DLCO (Diffusing Capacity) Testing

What DLCO Measures

DLCO fills in this part of the picture, giving a more complete profile when combined with the other components described on the pulmonary function testing page.

Two Jobs, Two Measurements

The lungs perform two related but separate jobs, and DLCO is built to isolate the second one.

Several things can interfere with this second job even when the first job, moving air, is working fine:

• Reduced surface area: less alveolar tissue is available for oxygen to cross into the blood
• Thickened or scarred tissue: the barrier oxygen must cross becomes slower and less efficient
• Reduced blood flow: less blood passes through the capillaries to carry oxygen away

DLCO testing is designed to detect a reduction in this overall gas-transfer process, though it does not by itself distinguish which of these underlying mechanisms is responsible. That determination depends on the broader clinical picture.

How the Test Works: The Carbon Monoxide Principle

DLCO testing uses carbon monoxide as a tracer gas, which can understandably raise a question for patients: is that safe? The answer is yes. The amount used is extremely small and is mixed with other gases for a single test breath.

Carbon monoxide is chosen for technical reasons: it binds readily and predictably to hemoglobin in the blood, which makes its uptake from a single breath precisely measurable using specialized testing equipment. The trace amount used is not clinically meaningful and is not a health concern; it is a measurement tool, not a treatment or exposure of any real consequence.

What Happens During the Test

1. 01→Inhale the Gas MixtureYou take a single deep breath of a gas mixture containing a trace, safe amount of carbon monoxide.↓
2. 02Hold Your BreathYou hold your breath briefly, typically around ten seconds.↓
3. 03→ExhaleYou exhale, and the equipment measures how much tracer gas was absorbed compared to how much was inhaled.↓
4. 04Result CalculatedThe amount absorbed is compared against predicted normal values and, over time, against your own prior results.

The more efficiently your lungs transfer gas into the bloodstream, the more of the tracer gas is absorbed during the breath-hold. This single-breath technique is standardized so results can be compared meaningfully across visits.

How Results Are Interpreted

DLCO results are reported as a percentage of a predicted normal value, calculated using factors such as your age, sex, height, and sometimes race or ethnicity, the same general approach used for other pulmonary function measurements like spirometry and lung volumes. A result within the expected range suggests gas transfer is functioning normally, while a result below the expected range suggests some degree of impaired gas exchange.

As with every other component of pulmonary function testing, a DLCO result is meaningful as part of an overall pattern, not as a stand-alone number. The same percentage-of-predicted value can arise from very different underlying processes, and DLCO alone cannot tell you which one applies to you.

This is why DLCO is always interpreted by a physician together with your symptoms, your other pulmonary function testing results, your medical history, and sometimes imaging studies, never as an isolated figure to be read on its own.

What Can Lower DLCO

DLCO can be reduced by several distinct underlying processes, each explored in more detail below.

Common Causes of a Reduced DLCO

Emphysema

Destruction of the walls between alveoli reduces the total surface area available for oxygen to cross into the bloodstream.

Interstitial Lung Disease

Thickening, inflammation, or scarring of lung tissue slows and reduces gas transfer across the barrier oxygen must cross.

Pulmonary Vascular Disease

Reduced blood flow through the lungs' capillary network limits how much oxygen can be carried away, even when lung tissue is unaffected.

Anemia

Lower-than-normal hemoglobin reduces how much tracer gas is taken up, independent of the lungs' tissue or blood vessels.

Low DLCO From Changes in Lung Tissue

Low DLCO Beyond the Lung Tissue Itself

Why Results Require Clinical Context

It bears repeating clearly: patients should not attempt to self-diagnose a specific lung condition from an isolated DLCO number. A low DLCO can arise from loss of alveolar surface area, tissue thickening or scarring, reduced blood flow through the lungs, low hemoglobin, or some combination of these, among other possibilities. The number alone does not distinguish between them.

Reaching a diagnosis requires a physician to weigh your DLCO result together with your reported symptoms, your other pulmonary function testing components, your medical history, and sometimes imaging studies such as a chest CT. If you have questions about what your own DLCO result means, that conversation belongs with the physician who ordered your testing.

Relationship to Full Pulmonary Function Testing

DLCO is typically ordered as part of a complete pulmonary function testing session, performed alongside spirometry and lung volume measurement, rather than as a stand-alone test.

Combining all three components gives a physician a fuller picture: spirometry describes airflow, lung volumes describe overall lung capacity, and DLCO describes how well gas actually transfers once air has reached the alveoli.

Ordering DLCO in isolation is less common and is generally reserved for specific situations identified by your physician, such as monitoring a previously diagnosed condition where gas transfer is the primary measurement of interest.