DLCO Interpretation and HRCT Timing in ILD Screening
In patients being screened for interstitial lung disease, obtain HRCT when DLCO is <80% predicted, as this threshold provides 83.6% sensitivity for detecting ILD and should trigger advanced imaging regardless of symptom presence. 1
Severity Grading of DLCO Reduction
Use the lower 5th percentile (LLN) as the threshold for abnormal DLCO, then grade severity as follows: 2
- Mild: 60% to <LLN predicted
- Moderate: 40-60% predicted
- Severe: <40% predicted
Critical Pre-Interpretation Adjustments
Always correct DLCO for hemoglobin before interpretation, particularly in connective tissue disease patients where anemia is common and falsely lowers DLCO without reflecting true gas exchange abnormalities. 2, 3, 4 Carboxyhemoglobin adjustment is equally important when monitoring for drug toxicity or in patients with variable exposures. 2, 4
Algorithmic Approach to HRCT Decision
Step 1: Initial DLCO Screening
- DLCO <80% predicted: Proceed to chest radiography 1
- DLCO ≥80% predicted: Consider clinical context; 25% of CTD-ILD patients are asymptomatic with reduced DLCO 5
Step 2: Combined Assessment
The combination of reduced DLCO (<80%) plus chest X-ray abnormality yields 95.2% sensitivity for ILD detection. 1 If either test is abnormal, proceed to HRCT. 1
Step 3: HRCT Indications
Obtain HRCT when: 1
- DLCO <80% predicted (even with normal chest X-ray)
- Any chest X-ray abnormality suggesting ILD
- Clinical suspicion remains high despite normal initial tests
The stepwise combination of DLCO, chest X-ray, and HRCT achieves 95.2% sensitivity and 77.4% specificity for ILD detection. 1
Pattern Recognition for Differential Diagnosis
Interpret DLCO alongside spirometry and lung volumes: 2
Normal Spirometry + Normal Volumes + Low DLCO
Suggests: 2
- Early ILD (most relevant for your screening context)
- Pulmonary vascular disorders
- Early emphysema
- Anemia (check hemoglobin correction)
Restrictive Pattern + Low DLCO
Strongly suggests parenchymal ILD (sarcoidosis, pulmonary fibrosis, CTD-ILD). 2, 3 This pattern distinguishes true ILD from chest wall or neuromuscular disorders, which maintain normal DLCO despite restriction. 2, 3
Obstructive Pattern + Low DLCO
Primarily indicates emphysema, though also seen in lymphangioleiomyomatosis. 2
Special Considerations for CTD-ILD Screening
One-quarter of CTD patients with ILD at initial diagnosis are asymptomatic, yet still demonstrate reduced DLCO (mean 60.0% vs 49.7% in symptomatic patients). 5 This underscores the critical importance of objective DLCO screening rather than relying on symptoms alone. 5
- DLCO is more sensitive than chest radiography for early ILD detection
- Mean DLCO values in CTD-ILD range 76-81% predicted across disease subtypes
- DLCO <80% warrants HRCT even in asymptomatic patients
Prognostic Significance Beyond Diagnosis
DLCO <45% predicted is associated with poor outcomes in pulmonary arterial hypertension, though this threshold applies specifically to that population. 2 More broadly, reduced DLCO predicts cardiopulmonary complications and mortality even when FEV1 remains normal, making it a critical risk stratification tool. 3
Common Pitfalls to Avoid
Do not use DLCO/VA (KCO) for primary interpretation, as the relationship between DLCO and lung volume is non-linear, making this ratio an imperfect normalization method. 3 However, when DLCO loss greatly exceeds volume loss (low DLCO with low DLCO/VA), this pattern strongly suggests parenchymal abnormalities rather than extraparenchymal restriction. 2, 3
Do not delay HRCT in high-risk populations (systemic sclerosis, myositis, occupational exposures) when DLCO is reduced, as early ILD detection is associated with improved outcomes and 25% of patients lack pulmonary symptoms. 5