A low DLCO, the measurement of how efficiently your lungs transfer carbon monoxide (and by extension oxygen) from inhaled air into your bloodstream, most often shows up as shortness of breath during physical activity, fatigue that seems out of proportion to your effort, and a reduced ability to exercise at previous levels. The tricky part is that these symptoms are vague enough to overlap with dozens of other conditions, and some people with a mildly low DLCO feel nothing at all. What makes a low result clinically meaningful is the pattern it forms alongside other test results and the underlying condition driving it down.
What DLCO Actually Measures
DLCO stands for diffusing capacity of the lungs for carbon monoxide. During the test, you breathe in a tiny, harmless amount of carbon monoxide, hold your breath briefly, then exhale. The equipment measures how much of that gas crossed from your air sacs into your blood during those few seconds. Two things determine the result: the surface area of your lung membrane available for gas exchange, and the volume of blood flowing through the tiny capillaries surrounding your air sacs.1PubMed Central. Differential effects of central blood volume loading and unloading on pulmonary diffusing capacity in humans Anything that damages the membrane, reduces the blood supply, or shrinks the working lung tissue can lower the number.
Results are reported as a percentage of a predicted value based on your age, sex, height, and sometimes race. A result below about 80% of predicted is generally considered low, though clinical significance depends heavily on context. One complication worth knowing: if your lungs can’t fully expand during the test because of a chest wall problem or muscle weakness, the raw DLCO number drops even if the lung tissue itself is fine. Adjusting for the volume of air you actually inhale during the test helps clinicians figure out whether the membrane itself is impaired or whether you simply couldn’t get enough air in.2PubMed. Importance of adjusting carbon monoxide diffusing capacity (DLCO) and carbon monoxide transfer coefficient (KCO) for alveolar volume
The Symptoms People Notice
The hallmark complaint in someone with a meaningfully low DLCO is exertional dyspnea, which is the medical way of saying you get short of breath when you do things that used to feel easy. Walking up a flight of stairs, carrying groceries, or keeping up on a brisk walk may leave you winded. At rest, your lungs can usually compensate because blood spends enough time in the capillaries for gas exchange to finish. During exercise, blood moves faster through the lungs, and if the membrane is damaged or the capillary bed is diminished, there isn’t enough time for oxygen to get across. That gap between resting comfort and exercise-related breathlessness is the classic signal.
Fatigue is the other dominant symptom. A large study of long-COVID patients found that both dyspnea and fatigue were present in roughly 41% of patients evaluated about six months after infection, and among those with symptoms, patients whose DLCO was below 80% of predicted walked significantly shorter distances on a six-minute walk test than those whose DLCO was normal.3PubMed. Dyspnea and fatigue in Long-COVID: definition of risk factors and of DLCO-based phenotypes in a multicenter study of 765 patients from Italy That fatigue isn’t the drowsiness you feel after a bad night’s sleep. It’s a deep, whole-body exhaustion that worsens with activity and doesn’t fully resolve with rest.
Other symptoms depend on severity and the underlying cause. Oxygen levels can drop during exercise, sometimes producing lightheadedness or a bluish tinge to the lips and fingertips. A dry, persistent cough shows up when interstitial lung disease is the driver. Chest tightness or pressure may occur if pulmonary hypertension is developing. But many people with a mildly reduced DLCO, especially if they are sedentary, may not notice anything until the impairment worsens or they happen to push themselves physically.
Interstitial Lung Disease
Diseases that scar or inflame the tissue between the air sacs are among the most common reasons for a low DLCO. In idiopathic pulmonary fibrosis (IPF), the scarring tends to concentrate in the lower parts of the lungs, and DLCO is one of the earliest markers to drop. Interestingly, research has found that IPF preferentially lowers the transfer coefficient (a measure of how well the membrane itself works per unit of lung volume) rather than the overall DLCO, which suggests the damage is focused on the alveolar-capillary membrane rather than on loss of total lung volume alone.4European Respiratory Journal. Idiopathic pulmonary fibrosis is associated with a preferential alteration of kCO rather than of DLCO The practical significance is that traditional spirometry may look relatively preserved in early IPF while gas exchange is already deteriorating.
