The Diffusing capacity of the Lung for Carbon Monoxide (DLCO) test is a specialized, non-invasive measurement within pulmonary function testing (PFT). It assesses the efficiency of gas exchange within the lungs. Specifically, the DLCO measures how readily gas moves from the air sacs (alveoli) into the bloodstream during a single breath hold. This result provides direct insight into the integrity of the alveolar-capillary membrane, the delicate barrier between the air and the blood, helping physicians diagnose and monitor various respiratory and cardiovascular conditions.
Preparing for the DLCO Test
Proper preparation is essential for accurate DLCO measurement, as common activities can temporarily alter the lung’s capacity to absorb gas. Patients must refrain from smoking for a minimum of four to six hours before the test, since carbon monoxide from smoke binds strongly to hemoglobin and skews results. Heavy or strenuous exercise should also be avoided for at least two hours prior, as physical exertion temporarily increases blood flow to the lungs.
Patients should consult with their physician regarding medication use, especially inhalers, as some may need to be withheld. If a patient uses supplemental oxygen, they must stop using it for 10 to 15 minutes before the test to ensure an accurate baseline reading. Avoiding alcohol consumption and heavy meals beforehand is also advised. Furthermore, individuals who have recently had eye or ear surgery should inform their technician, as the required breathing maneuvers involving pressure changes may present a risk.
The Step-by-Step Procedure
The DLCO procedure uses a highly controlled breathing maneuver to isolate the moment of gas transfer. The patient sits comfortably, and a nose clip is placed to ensure all airflow occurs through the mouth, which is sealed around a specialized mouthpiece. The process begins with a few moments of normal breathing to acclimate to the equipment.
The technician instructs the patient to exhale completely and forcefully, emptying the lungs down to residual volume. Following this full exhalation, the patient takes a single, rapid, deep inhalation, drawing the test gas mixture to total lung capacity. This mixture contains a safe, small concentration of carbon monoxide (typically 0.3%) and an inert tracer gas, such as helium or methane.
The patient must hold their breath perfectly still for exactly 10 seconds, allowing time for gas exchange. Carbon monoxide is used because its affinity for hemoglobin is approximately 200 times greater than oxygen, making minor changes in uptake easily measurable. After the breath hold, the patient exhales rapidly into the machine. The machine analyzes the concentration of carbon monoxide and the tracer gas remaining in the exhaled air. The difference between the inhaled and exhaled levels indicates how much gas the lungs successfully transferred into the bloodstream.
Understanding Your DLCO Results
The final DLCO result is reported as a raw number (mL/min/mmHg), but it is usually interpreted as a percentage of a predicted value. This predicted value is calculated based on the patient’s demographic data, including age, sex, and height, establishing what the result should be for a healthy individual. The normal range for DLCO generally falls between 75% and 140% of this predicted value.
A DLCO score below the normal range suggests a problem with the gas transfer apparatus. Physiologically, this reduction occurs due to a decrease in the available surface area for gas exchange (e.g., destroyed alveolar walls) or a thickening of the alveolar-capillary membrane, which slows diffusion. A low DLCO is often reported alongside other pulmonary function metrics, such as Forced Expiratory Volume in one second (FEV1) and Forced Vital Capacity (FVC), for a complete diagnostic picture.
Conversely, a high DLCO is less common but indicates certain physiological states. It occurs when there is an increase in the volume of blood within the pulmonary capillaries, providing more hemoglobin to absorb carbon monoxide. Conditions such as obesity, pulmonary hemorrhage, or cardiac shunts can lead to an elevated DLCO score. Since the test is affected by the amount of hemoglobin in the blood, the DLCO value is often adjusted for the individual’s hemoglobin concentration to accurately reflect lung function alone.
Conditions Indicated by Abnormal DLCO
Physicians frequently order the DLCO test to determine the underlying cause of unexplained shortness of breath or to monitor known pulmonary disease progression. A reduced DLCO suggests a disease process that has compromised the lung’s ability to exchange gases effectively. These conditions are categorized by the specific mechanical impairment they cause.
One category includes conditions that destroy the structure of the alveoli, reducing the total available surface area for diffusion. Emphysema, a component of Chronic Obstructive Pulmonary Disease (COPD), is a prime example where air sac destruction leads directly to a low DLCO. Another group involves diseases that cause scar tissue to form and thicken the alveolar-capillary membrane. This mechanism is characteristic of Interstitial Lung Diseases (ILDs), such as pulmonary fibrosis and sarcoidosis.
A third area involves conditions affecting blood flow to the gas exchange surface. Pulmonary hypertension (high blood pressure in the lung arteries) reduces the number of open, functioning capillaries available for gas exchange, leading to a low DLCO. The test is a sensitive tool for detecting and monitoring diseases that affect the pulmonary vasculature, often before changes are noticeable on other tests.