Diffusing Capacity of the Lungs for Carbon Monoxide (DLCO) Illustration

Diffusing Capacity of the Lungs for Carbon Monoxide (DLCO)

by | Updated: Apr 19, 2024

The diffusing capacity of the lungs for carbon monoxide (DLCO) is a crucial parameter in assessing pulmonary function and gas exchange efficiency.

It measures the ability of the lungs to transfer carbon monoxide from inhaled air to the bloodstream, reflecting the surface area and integrity of the lung’s alveolar-capillary membrane.

DLCO is an essential diagnostic tool in evaluating and monitoring various respiratory conditions, such as chronic obstructive pulmonary disease (COPD), interstitial lung disease, and pulmonary vascular disorders.

This article explains the significance of DLCO, its clinical applications, and its role in understanding and managing respiratory health.

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What is the Diffusing Capacity of the Lungs for Carbon Monoxide (DLCO)?

Diffusing capacity of the lungs for carbon monoxide (DLCO) refers to a test that assesses the ability of the lungs to transfer gas from inhaled air to the red blood cells in pulmonary capillaries.

It’s a measure of how well oxygen, and other gases, can move from the lungs to the bloodstream. This test is particularly sensitive to conditions that affect the alveolar-capillary interface.

Diffusing Capacity of the Lungs for Carbon Monoxide (DLCO) Test Vector


DLCO is often used in the diagnosis and monitoring of diseases that affect lung tissue, such as:

  • Chronic obstructive pulmonary disease (COPD): Including chronic bronchitis and emphysema.
  • Interstitial lung diseases: Such as pulmonary fibrosis, sarcoidosis, and asbestosis.
  • Pulmonary arterial hypertension
  • Lung hemoglobinopathies: Conditions affecting the blood’s ability to carry gases.
  • Other conditions: Including conditions that reduce the volume of the lung, like after a lobectomy.


During this pulmonary function test, the individual is asked to breathe in a small amount of carbon monoxide (CO), usually combined with helium or another tracer gas.

After holding their breath for a short time (typically 10 seconds), they exhale.

The concentration of CO in the inhaled and exhaled air is then compared to determine how effectively the CO is transferred from the lungs to the bloodstream.

Interpreting DLCO

A reduced DLCO can suggest a problem with gas exchange at the alveolar-capillary interface, but the precise cause needs further investigation.

It could be due to a reduced surface area for diffusion (like in emphysema or after surgical removal of a lung portion), thickened alveolar-capillary membrane (like in pulmonary fibrosis), or reduced red blood cell availability or function.

It’s also essential to adjust the DLCO for hemoglobin levels because hemoglobin can affect the test’s result.

Low hemoglobin (anemia) can falsely decrease DLCO, and high hemoglobin (polycythemia) can falsely increase DLCO.

There’s also a corrected DLCO for individuals who are current smokers since carboxyhemoglobin in their blood can affect the test’s outcome.

DLCO Practice Questions

1. What is diffusion?
The flow of particles from an area of higher concentration to an area of lower concentration.

2. What does the diffusing capacity test provide info about?
The transfer of gas between the alveoli and the pulmonary capillary blood.

3. What three general techniques can be performed in the DLCO measurement?
Steady state, rebreathing, and single breath.

4. What is the most common DLCO technique?
Single breath technique

5. What are the indications for DLCO testing?
(1) Evaluate or follow the progress of parenchymal lung disease, (2) evaluate pulmonary involvement in systemic diseases, (3) determine progression and differentiate between different types of obstructive lung disease, (4) evaluate cardiovascular diseases, (5) quantify disability associated with interstitial lung disease, and (6) evaluate pulmonary hemorrhage or polycythemia.

6. What does DLCO measure?
The transfer of CO across the alveolar-capillary membrane, which shows small differences between inspired and expired CO gas. It also measures how much CO crosses the membrane.

7. What is DLCO measured in conjunction with?
Spirometry and lung volumes

8. What units is DLCO measured in?

9. What does CO combine with, and how many times is it more readily available than O2?
It combines with Hb and is 210 times more readily available than O2.

10. What is the primary limiting factor to the diffusion of CO if the hemoglobin and ventilatory function are normal?
The status of the alveolar-capillary membrane.

11. What is DLCO directly related to?
Alveolar lung volume

12. What does the DLCO gas mixture consist of?
0.3% CO, 0.3% inert gas (He, Methane, or Neon), 21% Oxygen, and balance nitrogen.

13. The two major gases involved in lung diffusion must move through what two barriers?
The alveolar-capillary (A-C) membrane, and the blood plasma-red blood cell barrier.

14. The rate of diffusion across the primary liquid barriers is limited by what?
The surface area for diffusion, the distance the gas molecules must travel, the solubility coefficient of the gases in a liquid, the partial pressure difference between air and blood for each gas, and the density of each gas.

15. Why is CO more suitable for measuring diffusing capacity than other gases?
It has a great affinity for Hb, it’s soluble in blood, and its concentration in venous blood is insignificant.

16. The measurement of DLCO involves what?
The rate of consumption (uptake) of CO by the blood from the alveoli.

17. Diffusion across the A-C membrane depends on what?
The difference between the gas tension (or partial pressure) in the alveolus and in the plasma, the surface area available for diffusion, the distance the gas molecules must travel, and tissue characteristics.

18. What conditions can lower the DLCO value?
Respiratory muscle weakness, deformity preventing maximal inflation, reduced Hb, pulmonary embolus, increased CO or inspired O2 concentration, lung resection, emphysema, and interstitial lung disease.

