What the DLCO Test Reveals About Gas Transfer

The diffusing capacity of the lung for carbon monoxide, abbreviated DLCO, is one of the most information-dense values in pulmonary function testing. It asks how much of a tiny, safe test concentration of carbon monoxide disappears from an inhaled breath during a brief breath hold, and the disappearing amount is taken up by pulmonary capillary blood.

The result is often described as a measure of the lung membrane. That is incomplete. Uptake depends on available alveolar surface, thickness of the air-blood barrier, and pulmonary capillary blood volume. It depends on hemoglobin, inspired volume, and distribution of the test gas. It depends on performance of the maneuver. DLCO is a system measurement.

How the single-breath maneuver works#

After breathing normally, you exhale and then inhale a test mixture rapidly toward total lung capacity. The mixture contains a very low carbon monoxide concentration and an inert tracer gas. You hold your breath for about ten seconds and then exhale. The instrument discards gas from the conducting airways and analyzes an alveolar sample.

Carbon monoxide has strong affinity for hemoglobin, so its partial pressure in capillary blood remains very low during the maneuver; the fall in alveolar concentration relative to the tracer estimates transfer. The tracer also helps calculate alveolar volume, abbreviated VA, participating in the single breath.

The 2017 ERS and ATS technical standard defines equipment, calibration, and inspired-volume targets. It defines breath-hold calculation, sampling, repeatability, and quality criteria. Those details matter because a leak, slow inspiration, inadequate breath, or poorly timed sample can materially change the reported value.

Why carbon monoxide is used#

Oxygen transfer depends on ventilation, diffusion, perfusion, metabolism, and the oxygen dissociation curve. Directly extracting a membrane property from resting oxygen is difficult. Carbon monoxide at a trace concentration provides a practical probe because hemoglobin avidly binds it and capillary back-pressure is usually small.

The administered amount is far below toxic poisoning levels when the test is performed properly, though the method still needs special consideration in pregnancy, substantial baseline carboxyhemoglobin, inability to perform the maneuver, and acute illness. A laboratory follows local safety protocols and clinician judgment.

The name “diffusing capacity” can suggest pure diffusion, but pulmonary capillary blood volume and reaction with hemoglobin contribute, and some laboratories and international reports use transfer factor of the lung for carbon monoxide, or TLCO, for the same general measurement.

The main components of a DLCO result#

Your report usually gives measured DLCO, predicted value, percent predicted, and a z score or lower limit of normal. It may show VA and KCO. KCO is DLCO divided by VA, with units reflecting transfer per unit alveolar volume.

KCO is sometimes called DLCO/VA, which tempts people to treat it as a correction for a small breath. It is not. DLCO does not vary linearly with VA. When a healthy person inspires to a smaller lung volume, KCO can rise even though total transfer falls, so a “normal” or high KCO cannot automatically rescue a low DLCO obtained with inadequate inspired volume.

VA from the single-breath tracer is not necessarily the same as total lung capacity measured by plethysmography; in severe airflow obstruction or uneven ventilation, the test gas may not reach all lung units during the maneuver, so VA can be substantially lower.

What can lower gas transfer#

Emphysema destroys alveolar walls and capillary bed, reducing surface area. Interstitial lung diseases thicken or disrupt the air-blood interface and can reduce ventilated volume. Pulmonary vascular disease can lower capillary blood volume. These mechanisms can all produce a low DLCO but require different evaluation.

Anemia lowers carbon monoxide uptake because less hemoglobin is available to bind test gas. A low result should not be attributed to lung disease without considering a current hemoglobin value, and technical standards provide equations for adjustment, but your report should make clear whether and how it was applied.

Prior lung resection, incomplete inspiration, chest-wall or neuromuscular limitation, and severe uneven ventilation can reduce participating volume. Smoking shortly before testing raises carboxyhemoglobin and carbon monoxide back-pressure, often lowering the measured value. Supplemental oxygen before testing can also influence results. A normal chest image does not eliminate pulmonary vascular or early parenchymal causes, and conversely, a low DLCO alone does not justify a computed tomography scan or a specific diagnosis without probability, symptoms, and other tests.

What can raise DLCO#

Higher pulmonary capillary blood volume can increase uptake. Exercise, a left-to-right cardiac shunt, and some states of increased blood flow can raise DLCO. Polycythemia provides more hemoglobin binding capacity. Alveolar hemorrhage can raise uptake because carbon monoxide binds hemoglobin within the airspaces.

Asthma can have a normal or high DLCO, in contrast with emphysema, although this is not an absolute rule; obesity can affect lung volumes and pulmonary blood volume in ways that sometimes raise KCO or DLCO. Supine position can increase pulmonary blood volume compared with sitting. An elevated value is therefore not simply “excellent lungs.” It is a measurement requiring context, just as a low value is not a diagnosis.

