstream.
In this article, we explain what the test measures, what the numbers mean, how it is done, why pulmonary function tests (PFT) don’t always include it, and how to consider the DLCO value if you have pulmonary fibrosis.
What does the DLCO test measure?
The measurements made in a DLCO test indicate the amount of carbon monoxide (CO) that diffuses or crosses the alveolar-capillary membrane in your lungs. Carbon monoxide is used in the test because it binds strongly to hemoglobin in your blood. By using carbon monoxide, even small changes can easily be observed. By determining the amount of carbon monoxide that diffuses across the membrane, doctors get an idea of how well your lungs are able to transfer oxygen from your lungs to the bloodstream.
What does the DLCO number mean?
The DLCO number is given in milliliters per minute per millimeter of mercury (ml/min/mmHg). Its value is the volume of carbon monoxide (CO) transferred to the blood per minute at a particular carbon monoxide content within the inhaled air. The DLCO number is compared to what is expected for someone of the same age, height, and gender with healthy lungs, which is expressed as a range of values. A DLCO value that is lower than the lower limit of expected or “normal” is seen as an abnormal DLCO test result.
For someone with pulmonary fibrosis, the DLCO value is usually lower than for a similar person without any lung issues. This is because the scarring in the lungs causes the membrane to become thickened and stiff. The scarring makes it harder for both carbon monoxide and oxygen to cross the alveolar membrane. A lower DLCO value means that your lungs are less efficient at transferring oxygen to the bloodstream. If your blood is not getting enough oxygen, as could happen with a lower DLCO value, it can lead to symptoms like shortness of breath and fatigue.
How is the DLCO test done?
The DLCO test is often performed along with spirometry during a pulmonary function test (PFT). The test is usually given using the single-breath technique. The test generally proceeds this way:
- You will be asked to sit or stand in a booth that is connected to a machine called a spirometer. Or you may stand next to a spirometer with a clip placed on your nose.
- Breathing through a mouthpiece or mask, you will be instructed to take a deep breath and hold it for about 10 seconds. The air you inhale will be a mixture containing a very small amount of carbon monoxide.
- Then, you will be instructed to exhale into the spirometer.
- The spirometer reading will indicate the amount of carbon monoxide that has diffused from the inhaled gas into your bloodstream.
- The healthcare professional will record and analyze the results.
It is important to follow any instructions given by your healthcare provider before the test. You may be asked to avoid smoking and taking certain medications prior to the test. Some testers recommend that you wear loose clothing on the day of the test. Following these instructions will increase the accuracy of the results.
Why the DLCO test may not be included in your PFT
- Lack of equipment: DLCO testing requires specialized equipment, including a gas analyzer, and a carbon monoxide source, in addition to the spirometer. Some healthcare facilities may not have this equipment.
- Lack of trained personnel: Performing a DLCO test requires technical expertise and training. If the personnel responsible for performing PFTs are not trained in DLCO testing, your doctor may omit it from the PFT.
- Patient factors: Patients who desaturate too quickly may not be able to perform the test. These are usually patients who are on high levels of supplemental oxygen. Other patients may be unable to perform the DLCO test due to factors such as the inability to hold their breath or chest or abdominal pain.
What to consider when you see your result
When you look at your PFT scores, the three we most often look at are FVC (forced vital capacity, the forced exhaled volume per breath) and FEV1 (forced expiratory volume in 1 second), and the DLCO value. The FVC and FEV give your doctor an idea of how much air you are able to move in and out of your lungs. The DLCO test gives an idea of how well your lungs are transferring oxygen into your bloodstream and carbon dioxide out of the bloodstream for you to exhale.
The percentage (%) numbers are easier to understand than the raw values because percentages compare your result to someone similar to you but with healthy lungs. For people with pulmonary fibrosis, the DLCO percentage is often lower than the FVC and FEV1 percentages, though this is not always true. A low DLCO value alone is not enough to diagnose pulmonary fibrosis or a permanent change in your overall condition. A decrease in the DLCO value over time may indicate that the condition is getting worse, while an improvement in the DLCO value may suggest that treatments are helping.
While a DLCO value lower than 40% is generally considered severe, a person can live with a DLCO percentage in the teens or even in the single digits. Looking at your own results without guidance can be confusing and may cause unnecessary anxiety. Always ask your doctor to explain the results to you.
Summary
Overall, the DLCO test is an important component of pulmonary function tests and provides valuable information about the lungs’ ability to pass oxygen through the membrane of the lungs. The DLCO test is just one component of a complete pulmonary function test, which typically includes several other measurements and your doctor’s knowledge of any accompanying diseases. Always your healthcare professional to discuss the results with you and ask for their interpretation.