DLCO Lung Diffusion Test: Purpose, Normal Range and Results
Updated: 2 days ago
Written and medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: November 2026 · How we review
Quick Answer
A lung diffusion test (DLCO) is a pulmonary function test that measures how well oxygen moves from the lungs into the blood. You breathe in a safe, tiny amount of carbon monoxide, hold your breath for 10 seconds, then breathe out into a machine. A result of 75% to 140% of the predicted value is normal.
What the research says
Low DLCO predicts outcomes in mild COPD: in 360 people with mild COPD followed for about 9 years, those with DLCO below 60% of predicted had a death rate of 23% vs. 9% (de-Torres JP et al., Chest, 2021) [2]
Changes over time matter: in 104 people with lung fibrosis, the change in DLCO over 6 months predicted survival as well as the biopsy diagnosis (Latsi PI et al., Am J Respir Crit Care Med, 2003) [3]
Key takeaways
DLCO measures how well oxygen passes from your lungs into your blood.
You inhale a tiny amount of carbon monoxide and hold your breath for 10 seconds.
Normal is 75% to 140% of the predicted value.
A low DLCO can signal lung, heart or blood vessel problems.
Avoid smoking for 4 to 6 hours beforehand.
What is a DLCO test?
Oxygen passes from tiny air sacs in your lungs into the blood vessels around them. Carbon monoxide crosses the same barrier and binds strongly to red blood cells, so measuring how much is absorbed shows how well this transfer works. DLCO is often done with spirometry and body plethysmography for a full picture of lung function. Sources: [1]
Why it's done
DLCO shows how well your lungs transfer oxygen.
Explain shortness of breath, coughing or wheezing.
Assess lung damage or screen for lung disease.
Check lung function before surgery or a strenuous exercise program.
See how a disease or treatment affects your lungs. Sources: [1]
How to prepare
Don't smoke for 4 to 6 hours before the test.
Don't use supplemental oxygen for 10 minutes beforehand, if it's safe.
Avoid exercise right before the test. Sources: [1]
What happens during the test
The test is repeated at least once.
A soft clip is placed on your nose.
You breathe through a mouthpiece connected to a machine.
After a few normal breaths, you breathe out fully, then breathe in deeply.
You hold your breath for 10 seconds, then breathe out.
The test is repeated at least once after a few minutes. Sources: [1]
Risks
Risks are very minimal. Some people feel light-headed. The amount of carbon monoxide is very small and safe. Sources: [1]
What the results mean
Results are interpreted with spirometry and other tests.
DLCO (% predicted) | Meaning |
75% to 140% | Normal |
60% to 75% | Mildly reduced |
40% to 60% | Moderately reduced |
Below 40% | Severely reduced |
Low DLCO | High DLCO |
Smoking, COPD, emphysema | Asthma |
Interstitial lung disease, cystic fibrosis | Obesity |
Pulmonary embolism, pulmonary arterial hypertension | Exercise before the test or high altitude |
Heart failure, anemia, some autoimmune disorders | Lung bleeding, cardiac shunt, polycythemia vera |
Your provider interprets DLCO with spirometry, and more tests or imaging may follow. Research shows a low DLCO can predict worse outcomes even in mild COPD, and changes over time help track lung fibrosis. Sources: [1, 2, 3]
Conclusion
The DLCO test shows how well your lungs pass oxygen into your blood. It helps detect and track conditions that damage the air sacs or lung blood vessels, often before other tests change.
Keep learning: read about pulmonary function tests and spirometry.
Frequently asked questions
What does DLCO stand for?
Diffusing capacity of the lungs for carbon monoxide.
What is a normal DLCO?
75% to 140% of the predicted value.
Is the carbon monoxide dangerous?
No. The amount is tiny and safe.
How long do I hold my breath?
About 10 seconds.
Can I smoke before the test?
No. Avoid smoking for 4 to 6 hours beforehand.
What does a low DLCO mean?
Oxygen isn't moving into your blood as well as expected.
Can anemia affect DLCO?
Yes. Anemia can lower DLCO.
Is DLCO part of a pulmonary function test?
Yes, it's often done with spirometry.
Does it hurt?
No, though some people feel light-headed.
What happens if my DLCO is low?
More tests or imaging may follow.
Related Reading
Pulmonary Function Tests (PFTs): Types, Preparation and What Your Results Mean the full set of lung function tests.
Spirometry: What the Lung Function Test Measures, How It's Done and What Results Mean a test of how much and how fast you breathe.
Body Plethysmography: What It Measures, How It's Done and Results a test of total lung volume.
What Is Chronic Obstructive Pulmonary Disease (COPD)? Symptoms, Causes, Stages, and Treatment a condition that lowers DLCO.
Pulse Oximetry: Normal Oxygen Levels, How It Works and What Affects Accuracy a quick check of oxygen levels.
References
Cleveland Clinic. Lung Diffusion Testing (DLCO): Purpose, Range & Results. Cleveland Clinic. Last updated February 8, 2024. https://my.clevelandclinic.org/health/diagnostics/15445-dlco
de-Torres JP, O'Donnell DE, Marín JM, et al. Clinical and prognostic impact of low diffusing capacity for carbon monoxide values in patients with Global Initiative for Obstructive Lung Disease I COPD. Chest. 2021;160(3):872-878. PMID 33901498. https://doi.org/10.1016/j.chest.2021.04.033
Latsi PI, du Bois RM, Nicholson AG, et al. Fibrotic idiopathic interstitial pneumonia: the prognostic value of longitudinal functional trends. Am J Respir Crit Care Med. 2003;168(5):531-537. PMID 12791580. https://doi.org/10.1164/rccm.200210-1245OC
Medical disclaimer
This article is for general education only and does not replace advice from a qualified healthcare professional. Talk to your doctor about your own diagnosis and treatment, and seek emergency care right away for severe or sudden symptoms.
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