Patterns with spirometry and lung volumes#

Obstruction plus low DLCO can support emphysema or another process that reduces gas exchange, while obstruction with preserved or high DLCO may fit asthma or chronic bronchitis more readily. These are patterns, not verdicts. Mixed disease is common.

Restriction plus low DLCO can suggest parenchymal lung disease. Restriction with preserved DLCO may point toward extrapulmonary causes such as obesity, chest-wall limitation, or neuromuscular weakness. True restriction requires reduced total lung capacity; a low forced vital capacity on spirometry alone can also result from air trapping or submaximal effort.

An isolated low DLCO with otherwise normal spirometry and volumes deserves attention. Possibilities include early emphysema, early interstitial disease, pulmonary vascular disease, anemia, or a flawed maneuver. The next step depends on symptoms, probability, and prior results. It depends on examination, hemoglobin, imaging, and cardiovascular assessment. The 2022 ERS and ATS interpretive standard emphasizes integrated patterns and uncertainty rather than one-value disease labels.

Reference equations and z scores#

Expected DLCO changes with age, sex, height, and population. Older reports often used fixed percent-predicted cutoffs such as 80 percent. A fixed threshold misclassifies people because normal variability is not constant across age and body size.

The Global Lung Function Initiative equations provide age-spanning reference values and z scores. A z score expresses how far the measurement lies from the predicted mean in standard-deviation units. The lower limit of normal is usually based on the lower fifth percentile rather than an arbitrary percent.

Reference equations do not eliminate interpretation problems. The laboratory must use compatible equipment and methods, and the reference sample may fit some populations better than others. Trends should ideally be compared using the same laboratory, method, correction practices, and reference framework.

Hemoglobin, altitude, and smoking corrections#

Hemoglobin adjustment estimates what DLCO would be at a standard hemoglobin concentration. It does not treat anemia or prove that anemia is the only cause. If hemoglobin was measured months earlier, the correction may be misleading.

At altitude, lower inspired oxygen changes carbon monoxide uptake. Barometric pressure is part of accurate testing. Supplemental oxygen should be managed according to protocol before the maneuver when safe. Your report should document those conditions.

Recent cigarette or other smoke inhalation can raise carboxyhemoglobin. Standards often advise avoiding smoking before testing and recording status. Chronic smoking also causes lung disease, so the acute measurement effect and long-term structural effect should not be confused.

Quality control before interpretation#

Before you explain an abnormal value, check whether inspiration reached the required proportion of vital capacity, whether inhalation was rapid, whether the breath hold was stable, whether leaks or Valsalva or Mueller maneuvers occurred, and whether repeated acceptable efforts agreed.

A Valsalva maneuver raises intrathoracic pressure and can reduce pulmonary capillary blood volume. A Mueller maneuver can increase it. Cough, hesitation, or an irregular exhalation can distort sampling. Good coaching is part of measurement quality.

The ATS patient information explains that pulmonary function tests require maximal effort and repeated maneuvers. “Patient effort” should not be used dismissively. The report should specify which quality criterion was not met and how it could bias the result.

DLCO is not pulse oximetry#

Pulse oximetry estimates the percentage of hemoglobin currently saturated with oxygen. You can have normal resting saturation and a low DLCO, because the reserve only shows up during exercise or because compensation holds the resting value. A person can have low saturation from hypoventilation with a DLCO that is not severely reduced.

Exercise oximetry, arterial blood gas analysis, cardiopulmonary exercise testing, echocardiography, or imaging may answer additional questions. Their use should follow a defined clinical problem, not an automatic cascade from one low number.

DLCO also does not predict breathlessness perfectly. Dyspnea reflects ventilatory demand, mechanics, and circulation. It reflects muscles, conditioning, perception, and comorbid illness. A mild reduction can matter in one context and be incidental in another.

How to read a report step by step#

Start with your symptoms and the reason for the test. Verify test quality and corrections. Read the z score and lower limit rather than only percent predicted. Compare DLCO with VA, KCO, spirometry, and measured lung volumes. Check hemoglobin, smoking timing, oxygen use, altitude, and prior values.

Then build a short differential by mechanism: surface area, membrane thickness, or capillary blood volume. Consider hemoglobin, participating volume, or technique. Use additional testing only when it changes a plausible decision.

Serial decline can be more informative than a single mildly low result, but biological and technical variability matter. A meaningful change must exceed expected repeatability and be interpreted alongside symptoms, treatment, and method consistency.

When evaluation is urgent#

DLCO itself rarely creates an emergency. New severe shortness of breath, chest pain, or fainting requires prompt assessment regardless of a pulmonary-function appointment. So does coughing blood, confusion, or a markedly low oxygen reading. A chronic abnormal value belongs in a planned evaluation.

References#

1. 2017 ERS and ATS DLCO technical standard
2. 2022 ERS and ATS interpretive strategies
3. GLI carbon monoxide transfer reference values
4. ATS patient guide to pulmonary function tests
5. 2005 ATS and ERS single-breath standardization
6. DLCO and KCO interpretation review