What the DLCO Test Reveals About Gas Transfer

The DLCO test, the single-breath diffusing capacity for carbon monoxide, measures how efficiently gas moves from the air inside the alveoli into the red cells flowing through the lung's smallest vessels. You breathe out fully, take one deep breath of a harmless mixture containing a trace of carbon monoxide and an inert tracer, hold it for about ten seconds, and exhale. The fall in carbon monoxide across that single breath, corrected for the volume the gas reached, tells the laboratory how well the blood-gas barrier is working. A reduced DLCO is one of the most useful abnormal numbers in pulmonary testing because it narrows the problem to three broad territories: lost gas-exchange surface, as in emphysema; a thickened or scarred membrane, as in interstitial lung disease; or too little blood in the pulmonary capillaries, as in pulmonary vascular disease.

Why carbon monoxide, and how the breath is measured

Carbon monoxide is the test gas for a specific reason. Hemoglobin binds it avidly, so under normal conditions almost none accumulates in the plasma to push back. The rate at which carbon monoxide disappears from the alveoli is therefore governed by how easily it crosses the membrane, not by how fast blood flows past. In physiological terms the uptake is diffusion limited, which is exactly the property that makes it a probe of the barrier itself.

The 2017 ERS/ATS technical standard for single-breath carbon monoxide uptake (Graham and colleagues, European Respiratory Journal) rebuilt much of the method around modern rapidly responding gas analysers, which now sample carbon monoxide and the tracer continuously through the exhaled breath rather than from a single collected bag. The standard asks for a full inspiration of at least 90 percent of the person's known vital capacity, a breath-hold near ten seconds calculated by the Jones-Meade method, and two acceptable maneuvers that agree closely before a value is reported. Those details are not bureaucratic. A shallow breath, a leak, or a short hold can each drag the number down and mimic disease.

What the single number is actually made of

DLCO is a product, not a stand-alone reading. It equals the transfer coefficient, KCO, multiplied by the alveolar volume, VA, that the test gas reached. Pulling it apart is what turns a low DLCO from a red flag into a direction.

• VA estimates how much lung the breath actually filled, measured by dilution of the inert tracer.
• KCO describes the efficiency of transfer per unit of that volume.

Two people can share the same low DLCO for opposite reasons. One may have reached very little lung, giving a low VA, but transferred gas well across what remained. Another may have filled the chest normally yet transferred poorly across a damaged barrier. The pattern of VA and KCO, read alongside spirometry and lung volumes, is where the diagnostic value lives.

Hemoglobin is part of the measurement

Because carbon monoxide uptake depends on hemoglobin being present to capture it, the result has to be interpreted against the blood. Anemia lowers a measured DLCO even when the lungs are healthy, and the standard therefore reports a hemoglobin-corrected value. Recent smoking matters too, since circulating carboxyhemoglobin occupies binding sites and creates a back-pressure that blunts uptake. A DLCO read without noting hemoglobin or recent smoking can suggest a lung problem that is really a blood problem.

Reading a low DLCO: three directions

Emphysema. Destruction of alveolar walls removes both surface area and the capillaries embedded in them. DLCO falls, and because the loss is of the exchanging tissue itself, KCO usually falls as well. On spirometry this sits with airflow obstruction, and a low DLCO helps separate emphysema from asthma or chronic bronchitis, where gas transfer is often preserved.

Interstitial lung disease. Fibrosis and inflammation thicken the membrane and shrink the volume available, so DLCO drops alongside a restrictive pattern and a reduced VA. Here the number tracks the extent of involvement and is one of the measurements followed over time to judge whether a fibrotic process is stable or advancing.

Pulmonary vascular disease. When the problem is in the vessels, as in pulmonary arterial hypertension or chronic thromboembolic disease, the capillary blood available to absorb carbon monoxide falls while the airways and lung volumes can look near normal. The classic clue is an isolated low DLCO with otherwise unremarkable spirometry, which is why an unexplained reduction should prompt thought about the circulation rather than the airways alone.

A fourth possibility deserves attention before any of these: the reduction may reflect a poor-quality maneuver or uncorrected anemia rather than any lung disease at all.

Grading the result and its limits

Modern interpretation avoids the old habit of calling anything under 80 percent of predicted abnormal. The 2022 ERS/ATS technical standard on interpretive strategies for routine lung function tests (Stanojevic and colleagues, European Respiratory Journal) recommends expressing DLCO as a z-score against reference equations, with values below roughly minus 1.64 marking the lower limit of normal and wider deviations graded as more severe. This reduces the misclassification that plagued fixed percentage cutoffs, especially at the extremes of age and height.

For all its usefulness, DLCO is a summary, not a diagnosis. It reflects surface area, membrane thickness, capillary blood volume, and hemoglobin all at once, so it localizes the problem without naming the cause. Newer combined nitric-oxide and carbon-monoxide methods, reporting DLNO with DLCO, can begin to separate the membrane component from the blood component, but the single-breath DLCO remains the workhorse precisely because one ten-second breath says so much about the whole gas-exchange surface. This article is educational and is not medical advice.