Diffusing capacity of the lung for carbon monoxide (DLCO) is a lung function test that may help detect COPD early and predict a person’s outlook.

DLCO is a lung diffusion test that measures how efficiently oxygen is moving from the lungs to the bloodstream. It can show how efficiently carbon monoxide and oxygen pass from the lungs to the blood.

DLCO can help doctors determine how well the lungs are working in people with COPD. This article discusses the importance of DLCO testing in COPD, how to interpret results, and other COPD tests.

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DLCO is a type of pulmonary function test, which helps doctors see how well the lungs are functioning.

DLCO measures the amount of carbon monoxide that transfers from the alveoli to red blood cells per minuteTrusted Source.

The alveoli are small air sacs in the lungs. A network of capillaries connects the alveoli to the bloodstream, creating the alveolar-capillary membrane, where the exchange of gases takes place.

When people breathe in and out, oxygen and carbon dioxide pass Trusted Sourcethrough the alveoli betweenTrusted Source the lungs and the bloodstream.

When people breathe in, oxygen moves through the alveoli to the blood, so that oxygen-rich blood can supply the body.

Carbon dioxide from the blood moves through the alveoli to the lungs, to eliminate the gas when people breathe out.

DLCO shows how well the lungs can move oxygen from inhaled air to the blood.

DLCO uses carbon monoxide rather than oxygen because carbon monoxide is more strongly boundTrusted Source to hemoglobin than oxygen. Hemoglobin is the protein in red blood cells that transports oxygen.

Carbon monoxide allows doctors to better identify any issues with the functioning of the alveolar-capillary membrane. The amount of carbon monoxide is very low, and DLCO testing is generally safe with little risk.

To take a DLCO test, people will wear a nose clip and mouthpiece, which connects to a mixture of gases, including carbon monoxide, oxygen, and nitrogen.

The mixture also contains either helium or methane, which work as tracer gases. Doctors measure the amount of tracer gas people exhale to find out how much the body has absorbed. This can show how well the lungs can transfer gases and how much oxygen they can take in.

A doctor will ask people to inhale fully and quickly, hold their breath for 10 seconds, and then exhale to provide a sample. Doctors then measure the level of carbon monoxide and tracer gases in the sample.

According to a 2019 study, impaired DLCO may link to an increase in COPD symptoms, decreased capacity for exercise, and increased risk of severe COPD exacerbations.

The study suggests DLCO is an important but under-used method of assessing COPD.

A key indicator of lung function

Diffusing capacity refers to how well the lungs and blood transfer oxygen and carbon dioxide. Diffusing capacity may link to pulmonary hypertension. Pulmonary hypertension is high blood pressure in the lungs.

Impaired DLCO can be a key indicator of a decline in lung function and may link to emphysema in COPD.

Emphysema is a type of COPD, where damage occursTrusted Source to the alveoli and makes it more difficult for the lungs to transfer oxygen and carbon dioxide in and out of the body.

Can help improve COPD outcomes

DLCO may play a role in assessing key factors in COPD, including severity of the disease, exacerbations, and mortality risk.

DLCO can helpTrusted Source doctors monitor the progression of lung diseases, response to treatment, and may help predict mortality.

Though small-scale, some research suggests a link between DLCO and frequent COPD exacerbations. There are conflicting results as to whether DLCO is a predictor of COPD mortality.

Can detect early stage COPD

DLCO gives doctors more information about lung function than spirometry, a common lung function test.

DLCO can detect certain changes in the lungs earlier than spirometry can detect them. A low DLCO result may indicate lung issues at an earlier stage, allowing people to begin treatment sooner, which may improve outcomes.

DLCO results are based on a predicted value, which is the expected result for each individual. DLCO results may be as followsTrusted Source:

  • Normal DLCO: between 75% to 140% of the predicted value
  • Mild DLCO impairment: 60% to 74% of the predicted value
  • Moderate DLCO impairment: 40% to 59% of the predicted value
  • Severe DLCO impairment: lower than 40% of the predicted value

People with emphysema and a DLCO lower than 60% of the predicted value have an increased mortality rate from all causes.

People with COPD and low DLCO have an increased risk of exacerbations, decreased capacity for exercise, and increased mortality due to COPD.

A DLCO result that is significantly lower than the predicted value may mean an increased risk of pulmonary hypertension due to lung disease, hospitalization, and mortality.

Other COPD tests may includeTrusted Source:

  • Spirometry: This test measures the amount and speed of air people can breathe out.
  • Arterial blood gas: This test measures the levels of oxygen and carbon dioxide in the blood.
  • Chest CT scan: This scan creates 3D images of the chest to find out the cause of symptoms.

According to a 2024 literature review, carbon monoxide diffusion testing may help:

  • detect COPD early
  • accurately stage COPD
  • monitor COPD progression
  • predict treatment response and overall outlook

Future research into diffusion capacity may improve the diagnosis and treatment of COPD, help improve gas exchange in the lungs, and help reduce disease progression.

The review suggests areas of future research include:

  • advances in measurement and diagnostic tools
  • exploring biomarkers of carbon monoxide diffusion capacity
  • new treatment methods

DLCO is a noninvasive test for lung function that indicates how well oxygen is passing between the lungs and the bloodstream.

DLCO may help detect COPD earlier, monitor disease progression, and help predict treatment response.