The Diffusing Capacity of the Lung for Carbon Monoxide (DLCO) test is a common pulmonary function assessment. It evaluates how efficiently gases transfer from the air sacs within the lungs into the bloodstream. This test provides information about the lungs’ gas exchange capabilities, helping healthcare providers understand overall lung function.
Understanding Lung Diffusion
Gas exchange in the lungs occurs in tiny air sacs called alveoli, which are surrounded by a network of capillaries, forming the alveolar-capillary membrane. When a person inhales, oxygen travels from the alveoli across this thin membrane and into the red blood cells within the capillaries, where it binds to hemoglobin. Simultaneously, carbon dioxide moves from the blood into the alveoli to be exhaled. This continuous transfer of gases is fundamental for the body’s metabolic processes.
The DLCO test uses a small, safe amount of carbon monoxide (CO) to measure this transfer efficiency. Carbon monoxide binds to hemoglobin much more readily than oxygen, making it a sensitive indicator of gas transfer from the lungs into the blood. A tracer gas, such as methane or helium, is also included in the inhaled mixture; this gas is not absorbed by the blood and helps measure the amount of CO absorbed. By measuring the absorbed CO, clinicians can assess the integrity of the alveolar-capillary membrane.
Why the Test is Performed
Healthcare providers order a DLCO test to diagnose and manage various lung conditions. It helps identify the cause of symptoms like shortness of breath, persistent cough, or wheezing. For instance, the test is valuable in assessing conditions such as emphysema, where damage to the alveoli reduces the surface area for gas exchange.
The DLCO test also evaluates interstitial lung diseases (ILDs), including pulmonary fibrosis and sarcoidosis, which involve scarring and thickening of lung tissue. These changes can hinder gas transfer, leading to a reduced DLCO. The test helps understand how an underlying disease affects lung function and can detect changes earlier than other common lung function tests like spirometry.
Beyond diagnosis, the DLCO test monitors the progression of lung diseases and assesses treatment effectiveness. It is also part of pre-surgical evaluations to determine a patient’s lung capacity and how well they might tolerate certain procedures.
How the Test is Administered
The DLCO test is performed in a pulmonary function laboratory. During the test, the patient sits comfortably with a clip on their nose to ensure all breathing occurs through the mouth. A mouthpiece connected to a machine is then placed in their mouth.
The patient is instructed to take a few normal breaths, then exhale completely to residual volume. Then, they take a single, deep breath of a special gas mixture. This mixture contains a small, safe amount of carbon monoxide, along with tracer gases like methane or helium, nitrogen, and oxygen. The patient holds their breath for about 10 seconds, allowing time for gas exchange to occur.
After the breath-hold, the patient exhales quickly into the machine. The equipment measures the amount of carbon monoxide and tracer gas remaining in the exhaled air. By comparing inhaled and exhaled gas concentrations, the machine calculates how much carbon monoxide was absorbed by the blood. The test is safe and may be repeated multiple times to ensure accurate results.
What DLCO Results Indicate
DLCO results are presented as a percentage of a predicted normal value, based on factors such as age, sex, height, and hemoglobin levels. A normal DLCO value falls between 75% and 140% of the predicted value, indicating efficient gas exchange and effective transfer of gases into the bloodstream.
A low DLCO value signifies impaired gas transfer. This can be due to issues such as damage to the alveoli, as seen in emphysema, or thickening of the alveolar-capillary membrane, which occurs in interstitial lung diseases like pulmonary fibrosis. A reduced DLCO can also be associated with conditions affecting the pulmonary vasculature, such as pulmonary hypertension, or with anemia due to lower hemoglobin levels. A severely reduced DLCO can indicate more advanced lung disease.
While less common, a high DLCO value can also provide insights. Elevated DLCO may be observed in conditions like asthma, obesity, or polycythemia, where there is an increased volume of blood in the lungs or a higher hemoglobin concentration. It can also occur with pulmonary hemorrhage due to blood in the alveoli. Interpreting DLCO results requires considering them alongside other pulmonary function tests and the patient’s overall medical history for a comprehensive understanding of their lung health.