The carbon monoxide diffusion lung capacity (DLCO) provides crucial information about the lungs’ ability to transfer oxygen from inhaled air into the bloodstream. For this reason, this test is essential for the diagnosis and monitoring of various pulmonary diseases. In this blog, we analyze its advantages and provide examples of its benefits in clinical practice.
By América Torres
What is Carbon Monoxide Pulmonary Diffusion?
The DLCO test, also known as the carbon monoxide transfer factor (TLCO), measures the amount of carbon monoxide (CO) transferred per minute from the alveolar gas to the red blood cells (RBCs). Generally, the test is performed as follows:
- The patient remains seated, using a nasal clip and mouthpiece, breathing normally until the DLCO equipment operator instructs the patient to exhale completely to the residual volume.
- The mouthpiece is connected to a test gas mixture containing 0.3% CO, a tracer gas (helium or methane), oxygen, and nitrogen.
- The patient then inhales rapidly until reaching total lung capacity (TLC) within a maximum of 4 seconds.
- Upon reaching TLC, the patient holds their breath for 10 seconds and then exhales completely.
- The test is repeated after a minimum of 4 minutes, but the patient should not perform more than 5 attempts per session, as additional attempts may reduce DLCO by up to 3.5%.
- At least 2 attempts should fall within 2 mL/min/mm Hg (0.67 mmol/min/kPa).
Pulmonologists use this test because the inhaled CO has a high affinity for hemoglobin (Hgb). Since this gas is diluted in Hgb, its absorption is mainly determined by changes in the alveolar-capillary membrane rather than by blood flow, allowing for the evaluation of membrane integrity.
Utility of the Carbon Monoxide Pulmonary Diffusion Test
DLCO (carbon monoxide diffusion capacity) is a fundamental test in the evaluation of pulmonary function, as it measures the efficiency with which the lungs transfer oxygen into the bloodstream. As mentioned earlier, alterations in the properties of the respiratory membrane, hemoglobin (Hgb) levels, and capillary blood volume are key factors that contribute to variations in DLCO, making this test an indispensable tool for diagnosing and monitoring various pulmonary diseases.
DLCO is especially useful for evaluating patients with dyspnea, hypoxemia, emphysema, and interstitial lung disease (ILD). Additionally, it serves as an early indicator of conditions like idiopathic pulmonary fibrosis (IPF), even before changes are detected in spirometry results. This ability to detect diseases in early stages makes carbon monoxide pulmonary diffusion a key resource in pulmonary medicine.
In addition to its diagnostic function, DLCO is valuable for monitoring disease progression, assessing the patient’s response to treatment, and predicting mortality. Any condition that reduces oxygen uptake, such as pulmonary diseases, will also affect CO uptake, highlighting the importance of this test in early detection and clinical decision-making.
Applications of DLCO
DLCO is a valuable tool for early intervention and more effective management of pulmonary diseases. It helps identify the underlying cause of hypoxemia and dyspnea, assess the severity of obstructive and restrictive pulmonary diseases, analyze pulmonary vascular diseases, and predict preoperative risk in patients. In fact, it is such a versatile test that, when combined with spirometry, it provides pulmonary specialists with crucial information about both parenchymal and non-parenchymal lung diseases.
Among the conditions that cause an elevation in DLCO, we can mention:
- Asthma
- Polycythemia
- Mild left heart failure due to increased pulmonary capillary blood volume
- Pulmonary hemorrhage
Likewise, a decreased DLCO may be associated with various conditions, for example:
- Normal lung volumes: anemia, pulmonary vascular disease, early ILD.
- Obstructive pathologies with low DLCO, with or without concurrent restriction: bronchiolitis, cystic fibrosis, emphysema, ILD in COPD patients, sarcoidosis, pulmonary fibrosis combined with emphysema.
- Restrictive disorders with low DLCO: ILD, pneumonitis.
- Decreased DLCO with normal spirometry: pulmonary vascular disease, early ILD, anemia, elevated carboxyhemoglobin (COHb) levels caused by smoking.
Below, we provide some examples that demonstrate why DLCO is an indispensable tool for diagnosing and treating various pulmonary conditions.
The Value of DLCO in the Diagnosis and Follow-up of Post-COVID Syndrome
COVID-19 primarily affects the alveoli, as the virus attacks type II pneumocytes, which can cause abnormalities in the alveolar-capillary membrane. This is known as post-COVID syndrome, and patients with this condition complain of symptoms such as dyspnea and fatigue, even though spirometry results are often normal.
In these cases, the diffusion test is the ideal method to confirm the diagnosis of post-COVID syndrome and is useful for monitoring pulmonary rehabilitation after this illness.
Association of Reduced DLCO and Small Airway Dysfunction with Worse Prognosis
The clinical trial “Association between impaired diffusion capacity and small airway dysfunction: a cross-sectional study” by Zhou K. et al. explored the association between small airway dysfunction (SAD) and reduced DLCO, and its correlation with a worse prognosis. The researchers involved in the study identified individuals with both dysfunctions in clinical practice and lacked the elements to clarify whether these conditions were indicators of worsening in those patients or if they required specific management.
The study included 581 subjects with the aim of exploring the relationship between small airway dysfunction and impaired DLCO. Groups with both dysfunctions, with one dysfunction, and with no dysfunction were compared. The researchers performed evaluations of diffusion capacity, questionnaires, exacerbations, and tests of spirometry, oscillometry, and computed tomography (CT).
Small airway dysfunction, defined by spirometry and CT, was much more frequent in subjects with reduced DLCO compared to those with normal DLCO. Furthermore, patients with both dysfunctions experienced more exacerbations in the previous year than the control subjects.
Therefore, the researchers concluded that reduced DLCO is associated with small airway dysfunction. As a result, patients with both conditions may have a more compromised health status and require additional treatment.
PowerCube Diffusion+: The SCHILLER-GANSHORN Solution
The PowerCube Diffusion+ from SCHILLER-GANSHORN was the first CO diffusion system on the market to utilize SharpFlow (ultrasonic) technology. This advanced device features a high-speed, long-lasting sensor that requires no maintenance and allows for adjustable discard and sampling volumes, enabling measurements in patients with a vital capacity as low as 0.5 liters. Additionally, its high-precision analyzers provide a detailed visualization of CO and helium washout curves. Its demand valve ensures efficient use of sample gas, reducing operational costs.
The PowerCube Diffusion+ system can be integrated into the PowerCube Body+ Body Plethysmograph, but we also offer a standalone version. This makes it an excellent solution for pulmonary function clinics or laboratories that need to assess lung volumes but have limited space.
See It in Action!
Schedule a personalized session with a SCHILLER specialist. They will carefully listen to your needs and demonstrate how the PowerCube Diffusion+ can optimize your practice. This session is completely free, with no purchase obligation, and will provide you with all the information you need to make the best decision for your clinic, office, or hospital. Click the button and let us help you take the next step!
REFERENCES
[1] Modi P, Goldin J, Cascella M. Diffusing Capacity of the Lungs for Carbon Monoxide. [Updated 2024 Oct 6]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK556149/
[2] Zhou K, Wu F, Lu L, Tang G, Deng Z, Dai C, Zhao N, Wan Q, Peng J, Wu X, Zeng X, Cui J, Yang C, Chen S, Huang Y, Yu S, Zhou Y, Ran P. Association between impaired diffusion capacity and small airway dysfunction: a cross-sectional study. ERJ Open Res. 2025 Jan 13;11(1):00910-2023. PMCID: PMC11726590 DOI: 10.1183/23120541.00910-2023
