DLCO Test Diffusing Capacity of the Lungs for Carbon Monoxide

Diffusing Capacity of the Lungs for Carbon Monoxide (DLCO)

by | Updated: Aug 22, 2023

The DLCO test refers to the diffusing capacity for carbon monoxide in the lungs. It’s a type of pulmonary function test (PFT) that helps to assess how well gas is exchanged between the lungs and the bloodstream.

In this article, we will discuss how and why the DLCO test is performed, and what the results mean. We also included helpful practice questions on this topic.

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What is a DLCO Test?

DLCO stands for the diffusing capacity for carbon monoxide in the lungs. A DLCO test is performed to assess the extent to which carbon monoxide can diffuse from atmospheric air into the bloodstream.

During the test, carbon monoxide is used because it has a high affinity for hemoglobin. This helps medical professionals determine how well oxygen can diffuse through the lungs, into arterial blood, and be transported throughout the body.


A DLCO test may be ordered for patients with:

  • Emphysema
  • Pulmonary fibrosis
  • Pulmonary edema
  • Acute respiratory distress syndrome (ARDS)

Patients with such conditions may show poor diffusion during the DLCO test, which correlates with poor diffusion of oxygen.

How is the DLCO Test Performed?

During the DLCO test, the patient inhales a low concentration of carbon monoxide (CO) and tracer gas. Then, the patient performs a breath-hold maneuver for 10 seconds.

This allows physicians and respiratory therapists to assess the patient’s diffusion capacity in the lungs. The DLCO test is often performed to assess pneumoconiosis, evaluate obstructive diseases, and monitor changes in the patient’s lung function.

DLCO Test Practice Questions:

1. What is diffusion?
It is 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?
Transfer of gas between the alveoli and the pulmonary capillary blood.

3. What three general techniques can be performed in the measurement of DLCO (Diffusion Capacity of the Lung for Carbon Monoxide)?
Steady state, Rebreathing, and Single Breath.

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

5. What are the 6 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. It 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?
mL/min/mm Hg.

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

10. What is the primary limiting factor to diffusion of CO if the hemoglobin and ventilatory function are normal?
Status of alveolar capillary membranes.

11. Other than the primary limited factor to the diffusion of CO, what other factors affect diffusing capacity?
Alterations in Hb and the capillary blood volume.

12. What is DLCO directly related to?
Alveolar Lung Volume (VA).

13. What are the 4 techniques for DLCO?
Single breath or breath hold, Rebreathing technique, Slow exhalation single-breath intra-breath method, and Stead State Method.

14. How is the Single Breath-Hold technique performed?
Normal resting breath for several breaths. Full exhalation, RAPID inhalation to VC. At VC, hold breath 10 seconds at TLC. At the end of breath hold, passively exhale until empty.

15. What does the DLCO gas mixture consist of?
0.3% CO, 0.3% Inert Gas (He, Methane or Neon), 21% Oxygen, and Balance Nitrogen.

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16. The two major gases involved in lung diffusion (O2 and CO2) must move through what two barriers?
The alveolar-capillary (A-C) membrane, and the blood plasma-red blood cell barrier.

17. The rate of diffusion across these primarily liquid barriers is limited by?
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 (gradient) between air and blood for each gas, the density of each gas.

18. Why is CO is more suitable to measure diffusing capacity than other gases?
It has a great affinity for Hb (210 times that of O2), It is soluble in blood, its concentration in venous blood is insignificant.

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

20. 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.

21. What are the conditions that can lower DLCO value?
Respiratory muscle weakness or deformity preventing maximal inflation, Reduced Hb, Pulmonary emboli, Increased CO or inspired O2 concentration, Lung resection, Emphysema, and Interstitial lung disease.

22. What are the conditions that can increase DLCO value?
Increased Hb (polycythemia), decreased intrathoracic pressure (resistance breathing), exercise, asthma, and supine position.

