A decreased diffusion capacity of the lung means your lungs are less efficient than expected at moving gases between inhaled air and your bloodstream. The test that measures this, commonly called DLCO, uses a tiny amount of carbon monoxide to gauge how well oxygen would cross the thin membrane separating your air sacs from the blood vessels surrounding them. When the result comes back low, it signals that something is interfering with that gas exchange, whether the membrane itself is damaged, there is less surface area available, or the blood supply on the other side is compromised. The causes range from emphysema and scarring diseases to anemia and pulmonary vascular problems, and sorting out which one matters is a key part of what your doctor does next.
What the Test Actually Measures
The DLCO test asks you to take a single deep breath of air containing a very small, safe concentration of carbon monoxide, hold it briefly, and exhale. The equipment measures how much CO your lungs absorbed during that breath hold. Clinicians use carbon monoxide for this purpose because it binds so strongly to hemoglobin that its uptake is limited almost entirely by the membrane between your air sacs and capillaries, not by blood flow. That makes it a clean readout of the membrane’s condition.1PubMed. Diffusing Capacity of the Lungs for Carbon Monoxide Oxygen would be a logical choice, but its weaker binding to hemoglobin and the body’s constant consumption of it make the measurement messier.
Three physical properties drive the result: the surface area of the membrane, its thickness, and the pressure difference pushing gas across it. Anything that shrinks the available surface area (destroyed air sacs, for instance), thickens the membrane (scar tissue or fluid), or reduces the blood volume on the capillary side will lower the number. The result is reported as a percentage of a predicted value based on your age, sex, height, and hemoglobin level. Below roughly 80 percent of predicted is generally considered abnormal, though the clinical significance depends heavily on how far below that threshold you fall and what other test results look like.
Lung Diseases That Reduce Diffusion Capacity
The most common reason for a low DLCO is lung disease that damages either the air sacs themselves or the tissue between them. In emphysema, the walls between air sacs break down, leaving fewer, larger spaces with less total surface area for gas exchange. This distinguishes emphysema from chronic bronchitis, where the airways are inflamed and mucus-clogged but the air sacs may remain relatively intact. Clinicians have long used the DLCO test to help separate these two patterns.1PubMed. Diffusing Capacity of the Lungs for Carbon Monoxide
Interstitial lung diseases, a family of conditions that cause scarring or inflammation in the tissue surrounding the air sacs, are another major category. Idiopathic pulmonary fibrosis (IPF) is the best-known example. In IPF, progressive scarring thickens the membrane and stiffens the lung, steadily reducing DLCO over time. One study of newly diagnosed IPF patients found that a decline in DLCO of 10 percent of predicted or more over the first year was independently linked to a higher risk of death, even after accounting for other factors.2Scientific Reports. Prognostic implication of 1-year decline in diffusing capacity in newly diagnosed idiopathic pulmonary fibrosis That same research showed something subtle: some patients had a falling DLCO even while their lung volume measurements stayed stable, suggesting the test picks up damage to the microscopic blood vessels and membrane that other tests miss.
Pulmonary Vascular Problems
A low DLCO does not always mean the lung tissue itself is diseased. The capillary network wrapping around the air sacs is just as important, and when those tiny vessels are destroyed, blocked, or remodeled, gas exchange suffers. Pulmonary arterial hypertension (PAH) is the textbook example. In PAH, rising pressure in the lung’s blood vessels damages and narrows them over time. A study of patients with idiopathic PAH found that those with a severely reduced DLCO had worse exercise performance and significantly poorer survival, despite having similar blood-pressure readings in their lungs compared to patients with better diffusion capacity.3European Respiratory Journal. Severely reduced diffusion capacity in idiopathic pulmonary arterial hypertension: patient characteristics and treatment responses In that cohort, a very low DLCO was also linked to older age, a history of tobacco exposure, and more abnormalities visible on CT scans.
Chronic blood clots in the lung vessels (chronic thromboembolic pulmonary disease) produce a similar pattern. Because the clots physically obstruct capillary beds, gas has less vascular surface area to cross into. Heart failure with preserved ejection fraction can also lower diffusion capacity. In patients with pulmonary hypertension related to this form of heart failure, a DLCO below about 45 percent of predicted carried a dramatically worse prognosis: roughly a 36 percent survival rate at three years compared with nearly 88 percent for those above that threshold.4PubMed. Diffusion Capacity and Mortality in Patients With Pulmonary Hypertension Due to Heart Failure With Preserved Ejection Fraction
When Diffusion Capacity Is the Only Abnormal Finding
Sometimes DLCO comes back low while spirometry (the test measuring airflow) and lung volume measurements all look normal. This “isolated” low DLCO is surprisingly common, and it deserves attention rather than dismissal. It can be the earliest physiological clue to conditions like pulmonary hypertension, early interstitial lung disease, or chronic thromboembolic disease, all of which may not yet show up on simpler lung function tests.5PubMed Central. Why we should never ignore an “isolated” low lung diffusing capacity Patients in this situation often come to the clinic with unexplained shortness of breath during exercise, and failing to investigate can delay diagnoses that genuinely change outcomes.
