How DLCO is Measured
DLCO is measured using a single-breath technique where the patient inhales a test gas mixture containing a small amount of carbon monoxide, holds their breath for 10 seconds, and then exhales—a simple maneuver requiring minimal patient effort. 1, 2
The Measurement Technique
The single-breath method is used almost exclusively in the United States and has been the standard for over 50 years. 3 The test procedure involves:
- Patient breathes quietly at rest, then inhales the test gas mixture to total lung capacity 1
- Breath-hold period of exactly 10 seconds to allow CO diffusion across the alveolar-capillary membrane 1, 2
- Patient exhales completely, and the exhaled gas is analyzed 1
- The test measures CO uptake from alveolar space into pulmonary capillary blood, following the same pathway as oxygen 4
What the Test Actually Measures
DLCO examines two fundamental components of gas transfer:
- Alveolar volume (VA), measured simultaneously via single-breath helium dilution during the DLCO maneuver 1, 2
- Transfer coefficient (KCO or DLCO/VA), which represents diffusing capacity per unit of alveolar volume 5, 2
The test captures the underlying pathophysiology guiding oxygen transfer from alveoli into blood. 2
Special Considerations for Patients with Allergies or Autoimmune Disease
The measurement technique itself does not change for patients with allergic reactions or autoimmune diseases—the same single-breath maneuver is used. However, interpretation requires specific adjustments:
- Hemoglobin correction is mandatory in autoimmune patients who may have anemia of chronic disease, as anemia reduces hemoglobin available for CO binding and falsely lowers DLCO 5, 6
- Carboxyhemoglobin adjustment is important for accurate results, particularly in patients being monitored for drug toxicity (common in autoimmune disease treatment) 5
- The test is safe and involves only trace amounts of CO—no contraindications exist for allergic or autoimmune patients 1
Technical Quality Control
Strict adherence to lung function standards and guidelines reduces test variability. 5 The measurement requires:
- Meticulous quality control to ensure reliable results, especially when tracking disease progression over time 5
- Proper breath-hold time of 10 seconds, as this directly affects the measured value 1
- Adequate patient cooperation for the inhalation to total lung capacity, though the maneuver requires very little effort compared to spirometry 1
Billing and Documentation
Only CPT code 94720 should be used for DLCO measurement. 3 Code 94725 (membrane diffusing capacity) has no established clinical indications and should not be billed. 3