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

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

25. How many times should the Single Breath-Hold Technique be repeated?
What is the max amount of times it should be repeated and why?
Minimum of twice. Maximum of 5 times because carboxyhemoglobin increases from inhalation of test gas.

26. How much of a delay should there be for Single Breath-hold Technique between repeated maneuvers and why?
4-5 minutes to allow washout of the tracer gas from the lungs.

27. What is the criteria for inspiration from RV to TLC for Single breath-hold Technique?
Must be rapid and should occur <2.5 seconds in healthy patients and <4 seconds in patients with obstructive lung disease.

28. The volume inspired should be at least what?
It should be at least 85% of the recorded best vital capacity. 

29. How long should breath hold time be within for single breath-hold maneuver? And there should be no evidence of what?
8-12 seconds. No evidence of leaks, Valsalva or Muller maneuver.

30. What is the acceptable criteria for alveolar sample volume in Single Breath-hold?

31. What is the acceptability criteria for exhalation during DLCO single breath-hold technique?
Rapid but not forced, 4 seconds or less, and appropriate clearance of gas mixture.

32. How long should the interval be between repeated DLCO Single-Breath Maneuver?
4 minutes.

33. What acceptable trials are reported for DLCO Single-Breath hold?
Average of 2 best or more.

34. How does poor inspiratory effort affect the DLCO?
If it’s less than 85 % of VC then it will DECREASE the DLCO.

35. What is the Valsalva maneuver? How does it affect DLCO?
The patient fails to relax during the breath hold, so the results in two or more DLCO measurements are widely varied. Positive Intrathoracic Pressure. Reduces pulmonary capillary blood volume and produces falsely low DLCO.

36. What is the muller maneuver? How does it affect DLCO?
Excessive negative intrathoracic pressure. Increases pulmonary capillary blood volume and may falsely increase DLCO.

37. What technique should you use if the patient can’t hold their breath?
Rebreathing Technique and DLCOrb.

38. How is the DLCOrb done? What is measured? What can this method be used during?
The patient rebreathes from a reservoir containing gas mixture for 30-60 seconds at a rate of 30 breaths per minute. The final CO, tracer and O2 concentrations in the reservoir are measured after interval. This method can be used during exercise.

39. What does DLCOib stand for?
Slow Exhalation Single-Breath Intrabreath.

40. How is DLCOib done? What monitors CO and CH4 gas concentrations? What can it be used during?
The patient inspires a vital capacity (IVC) of the gas mixture. Slowly and evenly exhales at approximately 0.5L/sec from TLC to RV. A rapidly responding infrared analyzer monitors CO an CH4 gas concentrations. It can be used during exercise.

41. How many measurements do you need for DLCOib?

42. How is the DLCOss done?
Patient breaths a gas mixture which contains 0.1%-0.2% of CO for 5-6 minutes. The last 2 minutes of exhaled CO, CO2 and O2 gas is collected.

43. What are the advantages of DLCOsb?
Easy calculations, simple and fast. Highly standardized and automated. Minimal COHb back-pressure effort.

44. What are advantages of DLCOrb?
Less sensitive to VA than DLCOsb, less sensitive to V/Q abnormalities, can be used with NO to measure DLno.

45. What are advantages of DLCOib?
Breath holding not required. Can be used during exercise.

46. What factors affect DLCO?
Restrictive Lung disease, Obstructive lung disease (severe), Inhalation of toxic gas or organic agents, Increased HR, CHR, Pulmonary Hypertension, Radiation Therapy, COHb, Decreased or Increased Hb and Hematocrit, Altitude about sea level, Body Position, and Obesity.

