Can pulmonary fibrosis show a decrease in Diffusing Capacity of the Lung for Carbon Monoxide (DLCO) on Pulmonary Function Tests (PFTs)?

Pulmonary Fibrosis and Decreased DLCO on PFTs

Yes, pulmonary fibrosis characteristically shows a decreased DLCO on pulmonary function testing, often representing the earliest and most sensitive abnormality detectable. 1

Characteristic PFT Pattern in Pulmonary Fibrosis

Decreased DLCO is a hallmark finding in pulmonary fibrosis and typically appears before other pulmonary function abnormalities become evident. 1, 2

• The European Respiratory Society confirms that DLCO reduction is the earliest and most sensitive PFT abnormality in interstitial lung disease (ILD), frequently preceding changes in lung volumes 2
• Patients with ILD, sarcoidosis, and pulmonary fibrosis usually demonstrate a low DLCO 1
• In idiopathic pulmonary fibrosis (IPF) specifically, mean DLCO at diagnosis is approximately 60% of predicted 3

Severity Grading of DLCO Reduction

The degree of DLCO impairment in pulmonary fibrosis can be categorized as: 1

• Mild: >60% and <lower limit of normal
• Moderate: 40-60% of predicted
• Severe: <40% of predicted

A DLCO <45% of predicted is associated with poor outcomes and increased mortality. 1

Associated PFT Findings

Beyond isolated DLCO reduction, pulmonary fibrosis demonstrates a characteristic restrictive pattern: 1, 3

• Restrictive ventilatory impairment confirmed by reduced total lung capacity (TLC), with proportional reduction of all lung compartments including TLC and residual volume 1
• At initial diagnosis of IPF, 73% of patients have a restrictive pattern with mean TLC of 72% of predicted 3
• Mean forced vital capacity (FVC) is approximately 71% of predicted, though 44% of patients have FVC <95th percentile at diagnosis 3
• Important caveat: One in four patients with IPF have normal TLC and more than half have normal FVC during initial evaluation, making DLCO reduction particularly valuable for early detection 3

Pathophysiologic Basis

The severe DLCO reduction in pulmonary fibrosis reflects: 1, 4

• Loss of normally functioning alveolar-capillary units 4
• Reduced effective surface area available for gas exchange 4
• Thickening of the alveolar-capillary membrane from fibrotic tissue 1
• Destruction of pulmonary capillary bed 1

As restriction severity increases, DLCO progressively decreases (74% of predicted in mild restriction to 39% in severe restriction), while DLCO/VA (DLCO corrected for alveolar volume) paradoxically remains within normal limits regardless of restriction severity 3

Diagnostic Interpretation Algorithm

When encountering decreased DLCO on PFTs: 1, 2

1. If restriction is present with decreased DLCO: Strongly suggests ILD/pulmonary fibrosis rather than chest wall or neuromuscular disorders (which show normal DLCO) 1

2. If spirometry and lung volumes are normal with decreased DLCO: Consider early ILD, early emphysema, pulmonary vascular disorders, or anemia 1

3. Next diagnostic steps for suspected pulmonary fibrosis: 2

   • High-resolution CT chest (primary imaging modality) 2
   • Arterial blood gases and pulse oximetry at rest and with exertion 2
   • Transthoracic echocardiography to evaluate for pulmonary hypertension 2

Prognostic Significance

DLCO serves as a critical prognostic marker in pulmonary fibrosis: 1, 5

• A threshold of 10% decrease in FVC or 5% decrease with corroborative 15% drop in DLCO predicts survival 1
• Changes in DLCO are highly predictive of long-term outcomes, especially over the next 2 years 1
• Five-year survival is 70.4% in preserved-DLCO group (>40% predicted) versus 20.0% in low-DLCO group (<40% predicted) 5
• The relative risk of mortality within 5 years is 2.70 (95% CI 1.46-4.99) in the low-DLCO group 5

Monitoring Recommendations

Serial PFTs every 3-6 months for at least 1 year are required to establish disease trajectory and determine treatment necessity 2

• DLCO measurements should be adjusted for hemoglobin and carboxyhaemoglobin levels, particularly when monitoring for drug toxicity or in patients undergoing chemotherapy 1
• Exercise testing may provide additional diagnostic value, as DLCO shows better correlation with blood gas abnormalities during exercise than at rest 6