Sarcoidosis, a condition in which clusters of inflammatory cells form in the lungs and other organs, also lowers DLCO through loss of functional alveolar surface area. DLCO is considered the most sensitive lung function parameter for detecting that loss, and its reduction correlates strongly with how much the disease has spread on imaging.5Lung & Breathing. Pulmonary function testing and sarcoidosis: A review – Section: Impairment in diffusion capacity One wrinkle with sarcoidosis is that anemia is common in these patients, and hemoglobin itself acts as the “sink” that absorbs carbon monoxide during the test. If your hemoglobin is low, the test result drops even if your lungs are unchanged. Clinicians should adjust the DLCO value for hemoglobin level in any patient with known or suspected anemia. DLCO also appears more sensitive than standard lung volume measurements (like FVC) in detecting early fibrotic changes in sarcoidosis, though its variability between tests makes it less ideal as a stand-alone monitoring tool.6PubMed Central. Pulmonary Function in Pulmonary Sarcoidosis
Pulmonary Vascular Disease
When the blood vessels within the lungs are the primary problem rather than the tissue between air sacs, DLCO still drops because there is less blood available in the capillaries to pick up the test gas. Pulmonary arterial hypertension and chronic thromboembolic pulmonary hypertension (CTEPH) are the main culprits in this category. In CTEPH, clots that never fully dissolved after a pulmonary embolism block parts of the pulmonary vasculature, and a secondary small-vessel disease often develops in the remaining open vessels. Studies of CTEPH patients after surgical treatment show that about three-quarters still have DLCO values below 80% of predicted, with a mean around 68%.7European Heart Journal. DLCO, a potential indicator of pulmonary microvasculopathy, and its association with exercise capacity, oxygenation, and hemodynamics in patients with chronic thromboembolic pulmonary hypertension aft That persistent impairment likely reflects ongoing small-vessel disease that imaging with dual-energy CT can sometimes detect as poor blood flow at the edges of the lungs.8PubMed Central. Small vessel disease in chronic thromboembolic pulmonary hypertension: a comprehensive narrative review of multimodality imaging evaluation
A low DLCO can also serve as a screening clue for exercise-induced pulmonary hypertension in patients with lung disease. In one study of patients referred for unexplained breathlessness, a DLCO below 46% of predicted identified exercise pulmonary hypertension with perfect sensitivity in those who had underlying parenchymal lung disease.9PubMed Central. Lower DLco% identifies exercise pulmonary hypertension in patients with parenchymal lung disease referred for dyspnea For patients without parenchymal disease, a higher threshold of 73% was more useful, though with lower sensitivity. The takeaway is that a very low DLCO in someone with known lung disease should prompt consideration of superimposed pulmonary vascular problems, not just worsening of the lung disease itself.
Autoimmune Diseases and DLCO as an Early Warning
Systemic sclerosis (scleroderma) is arguably the autoimmune condition most tightly linked to declining DLCO. In limited systemic sclerosis specifically, roughly 73% of patients have a DLCO below 80% of predicted, and the finding often precedes detectable interstitial lung disease on imaging. Low DLCO showed a sensitivity of 71% and specificity of about 88% for identifying early limited systemic sclerosis in patients who had Raynaud’s phenomenon but no other definitive diagnosis yet.10PubMed. Impaired carbon monoxide diffusing capacity as a marker of limited systemic sclerosis In scleroderma patients whose DLCO is severely reduced (below 55% of predicted) but who have no visible lung or heart disease on standard testing, about 20% go on to develop pulmonary arterial hypertension over the following decade.11PubMed. Predictive value of isolated DLCO reduction in systemic sclerosis patients without cardio-pulmonary involvement at baseline That makes an isolated, severely low DLCO a red flag that warrants close follow-up in anyone with scleroderma.
Rheumatoid arthritis can also quietly damage the lungs. Interstitial lung disease occurs in roughly 8 to 15% of RA patients and is frequently underrecognized because joint symptoms dominate the clinical picture.12PubMed Central. Progressive Decline of Lung Function in Rheumatoid Arthritis Associated Interstitial Lung Disease Research has found that a DLCO below roughly 53% of predicted could sensitively discriminate RA patients with ILD from those without it, suggesting the test has real utility as a screening tool in this population.13PubMed Central. Clinical features of rheumatoid arthritis-associated interstitial lung disease The message for anyone with an autoimmune condition who develops new or worsening breathlessness is that DLCO testing can catch lung involvement before imaging shows obvious damage.
