Dlco Test: Purpose, Test Details & Results

Purpose of Dlco Test

Understanding how well your lungs transfer oxygen to your blood is crucial, especially if you have unexplained breathlessness or a known lung condition. The diffusing capacity of the lungs for carbon monoxide (Dlco) test, sometimes called the transfer factor, is a specialized but straightforward way to gauge this vital function. It helps doctors get a more complete picture of lung health, offering insights that go beyond what routine spirometry tests can reveal. For many patients, this means earlier detection, more accurate diagnosis, and tailored management of respiratory diseases.

Why Measure Gas Exchange?

Unlike standard lung function tests that focus on airflow, the Dlco test zeroes in on how gases move across the delicate boundary between the air sacs (alveoli) and the blood vessels in your lungs. This is a crucial step in getting oxygen to your body’s tissues.

• Dlco uses carbon monoxide (CO)—a gas with a high affinity for hemoglobin—because it mirrors how oxygen diffuses but is easier to measure in small concentrations. The amount of CO transferred gives a direct readout of how effectively the lungs are performing gas exchange 3.
• The result is sensitive to changes in the lung membrane’s thickness, surface area, and blood flow—making it a valuable indicator of a wide range of lung and vascular diseases 2 3.

Key Clinical Applications

Chronic Obstructive Pulmonary Disease (COPD):

• Dlco helps distinguish between emphysema (common in smokers) and asthma. A low Dlco in a smoker with airway obstruction suggests emphysema, while a normal Dlco supports asthma 1 4.

Interstitial Lung Disease (ILD):

• In patients with restricted lung volumes, a reduced Dlco points toward ILD rather than chest wall or neuromuscular causes 4.

Pulmonary Vascular Disease:

• A low Dlco with otherwise normal spirometry raises suspicion for diseases affecting the lung’s blood vessels, such as pulmonary hypertension 4.

Diabetes-Related Microangiopathy:

• Dlco can reveal pulmonary microvascular involvement in people with diabetes, correlating with complications in other organs 5.

Prognostic Value

Dlco is more than a diagnostic tool—it is a powerful predictor of disease severity and future outcomes. A Dlco below 40% of predicted, or a significant decline over time, is associated with increased risk of severe symptoms, hospitalizations, and mortality, especially in COPD and ILD 1 4 12.

Dlco Test Details

The Dlco test might sound technical, but for patients, it’s usually a quick and straightforward experience. Understanding what happens during the test—and what factors can influence the results—can help demystify the process and prepare you for an accurate assessment.

How the Test Works

• Single-Breath Method:
  You’ll breathe in a small amount of a test gas mixture (containing a trace amount of CO and an inert gas like helium), hold your breath for about 10 seconds, and then exhale. The amount of CO absorbed by your blood during this time is measured to calculate your lung’s diffusing capacity 3 4 7.
• Simple and Painless:
  The test is non-invasive and requires minimal effort compared to other lung tests. It’s now widely available, including in outpatient clinics with portable devices 4 6.
• Calibration and Quality Control:
  Regular calibration and quality checks are essential to ensure the accuracy and reliability of Dlco measurements. Stable and well-maintained equipment, as well as biological or simulator controls, help avoid errors or drifts in the results 6 7.

What Affects Dlco Test Results?

Several factors—both technical and physiological—can influence the outcome. Understanding these helps in accurate interpretation.

• Hemoglobin Levels:
  Since CO binds to hemoglobin, anemia or polycythemia can falsely lower or raise Dlco values. Adjustments are often made for hemoglobin concentration 2 3 4.
• Lung Volume and Alveolar Ventilation:
  Reduced lung volumes (as in restrictive diseases) or poor inhalation during the test can lower Dlco 3 4.
• Smoking:
  Recent smoking increases blood CO and can lower the test result; patients are usually advised to abstain before testing 3.
• Altitude and Oxygen Concentration:
  High altitude and varying oxygen levels affect gas pressure gradients, influencing results 3.
• Technical Aspects:
  Proper coaching, equipment calibration, and patient cooperation are crucial for reliable results 4 6 7.

