What Does Mild DLCO Mean in PFT?
A mild DLCO reduction means your diffusing capacity is greater than 60% of predicted but below the lower limit of normal (LLN), indicating early or less severe impairment in the lung's ability to transfer gas from the alveoli to the bloodstream. 1
Severity Classification
According to the European Respiratory Society guidelines, DLCO reductions are graded as follows 1:
- Mild: >60% predicted AND <LLN
- Moderate: 40-60% predicted
- Severe: <40% predicted
Clinical Interpretation Depends on Context
The meaning of a mild DLCO reduction must be interpreted alongside spirometry and lung volumes to narrow the differential diagnosis 1:
With Normal Spirometry and Lung Volumes
A mildly reduced DLCO suggests 1:
- Anemia (check hemoglobin - DLCO adjustments for hemoglobin are critical)
- Early emphysema (particularly in smokers)
- Pulmonary vascular disorders (early pulmonary hypertension, chronic pulmonary embolism)
- Early interstitial lung disease (ILD)
With Airflow Obstruction (Low FEV1/FVC)
A mildly reduced DLCO strongly suggests 1, 2:
- Emphysema phenotype of COPD (in smokers with post-bronchodilator obstruction, low DLCO greatly increases probability of emphysema)
- Normal DLCO in this context makes chronic asthma more likely
With Restriction (Low FVC, Normal FEV1/FVC)
- Low DLCO: Increases probability of interstitial lung disease 1, 2
- Normal DLCO: Makes chest wall or neuromuscular restriction more likely
Important Adjustments and Caveats
Always adjust DLCO for hemoglobin and carboxyhaemoglobin levels 1, especially when:
- Monitoring for drug toxicity
- Patients undergoing chemotherapy (hemoglobin fluctuates)
- Evaluating smokers (elevated carboxyhaemoglobin)
Common pitfall: Do not use DLCO/VA (KCO) to "normalize" DLCO for lung volume - this relationship is not linear and can be misleading 1. Instead, examine DLCO/VA and VA separately for pathophysiologic insights.
Clinical Significance
Even mild DLCO reductions have clinical importance 3:
- Associated with increased COPD symptoms, reduced exercise performance, and higher severe exacerbation risk
- Every 10% predicted decrease in DLCO correlates with worse quality of life scores and reduced 6-minute walk distance
- DLCO <40% predicted or decline >4 units is associated with increased morbidity and mortality 2
A mild reduction warrants investigation for underlying causes, particularly when symptoms are present 4, as it may represent early disease requiring intervention or closer monitoring.