Normal lung diffusion capacity is typically defined as a result at or above the lower limit of normal (LLN) for your age, sex, and height, which means your result falls within the range expected for 95% of healthy people matching your profile. There is no single number that qualifies as “normal” for everyone, because your predicted value is calculated individually. Most pulmonary function labs report your result as a percentage of that predicted value, and results above roughly 75–80% of predicted are generally considered within the normal range.
What Lung Diffusion Capacity Measures
Lung diffusion capacity, formally called DLCO (diffusing capacity of the lungs for carbon monoxide), measures how efficiently gas crosses from the tiny air sacs in your lungs into your bloodstream. While the ultimate goal is to understand oxygen transfer, the test uses a small, harmless amount of carbon monoxide instead, because carbon monoxide binds to red blood cells so readily that it makes the measurement more precise.
This is different from spirometry, which measures how much air you can blow in and out and how fast. Spirometry tells you about airflow. DLCO tells you about gas exchange, the step where oxygen actually enters your blood. You can have normal spirometry results and still have an abnormal diffusion capacity, which is why doctors order both. In conditions like pulmonary fibrosis, DLCO is often the first lung function test to show an abnormality, sometimes before other tests detect a problem.
How Your Predicted Value Is Calculated
Because lung size and blood volume vary so much from person to person, your result is compared against a predicted value tailored to you. The main factors that determine your predicted DLCO are height, age, sex, and hemoglobin level.
- Height: Taller people have larger lungs with more surface area for gas exchange, so predicted DLCO increases exponentially with height.
- Sex: Men have roughly 15% higher diffusion capacity than women of the same height and age, largely due to differences in lung size and hemoglobin concentration.
- Age: DLCO rises each year until about age 20, plateaus, then begins declining after age 35 at a rate of roughly 0.6% per year.
- Hemoglobin: Since carbon monoxide binds to hemoglobin in red blood cells, your hemoglobin level directly affects the result. Each gram per deciliter of hemoglobin accounts for about a 6% difference in DLCO. Labs often adjust your raw result to a standard hemoglobin level (14.6 g/dL for men, 13.4 g/dL for women) so that anemia or high red blood cell counts don’t skew the interpretation.
The reference equations used to generate predicted values come from large population studies. The American Thoracic Society and European Respiratory Society recommend using equations from the Global Lung Initiative, which were developed from data on more than 12,000 individuals across 14 countries.
How to Read Your Results
Your report will show your measured DLCO, your predicted DLCO, and a percentage of predicted. It may also include a z-score, which expresses how far your result sits from the average for someone like you. A z-score above negative 1.645 is considered normal, meaning your result falls within the range that includes 95% of healthy individuals. A z-score below that threshold places you in the lowest 5%, which is flagged as potentially abnormal.
In clinical practice, many labs still use percentage of predicted as a shorthand. A common grading scale categorizes results like this:
- Normal: above 75–80% of predicted
- Mild reduction: 60–79% of predicted
- Moderate reduction: 40–59% of predicted
- Severe reduction: below 40% of predicted
These cutoffs vary slightly between institutions. The ATS and ERS prefer the statistical lower limit of normal over fixed percentage cutoffs, because a flat 80% threshold can misclassify older adults (whose normal range is naturally wider) or younger, tall individuals.
What a Low DLCO Means
A reduced diffusion capacity means gas is having trouble crossing from your lungs into your blood. This can happen for two broad reasons: the membrane between the air sacs and blood vessels is thickened or destroyed, or there’s less blood flowing through the lung capillaries to pick up the gas.
Conditions that damage or destroy the air sacs, like emphysema, reduce the total surface area available for gas exchange. Conditions that thicken the membrane, like pulmonary fibrosis or other interstitial lung diseases, slow the rate at which gas can cross. Pulmonary hypertension (high blood pressure in the lung arteries) reduces blood flow through the capillaries, which also lowers DLCO. In fact, when DLCO drops out of proportion to other lung function measurements, it can be an early signal of pulmonary hypertension developing alongside lung disease.
Anemia is another common and often overlooked cause of a low reading. Because the test depends on carbon monoxide binding to hemoglobin, fewer red blood cells means less gas gets absorbed, even if the lungs themselves are perfectly healthy. This is why the hemoglobin-adjusted value on your report matters: it separates a blood issue from a lung issue.
What a High DLCO Means
An above-normal DLCO is less common but does occur. In a study of patients with elevated readings, 62% had obesity, asthma, or both. Obesity increases blood volume in the chest, giving carbon monoxide more hemoglobin to bind to. Asthma can increase DLCO because the airways are hyperreactive but the gas-exchange membrane itself is intact, and some patients with asthma have larger lung volumes that boost the measurement.
Less common causes include polycythemia (an unusually high red blood cell count) and bleeding within the lungs, where leaked blood in the air sacs absorbs extra carbon monoxide during the test. A high DLCO on its own is rarely a cause for concern, but your doctor may investigate further if it doesn’t match your clinical picture.
What Happens During the Test
The standard method is the single-breath technique. You sit upright, breathe out fully, then take one deep breath of a test gas mixture containing a tiny amount of carbon monoxide (0.3%), a tracer gas like helium or methane, and the usual nitrogen and oxygen that make up air. You hold that breath for about 10 seconds, then exhale steadily into the mouthpiece. The machine compares how much carbon monoxide you breathed in to how much you breathed back out. The difference tells the lab how much crossed into your blood.
The tracer gas serves a separate purpose: it doesn’t cross into the blood at all, so the way it dilutes during the breath-hold reveals the total volume of air your lungs held. This measurement, called alveolar volume, helps the lab distinguish between a low DLCO caused by small lungs (for instance, after a lung surgery) versus one caused by actual damage to the gas-exchange surface.
Most labs will ask you to repeat the test two or three times with a few minutes of rest between attempts to ensure the results are consistent. The whole process takes about 15 to 30 minutes.