Recent Advances

• Portable Devices:
  Dlco testing, once limited to hospital labs, is now accessible in many clinics, thanks to portable and reliable devices 4 6.
• Combined Tests:
  Research is ongoing into combining Dlco with other diffusion tests (e.g., nitric oxide) for even more detailed lung assessments 7.

Dlco Test Results & Follow-Up

After the test, your doctor will interpret the Dlco results alongside your symptoms, history, and other lung function tests. The outcome often guides diagnosis, monitoring, and treatment plans—sometimes even predicting how your disease will progress.

Interpreting the Numbers

• Percent Predicted:

  • Results are usually reported as a percentage of the “predicted” value for your age, sex, height, and sometimes ethnicity.

  • 80% predicted = normal

  • 60–80% predicted = mild reduction
  • 40–60% predicted = moderate reduction
  • <40% predicted = severe reduction 9 12

• What Low Dlco Means:
  A reduced Dlco indicates that your lungs are less able to transfer gases, often due to:

  • Thickened or damaged alveolar membranes (as in interstitial lung disease or pulmonary fibrosis)
  • Loss of alveolar surface area (emphysema)
  • Poor blood flow in lung vessels (pulmonary hypertension, microangiopathy)
  • Anemia (if uncorrected in calculation) 2 3 4 5

Disease-Specific Patterns

• COPD:
  Lower Dlco is linked to more severe symptoms, worse exercise tolerance, and higher risk of hospitalizations—even when other tests (like spirometry) are similar 1 12.
• Interstitial Lung Disease:
  Dlco often drops before other tests become abnormal, making it an early marker. It also tracks disease progression and response to therapy 4.
• Pulmonary Vascular Disease:
  Dlco is highly sensitive to issues with blood flow through the lungs, such as in pulmonary embolism or pulmonary hypertension 4 5 9.
• Diabetes:
  Lower Dlco may reveal microvascular lung involvement in diabetic patients and correlates with complications in other organs 5.
• COVID-19:
  Many survivors show reduced Dlco for months after recovery, especially those with severe illness, ARDS, or higher CT severity scores. While many improve over a year, a significant number continue to have persistent impairment, highlighting the need for long-term follow-up 8 9 10 11.

Follow-Up and Prognosis

• Monitoring Disease Progression:
  Serial Dlco testing helps track progression or improvement, especially after interventions or changes in therapy 1 4 10.
• Prognostic Indicator:
  Persistently low Dlco, especially below 40% predicted, is associated with increased risk for severe symptoms, exacerbations, and mortality in diseases like COPD and ILD 1 4 12.
• When to Repeat:
  Your doctor may recommend regular Dlco testing if you have a chronic lung condition, are being treated for pulmonary vascular disease, or are recovering from a severe illness like COVID-19 4 8 10.

Conclusion

The Dlco test is a vital, patient-friendly tool that provides unique insights into lung health, going beyond what standard tests can reveal. It plays a key role in diagnosing, monitoring, and predicting outcomes in a wide range of lung and systemic diseases.

Key Takeaways:

• Dlco measures how efficiently your lungs transfer gases, reflecting the health of the alveolar-capillary membrane.
• It is indispensable for diagnosing and tracking diseases like COPD, interstitial lung disease, pulmonary vascular conditions, diabetes-related lung changes, and post-COVID-19 lung involvement.
• Test results are influenced by several factors—proper preparation and technique are essential for accuracy.
• Low Dlco is a marker of disease severity, predicts outcomes, and guides treatment decisions.
• Regular follow-up with Dlco testing is crucial for many chronic lung conditions, especially when symptoms persist or worsen.

Whether you’re a patient looking for answers or a healthcare professional seeking a deeper understanding, the Dlco test remains a cornerstone of modern respiratory care—helping to ensure that no aspect of lung health goes unnoticed.