Aug 27, 2025 Why DLCO Matters: Understanding Diffusion Capacity Testing
Diffusion Capacity Testing (DLCO) is a valuable diagnostic tool that helps clinicians determine the underlying reasons a patient struggles with distressing respiratory symptoms such as chronic coughing, wheezing, and shortness of breath. Many different conditions can produce these symptoms. DLCO provides critical information about how well or poorly the lungs diffuse gases like oxygen into the bloodstream. This is why DLCO matters: testing data provides unique, granular insights that doctors can use to diagnose whether a patient is struggling with allergy, asthma, COPD, or another respiratory condition. DLCO testing complements spirometry and lung volumes testing, providing additional information healthcare providers can use to arrive at the correct diagnosis faster, so patients can get the treatment they need to breathe more easily.
What is DLCO? The Pulmonary Diffusion Capacity Test Explained
Pulmonary diffusion is the mechanism enabling the blood to absorb oxygen from breathing. When a person inhales, the oxygen goes into the lungs, traveling into the tiny air sacs (alveoli). From there, the oxygen travels into the blood vessels (capillaries), which deliver the oxygen to the rest of the body. Several factors can determine how effectively a person’s diffusion process works, including the alveoli surface area, how much blood is traveling through the capillaries, hemoglobin concentration in the blood, excess fluid in the alveoli, and how thick the membrane is between the capillaries and alveoli. DLCO testing measures these aspects to help clinicians determine pulmonary function and the underlying reasons for any decline.
DLCO stands for “Diffusing Capacity of the Lungs for Carbon Monoxide” – which measures how capable the lungs are of transporting a tiny, non-toxic dose of carbon monoxide (CO) into the lung’s tiny alveoli, and from there into the bloodstream. This test uses CO because it binds more quickly to the hemoglobin than oxygen does. This enables more precise lung diffusion capacity measurements without impacting blood oxygenation.¹
Why DLCO Matters
DLCO testing with the MiniBox+ can be done easily during an office visit to help diagnose patients with respiratory conditions that spirometry alone cannot identify. It expands the depth of information clinicians can see, helping them provide better, accelerated care for patients with many different diagnoses, including:
- Allergies
- Anemia-related lung conditions
- Asthma
- Chronic Obstructive Pulmonary Disease (COPD)
- Dyspnea
- Emphysema
- Hypoxemia
- Interstitial lung disease (ILD)
- Idiopathic pulmonary fibrosis (IPF)
- Pulmonary hypertension
- Sarcoidosis
DLCO helps healthcare providers diagnose conditions like these more quickly because diseases that reduce oxygen diffusion will also cause a similar reduction in CO uptake. This provides an early warning indicator for clinicians to provide early intervention to improve outcomes for patients.
In cases such as idiopathic pulmonary fibrosis (IPF), for example, DLCO can catch telltale signs of the condition well before spirometry testing can. Healthcare providers can also use DLCO testing to monitor disease progression, observe how well patients are responding to various therapies, and catch red flags that predict mortality.
A recent study also determined that DLCO testing was a helpful warning indicator in patients with COPD, helping doctors predict and assess patient needs and treatment options more rapidly.²
The Diffusion Capacity Testing Process
The MiniBox+™ provides rapid, single-breath spirometry, lung volumes, and DLCO testing in a portable device that saves patients and healthcare providers time in getting an accurate diagnosis and an overall snapshot of pulmonary function. The compact, portable MiniBox+™ is easy to use for patients who cannot go into a cabin (those in wheelchairs, patients with obesity, or claustrophobia), and it can be used to test patients age 5 and up. The DLCO test explained simply:
- Providers should instruct patients to prepare for their appointment by abstaining from smoking or vigorous exercise for four to six hours before the test. Patients on supplemental oxygen should stop for 10 minutes prior to the test only if they can do so safely.
- During the appointment, the patient can sit comfortably as the provider attaches the nose clip and the patient puts the mouthpiece in their mouth, breathing normally. The nose clip ensures the isolation of inhaled gases, going only in and out through the mouth.
- The patient is instructed to exhale completely and then to inhale.
- A test gas mixture with 0.3% CO and a tracer gas for comparison flows through the mouthpiece.
- The clinician instructs the patient to inhale and exhale for specific measurements during the test to collect the most accurate data.
This simple outpatient test can provide precision information about how well a patient’s lungs are working, and, more importantly, why or why not. In addition to diagnosing a wide range of pulmonary diseases and conditions, DLCO testing can also determine lung function before surgery, help assess lung damage, and help clinicians track how well current treatments are working by indicating whether a condition is worsening or improving.
Diffusion Capacity Test Interpretation: How to Interpret DLCO Results
A DLCO test will collect and evaluate data on the following parameters:
1. Total DLCO
This parameter displays the lungs’ overall gas transfer abilities. This data point will reflect the alveoli surface area, the thickness of the membrane between the alveoli and the capillaries, and the amount of blood flowing through those capillaries.
2. Diffusion Capacity per Unit Volume of Lung – DLCO/VA
A measurement indicating how efficiently the lungs diffuse oxygen relative to their volume. Healthcare providers can use this measurement to detect lung capacity impairment caused by lung volume reduction or a specific disease process.
3. Alveolar Volume – VA
This measurement shows the alveolar air volume at the end of each breath. This measurement is important for accurately interpreting other DLCO test values.
4. Breath-Holding Time
Usually, a 10-second span of time during which the patient holds their breath. The MiniBox+™ uses this time to accurately calculate diffusion measurements.
These different measurements provide important metrics for evaluating pulmonary function, which can then be grouped into the following general measurements:
- >80% capacity: Lung function in this range is considered normal.
- 60-79% capacity: This measurement suggests early or mild lung impairment.
- 40-59%: Moderate pulmonary impairment falls into this range and is often a hallmark of chronic lung diseases.
- <40%: Severe reduction in pulmonary function that requires immediate care. Often present in patients with advanced lung diseases.
DLCO Patterns in Respiratory Patients
A complete pulmonary function test that includes diffusion capacity testing can help healthcare providers narrow down whether a patient’s difficulty breathing is related to an obstructive or restrictive lung disease, or whether there are elements of both disease processes. For example, a respiratory patient may suffer from asthma, an obstructive lung disease that narrows airways, obstructing the free flow of air in and out of the lungs, while also struggling with a restrictive lung condition that prevents them from completely filling their lungs with air, such as obesity or scoliosis (an abnormal s-shaped curve of the spine). A more common example may be when an allergist is helping a patient manage the frequent co-conditions of asthma and allergies. DLCO for allergists can also help them catch indications in patients who may be experiencing the early stages of COPD, which can improve treatment outcomes. DLCO testing also helps clinicians working with patients who may have occupational asbestosis or silicosis, just to name a couple of examples.
How the MiniBox+ Simplifies DLCO Testing and Interpretation
The compact, cabinless MiniBox+ enables clinicians to perform complete PFTs using plethysmography, including spirometry, lung volumes, and DLCO, quickly and easily right in the office without requiring a body box or outsourced testing. In addition to the short testing time for the patient, DLCO testing is fully automatic and can be performed by any member of a medical office staff. Results are automatically generated.
References
¹Modi, P., & Cascella, M. (2024). Diffusing capacity of the lungs for carbon monoxide (DLCO). PubMed; StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK556149/
²Diffusing Capacity of Carbon Monoxide in Assessment of COPD. (2019). Chest, 156(6), 1111–1119. https://doi.org/10.1016/j.chest.2019.06.035