DLCO and TLCO: Terminology Clarification
DLCO (diffusing capacity of the lung for carbon monoxide) and TLCO (transfer factor of the lung for carbon monoxide) are identical measurements with different names—they represent the exact same physiological test and provide the same clinical information for evaluating pulmonary parenchymal disease. [@2,3,4,5,6,7,8,9,10@]
Why Two Different Terms Exist
The terminology difference reflects geographic and historical preferences rather than any technical or clinical distinction:
- DLCO is the preferred term in North America, used by the American Thoracic Society 1
- TLCO is the preferred term in Europe, used by the European Respiratory Society 2
- Both terms describe the single-breath carbon monoxide uptake test that measures gas transfer from alveoli to pulmonary capillary blood [@11,13@]
The 2017 ERS/ATS technical standards explicitly acknowledge that "both DLCO (diffusing capacity) and TLCO (transfer factor) are valid terms to describe the uptake of carbon monoxide in the lung" 2. The European Respiratory Journal guidelines confirm this equivalence by stating "DL,CO: Diffusing capacity for the lungs measured using carbon monoxide, also known as transfer factor" [@2,3,4,5,6,7,8,9,10@].
Clinical Application for Parenchymal Disease Evaluation
Regardless of which term you use, the test provides identical diagnostic information:
For parenchymal lung disease specifically:
- Patients with interstitial lung disease, sarcoidosis, and pulmonary fibrosis characteristically show low DLCO/TLCO values [@2,5,6,7,8,10@]
- When spirometry shows restriction (low FVC with normal FEV1/FVC ratio), a decreased DLCO/TLCO strongly suggests parenchymal disease rather than chest wall or neuromuscular disorders [@2,5,6,7,8,10@]
- A loss of DLCO/TLCO that exceeds the loss of lung volume (low DLCO with low DLCO/VA or KCO) indicates parenchymal abnormalities [@2,3,4,5,6,7,8,9,10@]
Severity grading is standardized [@2,10@]:
- Mild: >60% predicted and <LLN
- Moderate: 40-60% predicted
- Severe: <40% predicted
Important Technical Considerations
The measurement technique, reference equations, quality control standards, and clinical interpretation are identical whether the report uses DLCO or TLCO terminology [@1,11@]. Both require:
- Hemoglobin adjustment for accurate interpretation [@2,3,4,5,6,7,8,9,10@]
- Correction for carboxyhaemoglobin when relevant [@2,3,4,5,6,7,8,9,10@]
- Evaluation alongside spirometry and lung volumes for proper diagnostic context [@2,5,6,7,8,10@]
Bottom line: Use whichever term your laboratory reports, but understand they are interchangeable. The clinical decision-making for parenchymal disease evaluation remains identical regardless of nomenclature.