Heart Failure and the Lungs
Chronic heart failure lowers DLCO through a mechanism that feels counterintuitive. You might expect that congested, fluid-logged lungs would have more blood in the capillaries, which would increase gas transfer. In early heart failure, that can happen briefly. But in advanced or long-standing heart failure, the constant back-pressure from the heart remodels the pulmonary blood vessels and thickens the alveolar-capillary membrane. Both the membrane component and the capillary blood volume component of DLCO decrease roughly in proportion, pointing to broad disruption of the pulmonary vascular bed.14PubMed Central. The alveolar-capillary membrane diffusing capacity and the pulmonary capillary blood volume in heart transplant candidates
In patients with advanced heart failure, DLCO per unit of lung volume correlates positively with the filling pressure on the left side of the heart, and that relationship persists even after accounting for smoking, diabetes, and chronic lung disease.15PubMed Central. Lung diffusion capacity in advanced heart failure: relation to central haemodynamics and outcome For a patient who has both heart failure and unexplained breathlessness out of proportion to what the heart alone seems to explain, a low DLCO helps quantify how much the lungs themselves have been affected by chronic cardiac disease.
Anemia and Other Non-Lung Causes
Because hemoglobin is what grabs carbon monoxide during the test, anemia lowers DLCO without any lung disease at all. This is one of the most common sources of a falsely low or misleadingly low result. In patients undergoing blood-cell transplantation, where severe anemia is frequent, the raw DLCO was markedly different from the anemia-corrected value. Among those with hemoglobin at or below 10 g/dL, the median uncorrected DLCO was about 84% of predicted, but after adjusting for hemoglobin the median jumped to about 111%.16PubMed Central. Adjusting diffusing capacity for anemia in patients undergoing allogeneic HCT: a comparison of two methodologies That is a massive gap and it demonstrates why any low DLCO should be interpreted alongside a recent hemoglobin level.
Other non-pulmonary causes of low DLCO include liver cirrhosis, which can create abnormal blood vessel channels in the lungs (hepatopulmonary syndrome), and obesity, which restricts lung expansion and reduces the effective surface area for gas exchange. A review of patients with isolated low DLCO and no clear lung disease on imaging found a mixed grab bag of explanations: pleural thickening, severe spinal curvature, persistent areas of collapsed lung, and combinations of cirrhosis with anemia or heart failure with mild emphysema.17CHEST. Etiologies of Isolated Reduced Diffusing Capacity of the Lungs for Carbon Monoxide In a small number of cases, no explanation was found at all even after extensive workup.
Drug Toxicity and Occupational Exposures
Certain medications damage the alveolar-capillary interface and produce a pattern of restrictive lung disease with falling DLCO. Chemotherapy agents, particularly bleomycin, are the most well-known offenders, but amiodarone (a heart rhythm drug), nitrofurantoin (an antibiotic used for urinary infections), and several targeted cancer therapies can do the same. The typical pattern on pulmonary function testing is decreased lung volumes alongside decreased DLCO, reflecting interference with the membrane across which gases move.18The Open Respiratory Medicine Journal. Drug Induced Interstitial Lung Disease – Section: Pulmonary Function Testings Anyone on a drug known to cause lung toxicity should have baseline DLCO measured before starting treatment and periodic follow-up testing, because a meaningful drop can precede symptoms by weeks.
Occupational dust exposure tells a similar story. Workers exposed to silica, asbestos, or coal mine dust show decreased spirometric parameters and DLCO even before chest X-rays reveal signs of pneumoconiosis, and this holds true in both smokers and nonsmokers.19PubMed. Respiratory impairments due to dust exposure: a comparative study among workers exposed to silica, asbestos, and coalmine dust A separate study found that among nearly 300 patients referred for asbestos or silica exposure evaluation, about one in five had a reduced DLCO as their only abnormal finding, with all other pulmonary function tests normal.20CHEST. Significance of Isolated Reduction in Diffusing Capacity in Patients With Occupational Dust Exposure An isolated low DLCO in someone with a significant dust exposure history is not something to dismiss, even if spirometry looks clean.
Post-COVID Lung Impairment
DLCO has become one of the most studied lung function markers in the aftermath of COVID-19. Among people recovering from non-severe COVID, about 17% had impaired DLCO at an average of roughly five months after the acute illness, and those with impaired diffusion were twice as likely to report ongoing respiratory symptoms compared to those with normal results.21Scientific Reports. Pulmonary diffusing capacity among individuals recovering from mild to moderate COVID-19: a cross-sectional study Individuals reporting moderate-to-severe dyspnea had nearly three times the odds of having impaired DLCO.