Systemic sclerosis, an autoimmune condition that affects connective tissue, illustrates this well. In a large study of over 800 scleroderma patients, about one in five had an isolated low DLCO with a normal lung volume. Among that subset, roughly 11 percent went on to develop isolated pulmonary hypertension, which carried severely reduced survival.6PubMed. Isolated diffusing capacity reduction in systemic sclerosis Pulmonary hypertension was strongly associated with an initial DLCO below 55 percent of predicted. That is why rheumatologists who follow scleroderma patients track DLCO regularly; it can be the canary in the coal mine for a complication that might not produce obvious symptoms until it is advanced.
A separate study of patients with limited systemic sclerosis found that about 73 percent had low DLCO values, even though frank interstitial lung disease was present in only 10 percent. The membrane component of the diffusion measurement was markedly reduced, pointing to damage at the capillary-membrane level rather than to scarring of the lung tissue.7PubMed. Impaired carbon monoxide diffusing capacity as a marker of limited systemic sclerosis
Non-Lung Factors That Affect the Result
Because the test depends on carbon monoxide binding to hemoglobin, anything that changes how much hemoglobin is available in the blood will shift the result. Anemia is the classic example. Studies have documented a clear positive correlation between blood hemoglobin concentration and DLCO: lower hemoglobin, lower measured diffusion capacity, even if the lungs themselves are perfectly healthy.8PubMed. Effect of low hemoglobin levels on the diffusing capacity of the lungs for CO Standardized testing protocols include correction equations that adjust the predicted DLCO for hemoglobin level, with separate formulas for adult males, adult females, and children.9European Respiratory Journal. Standardisation of the single-breath determination of carbon monoxide uptake in the lung If your lab did not correct for a known anemia, the raw number could be misleadingly low.
The amount of blood sitting in the lung capillaries at the moment of the test also matters. Research using lower-body negative pressure to shift blood away from the chest has shown that DLCO drops progressively as the central blood volume decreases, driven by reduced capillary blood volume rather than any change to the membrane itself.10PubMed Central. Differential effects of central blood volume loading and unloading on pulmonary diffusing capacity in humans In practical terms, this means that dehydration, recent blood loss, or even body position during the test can nudge the result. Conversely, conditions that engorge the lung capillaries, such as left-to-right heart shunts or polycythemia (too many red blood cells), can push DLCO above normal.
Altitude and pregnancy interact with this physiology in ways that can surprise both patients and physicians. In a study of Peruvian women, those living at high altitude had higher corrected DLCO values than their sea-level counterparts, reflecting the body’s adaptation to thinner air. During pregnancy, diffusion capacity remained stable through the first two trimesters at altitude but dipped in the third trimester compared with non-pregnant women.11Respiratory Physiology and Neurobiology. Pulmonary diffusing capacity in pregnancy at sea level and at high altitude The takeaway for anyone interpreting a DLCO result is that context matters enormously.
COVID-19 and Lasting Diffusion Impairment
The pandemic put DLCO testing in the spotlight because many COVID-19 survivors showed reduced gas exchange months after their acute illness. In one study that followed pneumonia survivors for six months, about 60 percent still had an abnormal DLCO (below 80 percent of predicted).12PubMed Central. Prediction of impaired lung diffusion capacity in COVID-19 pneumonia survivors The extent of lung involvement on CT during the acute infection was a strong predictor: the more lung tissue affected at the worst point of illness, the more likely diffusion capacity remained impaired afterward.
Some of this impairment does improve over time. A clinical trial of corticosteroid treatment for persistent post-COVID interstitial lung changes found that DLCO improved from baseline in both the treatment and control groups over the study period, suggesting at least partial recovery occurs with or without steroids.13PubMed Central. Corticosteroid treatment for post-COVID-19 persistent interstitial lung disease: a randomized clinical trial Still, the high prevalence of persistent low DLCO after moderate-to-severe COVID pneumonia means the test has become a standard part of post-COVID lung evaluation.
How Clinicians Use the Number
A single DLCO value rarely tells the whole story. Clinicians interpret it alongside spirometry, lung volumes, imaging, and clinical history. One important distinction is between DLCO (the absolute diffusion capacity) and DLCO divided by alveolar volume (sometimes written DLCO/VA or KCO). The absolute DLCO reflects total gas exchange across both lungs; dividing it by volume tells you how efficiently the available lung tissue is working per unit of volume. In conditions where lung tissue has been removed (after surgery, for instance), the absolute DLCO may be low simply because there is less lung, while DLCO/VA may be normal because the remaining tissue is healthy.
