What Does a Low DLCO Indicate in COPD? Causes Explained

A low DLCO test result means your lungs have a reduced capacity to transfer carbon monoxide—and, by extension, oxygen—from your air sacs into your bloodstream ^[1]. In people with chronic obstructive pulmonary disease (COPD), a low DLCO is most often associated with emphysema, which destroys the delicate structures where gas exchange happens ^[2][3]. However, a low score can also be related to anemia, high blood pressure in the lungs (pulmonary hypertension), or other lung conditions ^[4][5].

While a low DLCO is an important clue that helps explain why you might feel breathless, it is not a direct measure of your COPD “stage” and does not define your life expectancy. Furthermore, a low DLCO does not automatically mean you need supplemental oxygen; oxygen decisions are based on direct measurements of your blood oxygen levels, not your DLCO score ^[6].

How Emphysema Affects Your DLCO

DLCO stands for diffusing capacity of the lungs for carbon monoxide. During the test, you inhale a tiny, safe amount of carbon monoxide gas ^[1]. The test measures how easily that gas moves from your lungs into your red blood cells.

When COPD involves a lot of emphysema, the walls of the tiny air sacs (alveoli) and their surrounding blood vessels are damaged ^[2]. This reduces the total surface area available for gas to cross into the blood. Studies show that a lower DLCO strongly correlates with the amount of emphysema visible on a chest CT scan ^[3][7]. Keep in mind that a normal DLCO does not rule out COPD (especially if your COPD is mostly chronic bronchitis), and a low DLCO does not by itself prove you have emphysema.

How Doctors Interpret the Number

Receiving an abnormal lung-test result can be worrying, but the number is just one part of your evaluation. Doctors look at several factors to understand your DLCO:

• Percent Predicted and LLN: Your result is compared to what is expected for someone of your age, height, sex, and birth sex, often expressed as a percentage or checked against the “lower limit of normal” (LLN) ^[6].
• Context: The number is always interpreted alongside your breathing tests (spirometry), lung volumes, and symptoms ^[8].

When to Look Beyond Emphysema

Sometimes, a DLCO result is disproportionately lower than what doctors would expect based on your spirometry, specifically your FEV1 (the amount of air you can forcibly exhale in one second) ^[9]. When FEV1 is relatively normal but DLCO is very low, doctors broaden their search to include:

• Pulmonary hypertension: High blood pressure in the lung’s arteries ^[10]. If suspected, an echocardiogram (ultrasound of the heart) can estimate the probability of this condition, though a procedure called right-heart catheterization is required to confirm the diagnosis and measure exact pressures ^[11].
• Interstitial lung disease (ILD): A group of conditions that cause inflammation or scarring in the lung tissue ^[12].
• Other factors: Blood clots in the lungs, low overall lung volume, or poor test performance.

The Role of Anemia and Smoking

Your DLCO score relies on hemoglobin, the protein in red blood cells that carries oxygen and binds to the carbon monoxide used in the test. If you have anemia (low hemoglobin), less gas is absorbed during the test. This genuinely lowers your carbon monoxide uptake ^[5]. To make the interpretation fair, laboratories apply a mathematical correction based on a recent blood test to give you a “hemoglobin-adjusted” DLCO ^[13].

Additionally, smoking immediately before your test increases the carbon monoxide already in your blood (carboxyhemoglobin), which can lower your DLCO result ^[14]. Always follow the pulmonary lab’s instructions regarding smoking, inhaler use, and exercise before your test.

How DLCO Connects to Symptoms and Exercise

Breathlessness is complex and can be caused by airflow blockage, deconditioning, or heart issues. However, a low DLCO may help explain why you feel more breathless during daily activities ^[8][6].

People with a low DLCO are more likely to experience oxygen desaturation (a drop in blood oxygen levels) during physical exertion ^[6]. Because of this, your doctor might recommend a 6-minute walk test or cardiopulmonary exercise testing (CPET)—a specialized test on a bike or treadmill—to measure your actual oxygen levels during movement and determine if you would benefit from supplemental oxygen when active ^[6][15].

Tracking DLCO Over Time

For many people with COPD, DLCO declines slowly over time ^[16]. There is no single universal schedule for repeating this test; your doctor will individualize your testing timeline based on your symptoms, treatment changes, and baseline results. When checking for changes, it is best to compare tests done at the same laboratory, as different machines and calculation methods can cause slight variations ^[17].

Safety Warning: Routine testing is for tracking long-term changes. If you experience sudden or severe breathlessness, breathlessness at rest, chest pain, fainting, confusion, blue lips, or a drop in your oxygen saturation below your prescribed threshold, do not wait for a pulmonary function test. Seek emergency medical care immediately.