19. What conditions can increase the DLCO value?
Increased Hb (polycythemia), decreased intrathoracic pressure, exercise, asthma, and the supine position.

20. What causes increased DLCO values?
Increased pulmonary capillary blood volume, exercise, left-to-right intracardiac shunts, left heart failure, supine position, polycythemia, and asthma.

21. What causes decreased DLCO values?
Small lung volume (e.g., lung resection), pulmonary fibrosis, emphysema, pulmonary vascular and cardiovascular diseases, anemia, renal failure, and marijuana and/or cigarette smoking.

22. How many times should the single breath-hold technique be repeated?

23. How long should the delay be between repeated maneuvers for the single breath-hold technique?
4-5 minutes

24. How long should the breath-hold time be within for a single breath-hold maneuver?
8-12 seconds

25. What are the acceptable criteria for exhalation during DLCO single breath-hold technique?
It should be rapid but not forced and 4 seconds or less.

26. How long should the interval be between repeated DLCO single-breath maneuvers?
4 minutes

27. How does poor inspiratory effort affect the DLCO results?
If it’s less than 85% of VC, it will decrease the DLCO.

28. What technique should you use if the patient can’t hold their breath?
Rebreathing technique

29. What factors affect the DLCO?
Restrictive lung disease, obstructive lung disease, inhalation of toxic gas or organic agents, increased HR, CHR, pulmonary hypertension, radiation therapy, COHb, decreased or increased Hb and hematocrit, altitude above sea level, body position, and obesity.

30. How do restrictive lung diseases affect the DLCO?
They cause low DLCO results because of decreased diffusion.

31. How does the inhalation of toxic or organic agents affect the DLCO?
Causes decreased results

32. How does an increased HR affect the DLCO?
Causes increased results

33. How do CHF and pulmonary edema affect the DLCO?
They cause increased results.

34. How does pulmonary hypertension affect the DLCO?
Causes increased results

35. How does radiation therapy affect the DLCO?
Causes decreased results

36. How does COHb affect the DLCO?
Causes decreased results

37. How does a decreased Hb and hematocrit affect the DLCO?
They cause decreased results

38. How does an increased Hb and hematocrit affect the DLCO?
They cause increased results.

39. How does altitude above sea level affect the DLCO?
Causes increased results

40. How does the patient’s body position affect the DLCO?
Laying down causes increased results

41. How does obesity affect the DLCO?
Causes increased results

42. What changes as you increase altitude?
The PaO2 decreases.

43. What does DLCO measure?
It measures the extent to which oxygen passes from the alveoli into the blood.

44. What is the total amount of blood in the pulmonary capillaries?
60-140 mL

45. Fick’s law for diffusion is proportional to what?
Pressure gradient and surface area

46. Fick’s law for diffusion is inversely proportional to what?

47. What will an obstructive spirometry test show?
Low FEV1/FVC ratio

48. What does it mean if the DLCO and FEV1/FVC ratio are both low?
It could mean COPD or emphysema.

49. What does it mean if the DLCO is normal or high and the FEV1/FVC ratio is low?
It could mean asthma.

50. What are the normal values for FEV1, TLC, and DLCO?
80-120% of predicted

51. If spirometry tests are normal, but the DLCO is low, what is this indicative of?
ILD, anemia, or pulmonary vascular disease

52. If a spirometry test indicates a restrictive lung disease and the DLCO is low, what does this mean?
It could mean that the patient has an interstitial lung disease.

53. What diseases can cause a decreased DLCO?
Restrictive patterns, inhalation of toxic gases, radiation therapy, lung tumors, emphysema, chronic bronchitis, and asthma.

54. What is DLCO directly affected by?
Hemoglobin, carboxyhemoglobin, alveolar PCO2, pulmonary capillary blood volume, high altitude, and poor inspiratory effort.

55. How does hemoglobin affect the DLCO?
Low Hb means a decreased DLCO and high Hb means an increased DLCO.

56. How does having carboxyhemoglobin affect the DLCO?
COHb decreases the DLCO

57. How does having an increased PaCO2 affect the DLCO?
An increased PaCO2 increases the DLCO due to hypoventilation.

58. How does having an increased pulmonary capillary blood volume affect the DLCO?
Increased blood volume increases the DLCO.

59. How does a high altitude affect the DLCO?
A high altitude increases the DLCO because there is less oxygen.

60. Is DLCO the same as diffusion capacity?
Yes, DLCO is often referred to as the diffusion capacity for carbon monoxide in the lungs.

Final Thoughts

The diffusing capacity of the lungs for carbon monoxide (DLCO) plays a pivotal role in the assessment and management of respiratory disorders.

It provides valuable insights into the efficiency of gas exchange in the lungs, helping clinicians diagnose and monitor a wide range of pulmonary conditions.

Whether evaluating the impact of smoking on lung function, diagnosing interstitial lung disease, or assessing the progression of chronic respiratory illnesses, DLCO remains a critical parameter in respiratory care.

John Landry, BS, RRT

Written by:

John Landry, BS, RRT

John Landry is a registered respiratory therapist from Memphis, TN, and has a bachelor's degree in kinesiology. He enjoys using evidence-based research to help others breathe easier and live a healthier life.


  • Faarc, Mottram Carl Ba Rrt Rpft. Ruppel’s Manual of Pulmonary Function Testing. 11th ed., Mosby, 2017.
  • Faarc, Kacmarek Robert PhD Rrt, et al. Egan’s Fundamentals of Respiratory Care. 12th ed., Mosby, 2020.
  • Modi P, Cascella M. Diffusing Capacity Of The Lungs For Carbon Monoxide. [Updated 2023 Mar 13]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023.

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