Research comparing the two measures found that the absolute DLCO was actually a better predictor of whether patients desaturated during exercise than DLCO/VA. In a cohort mostly composed of people with interstitial lung disease or lung volume restriction, an absolute DLCO below about 55 percent of predicted was the optimal cutoff for predicting exercise desaturation.14PubMed Central. DLCO versus DLCO/VA as predictors of pulmonary gas exchange This is a good reminder that the simpler number often carries more clinical weight than the volume-adjusted one, even though both appear on the report.
Serial measurements over time are particularly valuable. In IPF, as noted earlier, the trajectory of DLCO decline predicts survival. In systemic sclerosis, a steadily falling DLCO flags the emergence of pulmonary hypertension. In COPD, the trend helps distinguish patients whose disease is predominantly airway-based from those developing significant emphysema. One snapshot may be ambiguous; a pattern over months or years is far more informative.
Can Diffusion Capacity Improve?
Whether a low DLCO is reversible depends entirely on what is causing it. If the culprit is anemia, correcting the hemoglobin level will bring the number back up. If fluid overload is engorging or compressing the capillary bed, treating the heart failure may help. If it is a drug side effect (certain chemotherapy agents are notorious for lowering DLCO), stopping or changing the medication sometimes allows partial recovery.
Even in chronic lung disease, some improvement is possible with rehabilitation. A study of COPD patients who completed a pulmonary rehabilitation program found statistically significant improvements in DLCO, along with better exercise tolerance and reduced breathlessness.15PubMed. Improvements in Lung Diffusion Capacity following Pulmonary Rehabilitation in COPD with and without Ventilation Inhomogeneity The gains were modest, but they are meaningful for quality of life in a population where exercise capacity is often severely limited.
In progressive scarring diseases like IPF, the realistic goal is usually to slow the decline rather than reverse it. Anti-fibrotic medications have been shown in large trials to reduce the rate at which lung function deteriorates, but they do not typically restore what has already been lost. Early detection, which is exactly what DLCO testing enables, gives these treatments their best chance of preserving function for longer.
Emerging Imaging That Complements the Test
One limitation of the standard DLCO test is that it gives a single number for both lungs combined. It cannot tell you whether the problem is diffuse or concentrated in one region. That is where newer imaging techniques are beginning to add value. Hyperpolarized xenon-129 MRI uses inhaled xenon gas to create three-dimensional maps of lung ventilation and gas exchange. Because xenon dissolves into tissue and blood, the technique can separately visualize gas in the airspaces, gas crossing the membrane, and gas reaching red blood cells.16PubMed. Hyperpolarized (129)Xe MRI and Spectroscopy: Quantitative Measurements, Results, and Emerging Opportunities
This regional information is potentially useful for catching disease earlier than a global DLCO measurement can. If scarring or vascular damage is patchy and confined to one part of the lung, the overall DLCO might still be in the low-normal range while xenon MRI reveals focal deficits. The technology also opens possibilities for guiding targeted treatments or planning surgery more precisely.17PubMed Central. The role of hyperpolarized 129xenon in MR imaging of pulmonary function Xenon MRI is still largely a research and specialty-center tool rather than something available in every hospital, but its clinical use is expanding as scanners and protocols become more standardized.
When DLCO Goes Unexpectedly High
Most clinical attention focuses on a low DLCO, but an unusually elevated result also tells a story. Conditions that increase the volume of blood in the pulmonary capillaries can push DLCO above the predicted range. Obesity, left-to-right cardiac shunts, exercise immediately before testing, and polycythemia are well-known causes. A more unusual example comes from hematologic malignancies. In a case of extreme white blood cell elevation (hyperleukocytosis), clinicians documented a markedly elevated DLCO. The proposed mechanism involved capillary engorgement and slowed red blood cell transit through the lung, which gave each red cell more time to pick up carbon monoxide. As the white cell count normalized with treatment, the DLCO drifted back toward expected values.18European Respiratory Journal. Elevated DLCO and pulmonary function evolution during treatment in a patient with hyperleukocytosis and leukostasis It is a rare scenario, but it illustrates how the test reflects the interplay between membrane properties and capillary blood dynamics, not just one or the other.
Pulmonary hemorrhage is another situation that can paradoxically raise DLCO. When blood leaks into the air sacs, the hemoglobin sitting in those spaces eagerly binds the inhaled carbon monoxide, making it look like the lungs absorbed more gas than they really transferred into the bloodstream. Clinicians aware of this artifact use it diagnostically: a rising DLCO in the right clinical context can actually help confirm active bleeding into the lungs.