The Diffusing Capacity of the Lung for Carbon Monoxide (DLCO) test is a specialized measurement within Pulmonary Function Tests (PFTs). The test measures how effectively gases move from the air sacs (alveoli) of the lungs into the bloodstream. It provides insight into the health of the alveolar-capillary membrane, the delicate structure separating air from blood in the capillaries. By assessing this transfer efficiency, the DLCO test helps identify conditions that impair the lungs’ primary function.
How the Test Measures Gas Exchange
The DLCO measurement relies on administering a small, safe amount of carbon monoxide (CO) as a tracer gas. The patient performs a single-breath maneuver: exhaling fully, inhaling a gas mixture containing a very low concentration of CO and a tracer gas, and holding their breath for about 10 seconds. After the breath-hold, the gas concentration in the final portion of the exhaled air is measured.
Carbon monoxide is utilized because it has an extremely high affinity for hemoglobin, binding to it about 210 times more rapidly than oxygen. This strong binding ensures that CO uptake is limited primarily by the capacity of the alveolar-capillary membrane to transfer the gas. The difference between the amount of CO inhaled and exhaled allows the calculation of how much gas was transferred into the blood, measuring the lungs’ diffusion capacity.
Conditions Indicated by Low DLCO Results
A low DLCO result indicates impaired gas exchange, meaning the surface area for transfer is reduced or the alveolar-capillary membrane is thicker and less permeable. This finding is a sensitive indicator of underlying lung or vascular pathology, often preceding changes seen in other lung function tests.
Parenchymal lung diseases, which affect the lung tissue itself, are major causes of reduced DLCO. Emphysema, a component of Chronic Obstructive Pulmonary Disease (COPD), causes the destruction of the alveolar walls, merging smaller air sacs into larger, less efficient ones. This loss of functional surface area for gas exchange directly translates to a lower DLCO.
Interstitial Lung Diseases (ILDs), such as pulmonary fibrosis, cause inflammation and scarring of the lung tissue, leading to a thickened alveolar-capillary membrane. This fibrotic thickening increases the distance the gas must travel to reach the blood, impeding diffusion. The DLCO test can often detect early-stage ILD even when other tests like spirometry appear normal.
Vascular issues within the lungs also significantly reduce the DLCO by decreasing the volume of blood passing through the pulmonary capillaries, thereby limiting the available hemoglobin to absorb the tracer gas. Conditions like Pulmonary Arterial Hypertension (PAH) or recurrent pulmonary emboli reduce the blood flow to the gas-exchanging capillaries, causing a drop in the measured DLCO.
A low DLCO can also be influenced by conditions outside the lungs, such as anemia (a low red blood cell or hemoglobin count). Since CO binds to hemoglobin, a low level of hemoglobin reduces the blood’s capacity to absorb the gas, leading to a falsely low DLCO reading. For this reason, DLCO results are often mathematically corrected for the patient’s hemoglobin level to accurately reflect the lung’s intrinsic function.
Understanding Normal and Elevated DLCO Values
A DLCO value within the predicted normal range (typically 80% to 120% of the reference value) suggests the gas exchange surface is functioning adequately. A normal result does not rule out all lung pathology, as diseases like isolated chronic bronchitis or asthma often present with a normal DLCO. If lung volumes are reduced due to chest wall or neuromuscular problems, the DLCO may appear low, but the transfer coefficient (DLCO/VA) may be normal, indicating preserved diffusion efficiency.
Elevated DLCO values (above 120% of the predicted value) are less common but carry diagnostic significance. This elevation is caused by an increase in pulmonary capillary blood volume. Conditions such as pulmonary hemorrhage (where blood leaks into the air sacs) or polycythemia (high concentration of red blood cells) increase the absorption of carbon monoxide. Increased pulmonary blood flow during exercise or in the presence of left-to-right cardiac shunts can also lead to an elevated DLCO measurement.
How DLCO Integrates with Spirometry
The DLCO result is rarely interpreted in isolation; it is most valuable when evaluated alongside spirometry measurements, such as the Forced Expiratory Volume in one second (FEV1) and the Forced Vital Capacity (FVC). This integrated approach allows clinicians to pinpoint the type and location of the lung impairment.
In an obstructive pattern, defined by a low FEV1/FVC ratio, a simultaneously low DLCO strongly suggests emphysema, where both airflow and the gas exchange surface are compromised. Conversely, an obstructive pattern with a normal DLCO is more consistent with conditions like chronic bronchitis or asthma, where the primary issue is resistance to airflow rather than damage to the alveolar membrane.
A restrictive pattern, characterized by low lung volumes (Total Lung Capacity), combined with a low DLCO points toward Interstitial Lung Disease (ILD). If the restrictive pattern is present but the DLCO is normal, the cause is likely extrapulmonary, such as a chest wall abnormality, obesity, or neuromuscular weakness, where the alveolar function remains intact. This combined interpretation of DLCO and spirometry provides a definitive framework for diagnosing complex lung diseases.