What is DLCO (Diffusing Capacity of the Lung for Carbon Monoxide) in a Pulmonary Function Test (PFT)?

DLCO in Pulmonary Function Testing

DLCO (Diffusing Capacity of the Lung for Carbon Monoxide) is a critical measurement in pulmonary function testing that assesses the ability of the lungs to transfer gas from inhaled air to the bloodstream, providing an integrated picture of gas exchange efficiency in the lungs. 1, 2

Definition and Measurement

• DLCO measures the uptake of carbon monoxide (CO) from the alveolar space into pulmonary capillary blood, following the same pathway as oxygen
• Also known as transfer factor (TL,CO) in some countries 1
• Measured through a simple breathing maneuver where the patient:
  • Breathes quietly
  • Inhales a test gas containing a small amount of CO
  • Holds breath for 10 seconds
  • Exhales 3

Clinical Significance

Interpretation Guidelines

DLCO results are typically reported as:

• Absolute value (mL CO/min/mmHg)
• Percentage of predicted value based on reference values
• Severity classification according to European Respiratory Society 1:
  • Mild: >60% predicted and below lower limit of normal (LLN)
  • Moderate: 40-60% predicted
  • Severe: <40% predicted

Clinical Patterns and Diagnostic Value

DLCO results should be interpreted in conjunction with spirometry and lung volumes 1:

• Normal spirometry + decreased DLCO: May indicate:

  • Anemia
  • Pulmonary vascular disorders
  • Early interstitial lung disease
  • Early emphysema 1

• Restrictive pattern + normal DLCO: May indicate:

  • Chest wall disorders
  • Neuromuscular disorders 1

• Restrictive pattern + decreased DLCO: May indicate:

  • Interstitial lung diseases 1

• Obstructive pattern + decreased DLCO: May indicate:

  • Emphysema
  • Lymphangioleiomyomatosis 1

• Elevated DLCO: May indicate:

  • Asthma
  • Obesity
  • Intrapulmonary hemorrhage
  • Polycythemia (less common) 1, 4

Related Measurements

KCO (Transfer Coefficient)

• KCO = DLCO/VA (diffusing capacity divided by alveolar volume)
• Also known as DL,CO/VA 1
• Provides information about gas exchange efficiency per unit of lung volume
• Helps distinguish between different pathophysiological processes:
  • Low DLCO + High KCO: May indicate inability to achieve full lung distension despite normal alveolar-capillary membrane (e.g., respiratory muscle weakness, pleural disease) 5
  • Low DLCO + Low KCO: May indicate reduced pulmonary capillary blood volume or reduced hemoglobin available for CO binding 5

Important Considerations

• Adjustments for hemoglobin and carboxyhemoglobin are important, especially when monitoring patients for drug toxicity or those with significant hemoglobin shifts 1
• DLCO measurements can be affected by:
  • Altitude
  • Testing technique
  • Patient factors (smoking status, recent exercise) 5
• A DLCO below 40% predicted or a decline of more than 4 units is associated with increased morbidity and mortality 3

Clinical Applications

• Second most important pulmonary function test after spirometry 3
• Helps differentiate between:
  • Emphysema vs. asthma in smokers with airway obstruction
  • Interstitial lung disease vs. chest wall restriction in patients with restrictive patterns
  • Various causes of unexplained dyspnea 3
• Provides objective assessment of disease severity and prognosis 3
• Guides clinical decision-making regarding anesthesia risk and management 5

DLCO testing provides valuable information that complements other pulmonary function tests and helps clinicians make important diagnostic and treatment decisions for patients with suspected or known respiratory disorders.