The Diffusing Capacity of the Lung for Carbon Monoxide (DLCO) test is a pulmonary function test used to assess how efficiently the lungs transfer gas from the air sacs into the bloodstream. This measurement provides insight into the gas exchange capabilities of the lungs that other standard breathing tests cannot offer. This article explains the physiological principle behind the DLCO measurement and details the specific procedure used to obtain this clinical value.
Defining the Measurement
The DLCO is a calculated value that indirectly assesses the ability of the lungs to transfer gas to the blood. It quantifies the volume of carbon monoxide (CO) that moves from the lung alveoli across the alveolar-capillary membrane and into the red blood cells each minute, for a given pressure difference. This measurement reflects the ease with which respiratory gases are transported into the pulmonary capillary blood. The test is a reflection of the total surface area available for gas exchange and the thickness of the barrier separating the air and blood.
The result measures the efficiency of the alveolar-capillary membrane, the interface where oxygen is transferred into the blood. Factors influencing this transfer include the lung’s available surface area, the thickness of the alveolar wall, and the volume of blood flowing through the pulmonary capillaries. The DLCO value is an indicator of the physiological health of the lung’s gas exchange apparatus.
The Testing Procedure
The DLCO measurement is performed using Krogh’s single-breath method. Patients are instructed to abstain from smoking or vigorous exercise beforehand to ensure accurate results. The procedure begins with the patient sitting comfortably, breathing through a mouthpiece connected to the machine with a nose clip in place.
First, the patient exhales completely to their residual volume, emptying the lungs. Immediately following this maximum exhalation, the patient rapidly inhales the specialized test gas mixture fully, reaching their total lung capacity. This deep inhalation is followed by a precisely timed breath-hold, typically lasting 10 seconds.
During the breath-hold, carbon monoxide from the test gas diffuses into the bloodstream. The patient then exhales quickly and completely back into the machine. The first portion of the exhaled gas, which represents the anatomic dead space, is discarded. A subsequent alveolar sample is collected and analyzed by the machine to determine the concentration of the test gases.
Understanding the Gas Mixture
The DLCO test relies on a specialized gas mixture containing a trace amount of carbon monoxide (CO), an inert tracer gas, oxygen, and nitrogen. The CO concentration is very low (around 0.3%) and is not enough to cause harm. CO is selected because its affinity for hemoglobin is approximately 200 to 250 times greater than that of oxygen. This high affinity ensures that CO immediately binds to hemoglobin upon crossing the membrane, effectively removing it from the gas phase. By measuring how much inhaled CO is “missing” from the exhaled sample, the machine calculates the amount that diffused into the blood.
The mixture also contains an inert tracer gas, such as helium or methane, typically at a concentration of about 10%. This tracer gas does not cross the alveolar-capillary membrane and remains in the air sacs. Measuring the dilution of the tracer gas in the exhaled sample allows technicians to calculate the alveolar volume (VA)—the volume of air in the lung alveoli at the start of the breath-hold. This calculated volume is then used in the final equation to determine the DLCO value.
Interpreting the Results
The final DLCO result is reported as a percentage of a predicted normal value, calculated based on the patient’s age, sex, and height. A normal DLCO result generally falls within the range of 75% to 140% of the predicted value. Values between 60% and 75% indicate a mild reduction, while those below 40% signify a severe reduction. A low DLCO indicates the lungs are less efficient at transferring gas into the blood. This reduction can be due to a loss of surface area, such as occurs in emphysema, or a thickening of the alveolar-capillary membrane, characteristic of conditions like pulmonary fibrosis.