When is DLCO Testing Indicated?
DLCO testing should be performed when evaluating patients with unexplained dyspnea, suspected interstitial lung disease, pulmonary vascular disease, emphysema, or when spirometry and lung volumes alone cannot explain the clinical picture. 1
Primary Diagnostic Indications
Unexplained Dyspnea with Normal or Abnormal Spirometry
- When spirometry and lung volumes are normal but dyspnea persists, DLCO can identify early parenchymal disease, pulmonary vascular disorders, early interstitial lung disease (ILD), or early emphysema. 1, 2
- More than 40% of patients with normal FEV1 (>80% predicted) may have reduced DLCO (<80% predicted), making this test critical when spirometry appears reassuring but symptoms persist. 2
- The correlation between FEV1 and DLCO is consistently poor, so reduced DLCO can occur even with otherwise normal spirometry values. 2
Differentiating Causes of Restrictive Patterns
- In the presence of restriction, a normal DLCO suggests chest wall or neuromuscular disorders, whereas a decreased DLCO indicates interstitial lung diseases. 1
- Patients with ILD, sarcoidosis, and pulmonary fibrosis typically demonstrate low DLCO. 1
- A restrictive pattern with normal or mildly reduced DLCO accompanied by elevated KCO (DLCO/VA) directs attention to extrapulmonary conditions such as respiratory muscle weakness, pleural disease, or rib cage abnormalities. 1
Evaluating Obstructive Lung Disease
- In the presence of airflow obstruction, a decreased DLCO strongly suggests emphysema rather than pure airway disease. 1
- DLCO helps distinguish emphysema-dominant COPD from other obstructive patterns and correlates with disease severity, exacerbation risk, and mortality. 3
- Airway obstruction with low DLCO is also seen in lymphangioleiomyomatosis. 1
Specific Clinical Scenarios
Pulmonary Vascular Disease
- Low DLCO is characteristic of chronic pulmonary embolism, primary pulmonary hypertension, and other pulmonary vascular diseases, with or without restriction of lung volumes. 1
- DLCO measurement assists in identifying pulmonary hypertension in COPD patients. 3
Monitoring Drug Toxicity
- DLCO adjustments for hemoglobin and carboxyhaemoglobin are particularly important when monitoring patients for possible drug toxicity, especially where hemoglobin is subject to large shifts (e.g., chemotherapy for cancer). 1
- Serial DLCO measurements help detect bleomycin-induced lung injury and other medication-related pulmonary complications. 4
Post-COVID-19 Assessment
- Consider routine DLCO testing in all patients with severe and critical COVID-19 at 3 months from diagnosis, regardless of symptoms. 1
- DLCO impairment has been found in up to 80% of patients at ICU discharge and 50-70% at 3-month follow-up after critical disease. 1
- Even among patients with mild-to-moderate COVID-19, abnormal DLCO was demonstrated in approximately 10%. 1
Conditions Associated with Elevated DLCO
While less common, high DLCO is associated with asthma, obesity, and intrapulmonary hemorrhage, which can provide diagnostic clues in the appropriate clinical context. 1
Important Interpretation Caveats
Hemoglobin Adjustment
- Always adjust DLCO for hemoglobin levels, as anemia can cause falsely low DLCO values that do not reflect true gas exchange abnormalities. 1, 2
Volume Considerations
- The relationship between DLCO and lung volume is not linear, so DLCO/VA or DLCO/TLC do not provide an appropriate way to normalize DLCO for lung volume. 1, 2
- A loss of DLCO much less than loss of volume (low DLCO but high DLCO/VA) suggests extraparenchymal abnormality such as pneumonectomy or chest wall restriction. 1
- A loss of DLCO much greater than loss of volume (low DLCO and low DLCO/VA) suggests parenchymal abnormalities. 1