47. How does restrictive lung disease affect DLCO?
Low results because of decreased diffusion.

48. How does Obstructive Lung Disease affect DLCO?
Severe – Decreased DLCO and generally normal.

49. How does inhalation of toxic or organic agents affect DLCO?

50. How does increased HR affect DLCO?

51. How does CHF and Pulmonary Edema affect DLCO?

52. How does Pulmonary Hypertension affect DLCO?

53. How does radiation therapy affect DLCO?

54. How does COHb affect DLCO?

55. How does a decreased Hb and hematocrit affect DLCO?

56. How does increased Hb and hematocrit affect DLCO?

57. How does altitude above sea level affect DLCO?

58. How does body position affect DLCO?
By laying down, it increases.

59. How does obesity affect DLCO?

60. What happens to diffusion during exercise?
HR and CO are increased so there is decreased time in the capillary, but the capillaries dilate to 3X normal size which allows for more partial pressure to equilibrate w/in a shorter time.

61. What happens to diffusion in emphysema or fibrosis?
Emphysema (decrease SA) or fibrosis (increased thickness), decreased volume so gas would not make it across the blood-gas barrier in a timely fashion thus the 0.75 is not adequate time for oxygen equilibration.

62. What changes as you increase altitude?
Lower alveolar PO2.

63. What does DLCO measure?
The extent to which oxygen passes from air sacs of lungs into the blood.

64. What is the total amount of blood in the pulmonary capillaries?
60-140 ml.

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

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

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

68. If the DLCO is low and FEV/FVC ratio is low, what is this indicative of?
COPD and emphysema.

69. What is the indication if the DLCO is normal or high and the FEV/FVC ratio is low?

70. What is the normal values of FEV1, TLC, DLCO?
80-120% of predicted.

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

72. If the spirometry test is indicative of a restrictive lung disease and the DLCO is low, what is this indicative of?
Interstitial lung disease.

73. If the spirometry test is indicative of a restrictive lung disease, but the DLCO is normal, what is this indicative of?
Neuromuscular or chest wall etiology.

74. What is the washout volume the patient must exhale before sample being analyzed?
0.75-1.0 L of washout.

75. What is the alveolar sample volume that must be analyzed?
0.50-1.0 L.

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76. Why is helium an inert gas used in the DLcoSb test?
Helium (He) is used to determine the concentration of CO in the lungs when its mixed with the RV, therefore, CHANGE in helium concentration reflects dilution by gas in the lungs at RV
this change determined CO concentration when it hits the lungs.

77. What is the Average DLcoSb value?
25 ml CO / min / mm hg.

78. What are some of the patient pretest preparations they should do or refrain from doing before DlcoSb test and why?
STOP smoking at least 1 hour prior to tests. STOP consuming alcohol 4 hours prior to the test. STOP strenuous activity like exercising. STOP eating at least 2 hours priors (decreases blood volume). It should sit 5 minutes before testing. Also, should DISCONTINUE supplemental 02 5 minutes before the test.

79. What diseases can cause a decreased DLCO?
Restrictive patterns, (thickening & scarring of tissue), Inhalation of toxic gases, radiation therapy, lung tumors, Emphysema, Chronic Bronchitis (may or may not be decreased) used to differentiate between emphysema, and Asthma (may or may NOT be decreased).

80. What is DLCO directly affected by?
Hemoglobin, Carboxyhemoglobin, Alveolar PCO2, Pulmonary Capillary Blood Volume, High altitude, and poor inspiratory effort.

81. How does hemoglobin affect the DLCO?
A low Hb means a reduced DLCO, and a high Hb means an elevated DLCO.

82. How does having carboxyhemoglobin affect the DLCO?
High COHb reduces DLCO (can’t carry anymore CO).

83. How does having an elevated PaO2 affect the DLCO?
Elevated PCO2 elevates DLCO (hypoventilation).

84. How does having an increased pulmonary capillary blood volume affect the DLCO?
Increased blood volume elevates the DLCO.

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

Final Thoughts

The DLCO test is a type of pulmonary function test (PFT) that helps to assess how well gas exchange is occurring in the lungs. It is indicated for patients with a variety of pulmonary conditions.

If you found this guide to be helpful, we have a similar article that covers the basics of pulmonary function testing that I think you’ll find useful. Thanks for reading and, as always, breathe easy, my friend.

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.


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