The Diffusing Capacity of the Lung for Carbon Monoxide (DLCO) is a measurement obtained during a pulmonary function test that assesses how effectively your lungs transfer gas from the air you breathe into your bloodstream. This test provides a quantitative number reflecting the overall functional surface area of the lungs and the health of the delicate barrier between the air sacs and the blood vessels. The DLCO result acts as an important indicator of the lung’s ability to perform its primary function: oxygen uptake. Understanding this value is important for individuals being monitored for chronic lung conditions or those experiencing unexplained shortness of breath.
Understanding the DLCO Test
The DLCO test measures the efficiency of gas exchange across the alveolar-capillary membrane, the thin barrier separating air in the lungs from blood in the capillaries. During the test, the patient inhales a specific gas mixture containing a small, harmless amount of carbon monoxide (CO) and a tracer gas like methane or helium. This single-breath technique requires the patient to inhale deeply, hold their breath for about ten seconds, and then exhale.
Carbon monoxide is used because it binds to hemoglobin in the blood far more readily than oxygen, making it an ideal stand-in to measure how quickly gas is absorbed into the circulation. By comparing the CO concentration in the inhaled air to the exhaled air, the machine calculates how much CO was absorbed by the blood. The tracer gas helps determine the volume of air that reached the gas-exchanging parts of the lungs. The final DLCO value, expressed in milliliters per minute per millimeter of mercury, represents the total functioning surface area available for gas exchange.
Interpreting a Normal Result
A DLCO result is presented as a percentage of a predicted value, rather than a standalone number. This predicted value is what a healthy person with the same characteristics would be expected to achieve, based on population studies. The widely accepted range for a normal DLCO result is typically between 80% and 120% of the predicted value. The 80% lower limit is a common clinical cutoff, though some laboratories may use a slightly wider range.
A result falling below the normal range is classified into categories of impairment. A result of 60% to 79% of the predicted value is generally considered a mild reduction, while a moderate reduction is between 40% and 59%. A DLCO value less than 40% of the predicted value indicates a severe reduction in the lung’s capacity to transfer gas. These classifications help clinicians understand the severity of the physiological problem.
Physiological Variables That Affect DLCO
The predicted DLCO value is calculated using a formula that accounts for several non-disease-related, physiological factors specific to the individual. Height is a significant factor, as taller individuals naturally have larger lung volumes and a greater surface area for gas exchange, leading to a higher predicted DLCO. Age is also a variable, as the DLCO value naturally decreases as a person gets older. Sex and race/ethnicity are built into the predictive equations because they correlate with differences in average lung size. The amount of hemoglobin in the blood must also be considered, as low hemoglobin (anemia) reduces the binding sites for carbon monoxide, lowering the DLCO; this factor is often mathematically corrected for in the final reported result.
Medical Conditions That Change DLCO Values
A significantly reduced DLCO result often points to two main types of problems: those affecting the gas exchange surface itself or those affecting the blood flow to the lungs. Conditions that destroy or reduce the surface area of the alveoli, such as emphysema (a form of COPD), cause a low DLCO because fewer functional air sacs are available. Interstitial Lung Diseases (ILDs), like pulmonary fibrosis, cause the alveolar-capillary membrane to thicken, making it more difficult for gas to pass through, which also results in a low DLCO.
Conditions that affect the pulmonary blood vessels, such as pulmonary hypertension or pulmonary embolism, reduce the amount of blood flowing to the gas-exchanging surfaces, which lowers the DLCO. Conversely, a DLCO result higher than the predicted value (sometimes above 120% or 140%) can also be clinically relevant. Elevated DLCO values are sometimes seen in people with asthma or in cases of acute pulmonary hemorrhage, where blood in the air sacs provides extra hemoglobin for the carbon monoxide to bind. High DLCO is also associated with large lung volumes, obesity, and conditions that increase the number of red blood cells, such as polycythemia.