What does a Diffusing Capacity of the Lung for Carbon Monoxide (DLCO) of 69% predicted and a Vital Capacity (VA) of 59% predicted on a Pulmonary Function Test (PFT) indicate?

Interpretation of DLCO 69% and VA 59% Predicted

These values indicate a mild reduction in diffusing capacity (DLCO 69% predicted) with a moderately-to-severely reduced alveolar volume (VA 59% predicted), suggesting either restrictive lung disease with parenchymal involvement or incomplete lung inflation during the test. 1

Severity Classification

DLCO Severity:

• Your DLCO of 69% predicted falls into the mild reduction category (>60% and <LLN) according to European Respiratory Society guidelines 1
• This is above the threshold of 60% that predicts increased respiratory complications in surgical candidates 1

VA Interpretation:

• VA of 59% predicted represents a moderate-to-severe reduction in the lung volume sampled during the single-breath test 1
• This low VA can occur from true restrictive lung disease, incomplete inspiration during testing, or air trapping that prevents gas distribution 1

Critical Diagnostic Consideration: The DLCO/VA Relationship

The pattern of DLCO reduction being proportionally less than VA reduction is diagnostically significant:

• When DLCO is reduced but DLCO/VA (KCO) is preserved or elevated, this suggests extraparenchymal restriction (chest wall disease, neuromuscular disorders, obesity, pleural disease, or pneumonectomy) rather than intrinsic lung parenchymal disease 1

• When both DLCO and DLCO/VA are proportionally reduced, this suggests parenchymal lung disease (interstitial lung disease, emphysema, pulmonary vascular disease) 1

• Calculate DLCO/VA: With your values, if DLCO/VA is normal or high, the lung tissue itself may be healthy but mechanically restricted 2

Essential Next Steps for Interpretation

You must obtain complete spirometry and lung volumes to properly interpret these values:

1. Check FEV1/FVC ratio to distinguish obstruction from restriction 1

   • If FEV1/FVC is normal (>LLN) and TLC is reduced (<LLN), this confirms true restrictive physiology 1
   • If FEV1/FVC is reduced, consider obstructive disease (emphysema if DLCO low, asthma if DLCO normal) 1

2. Measure TLC by body plethysmography - VA from single-breath DLCO systematically underestimates true TLC, especially with airflow obstruction, and cannot be used to diagnose restriction 1

3. Review test quality - A low VA may reflect poor test technique (incomplete inspiration, breath-holding <10 seconds) rather than true pathology 1

Differential Diagnosis Based on Pattern

If spirometry shows restriction (low TLC, normal FEV1/FVC):

• Normal DLCO/VA: Chest wall restriction, neuromuscular disease, obesity, pleural disease 1, 2
• Low DLCO/VA: Interstitial lung disease, pulmonary fibrosis, sarcoidosis 1, 3

If spirometry shows obstruction (low FEV1/FVC):

• Low DLCO: Emphysema phenotype of COPD 1, 4
• Normal DLCO: Chronic asthma more likely 4

If spirometry and lung volumes are normal:

• Isolated low DLCO: Consider pulmonary vascular disease, early interstitial lung disease, early emphysema, or anemia 1, 5, 4

Clinical Significance and Prognosis

• DLCO <60% predicted is associated with increased surgical risk (25% mortality, 40% pulmonary morbidity in one series), but your value of 69% is above this threshold 1

• DLCO <40% predicted is associated with significantly increased morbidity, mortality, and poor long-term survival 1, 4

• A decline of >4 units or >10% in DLCO over time indicates disease progression and warrants treatment intensification 4, 3

Common Pitfalls to Avoid

• Do not diagnose restriction based on VA alone - VA from DLCO testing underestimates TLC and requires confirmation with body plethysmography 1

• Always correct DLCO for hemoglobin and carboxyhemoglobin - anemia falsely lowers DLCO, and failure to correct can lead to misdiagnosis 1

• Examine DLCO and VA separately rather than relying solely on DLCO/VA, as their relationship provides pathophysiologic information 1

• Consider test quality - poor effort, incomplete inspiration, or inadequate breath-hold can artificially lower both DLCO and VA 1