An important nuance emerged from a large Italian multicenter study: about half of long-COVID patients who reported dyspnea or fatigue actually had normal DLCO. The low-DLCO group and the normal-DLCO group had different risk profiles. Low-DLCO dyspnea was linked to female sex, longer hospitalization, and corticosteroid use during the acute illness, and it improved with time. Normal-DLCO dyspnea was more associated with younger age and obesity, suggesting a different mechanism entirely, possibly deconditioning or autonomic dysfunction rather than true gas exchange impairment.3PubMed. Dyspnea and fatigue in Long-COVID: definition of risk factors and of DLCO-based phenotypes in a multicenter study of 765 patients from Italy For patients and clinicians navigating persistent post-COVID breathlessness, DLCO testing helps separate those whose lungs are genuinely impaired from those whose symptoms have other explanations.
Why DLCO Matters for Prognosis
Beyond diagnosing the cause of breathlessness, DLCO carries prognostic weight. In patients with pulmonary hypertension related to chronic lung disease, every 10% drop in predicted DLCO was independently associated with about a 31% increase in the risk of death.22PubMed Central. Survival in pulmonary hypertension due to chronic lung disease: Influence of low diffusion capacity of the lungs for carbon monoxide In newly diagnosed IPF, patients whose DLCO declined by 10% or more of predicted within the first year after diagnosis faced more than double the risk of future mortality compared to those whose DLCO remained stable.23Scientific Reports. Prognostic implication of 1-year decline in diffusing capacity in newly diagnosed idiopathic pulmonary fibrosis Tracking DLCO over time rather than relying on a single snapshot gives a much clearer picture of whether a lung disease is progressing.
Preoperative Risk Assessment
If you are facing lung surgery, your DLCO result directly affects what operations your surgical team considers safe. Current guidelines call for measuring both FEV1 (the volume of air you can forcefully exhale in one second) and DLCO before lung cancer surgery. When both values are above 80% of predicted, a pneumonectomy (removal of an entire lung) can generally proceed without further exercise testing.24PubMed Central. Pulmonary function tests in the preoperative evaluation of lung cancer surgery candidates. A review of guidelines If either value falls below that threshold, additional exercise testing is needed to estimate how well you’d tolerate the loss of lung tissue.
An expert consensus panel from the American Association for Thoracic Surgery identified low DLCO as one of the most important factors in determining whether a patient is high-risk for lobectomy for early-stage lung cancer, alongside the need for supplemental oxygen, frailty, and functional status.25PubMed. Definition and assessment of high risk in patients considered for lobectomy for stage I non-small cell lung cancer: The American Association for Thoracic Surgery expert panel consensus document Even in patients without chronic obstructive pulmonary disease, DLCO independently predicts pulmonary complications after lung resection, which is why thoracic surgeons recommend measuring it in all lung surgery candidates regardless of spirometric findings.26PubMed. Pulmonary complications after lung resection in the absence of chronic obstructive pulmonary disease: the predictive role of diffusing capacity
Technical Pitfalls That Can Skew Your Result
The test itself has a few quirks that can produce misleading numbers. The standard protocol calls for a breath-hold time of about ten seconds, but some patients, especially those with severe airflow obstruction, struggle to hold that long. In people with severe COPD, shortening the breath-hold time from ten seconds to six seconds produced a significantly lower DLCO value, meaning the result looked worse than it would have at the standard hold time.27ARROW@TU Dublin. Effect of Reduction of Breath-Holding Time In The Single-Breath Method To Assess Carbon Monoxide Diffusing Capacity In people with normal airways or interstitial lung disease, the shortened hold time didn’t significantly change the result. If you have COPD and your DLCO seems disproportionately low compared to your other test results, the breath-hold time used during the test is worth checking.
Recent smoking is another common confounder. Carbon monoxide from cigarettes occupies hemoglobin binding sites before the test even starts, so the lungs appear to transfer less gas than they actually can. Labs typically ask whether you’ve smoked in the past 24 hours and can apply a correction, but the adjustment is imperfect. High altitude, recent supplemental oxygen use, and even the time of day can introduce small variations. For all these reasons, a single low DLCO value in isolation is rarely the basis for major clinical decisions. Clinicians look at the trend over multiple measurements and interpret the result alongside spirometry, imaging, and blood work.