DLCO Test Interpretation and Clinical Guidance

Test Procedure and Technical Standards

• The test is performed by having the individual inhale a gas mixture that contains a small amount of carbon monoxide together with a tracer gas (helium or methane) 1.
• Both the diffusing capacity for carbon monoxide (DLCO) and the alveolar volume (VA) are measured, allowing calculation of the transfer coefficient (KCO) 1.

Quality and Reporting Standards

• The reported result must include the sample volume used for the measurement 1.
• DLCO values should be corrected to standard barometric pressure, which is especially important at higher altitudes 1.
• Modern analyzers employ rapid‑acting technology that provides “virtual” sample volumes automatically adjusted for the individual’s size 1.
• The term “DL/VA” is discouraged because it is frequently misunderstood; instead, DLCO/VA (KCO) should be reported 1.

Severity Grading of DLCO Reduction (European Respiratory Society)

• Mild reduction: DLCO ≥ 60 % of predicted and above the lower limit of normal 2.
• Moderate reduction: DLCO 40–60 % of predicted 2.
• Severe reduction: DLCO < 40 % of predicted 2.

Adjustments and Corrections (European Respiratory Society)

• DLCO must first be corrected for the individual’s hemoglobin concentration; anemia artificially lowers DLCO and can mask true gas‑exchange status 2.
• Adjustments for carboxyhemoglobin are required in smokers and in persons exposed to carbon monoxide 2.

Diagnostic Patterns

Normal Spirometry with Low DLCO

• Suggests anemia (after hemoglobin correction) 2.
• Indicates possible pulmonary vascular disorders such as pulmonary hypertension or chronic pulmonary embolism 2.

Restrictive Pattern (Reduced Lung Volumes) with Low DLCO

• Strongly points to parenchymal lung disease, including interstitial lung disease, sarcoidosis, or pulmonary fibrosis 2.

Restrictive Pattern with Normal DLCO

• Implies extraparenchymal causes such as chest‑wall disorders, neuromuscular disease, or obesity 2.

Obstructive Pattern with Low DLCO

• In adult smokers with post‑bronchodilator airway obstruction, a low DLCO markedly raises the likelihood of an emphysema phenotype of COPD 2.
• Lymphangioleiomyomatosis can also present with obstruction and a low DLCO 2.

Elevated DLCO

• May be seen in asthma, obesity, or intrapulmonary hemorrhage 2.

Interpretation of DLCO/VA (KCO)

• The relationship between DLCO and lung volume is non‑linear; therefore, DLCO/VA (KCO) is an imperfect method for normalizing DLCO 2.
• Low DLCO with high KCO: Suggests extraparenchymal abnormalities such as pneumonectomy or chest‑wall restriction 2.
• Low DLCO with low KCO: Indicates parenchymal abnormalities like interstitial lung disease or emphysema 2.
• The European Respiratory Society recommends evaluating DLCO/VA and VA separately rather than relying on their ratio for normalization 2.

Clinical Decision‑Making Steps (European Respiratory Society)

• Step 1 – Verify Hemoglobin Adjustment: Ensure the reported DLCO has been corrected for anemia or polycythemia 2.
• Step 3 – Pattern Recognition: Apply the diagnostic patterns described above, integrating spirometry, lung‑volume measurements, and the adjusted DLCO to reach a differential diagnosis 2.

Low DLCO/VA Ratio: Clinical Significance and Management

Understanding the Pathophysiology

• The European Respiratory Society emphasizes that the relationship between DLCO and lung volume is not linear, making DLCO/VA an imperfect normalization method 3, 4, 5
• When DLCO loss exceeds volume loss, this pattern strongly suggests parenchymal abnormalities rather than extraparenchymal restriction 3, 4
• Anemia reduces hemoglobin available for CO binding, which can lead to a low DLCO/VA ratio 3, 4

Differential Diagnosis by Clinical Pattern

• Emphysema is the primary consideration when low DLCO accompanies airflow obstruction, according to the European Respiratory Society 3, 4
• Interstitial lung diseases characteristically show low DLCO with restriction, as stated by the European Respiratory Society 3, 4
• The low DLCO/VA distinguishes interstitial lung diseases from chest wall or neuromuscular disorders, which typically maintain normal DLCO, as noted by the European Respiratory Society 3, 4

Diagnostic Algorithm

• Always adjust DLCO for hemoglobin levels first, as anemia falsely lowers DLCO without reflecting true gas exchange abnormalities, recommends the European Respiratory Society 3, 4, 6, 7
• Measure complete lung volumes with body plethysmography to confirm true restriction, as suggested by the European Respiratory Society and Praxis Medical Insights 6
• Obtain high-resolution CT imaging to evaluate for interstitial lung disease, pulmonary vascular disease, or early emphysema, as recommended by Praxis Medical Insights 6

Prognostic Significance

• Reduced DLCO predicts cardiopulmonary complications and mortality even when FEV1 is normal, making it a critical risk stratification tool beyond diagnosis, according to Praxis Medical Insights 6, 7

Equivalence and Terminology of DLCO/TLCO

Test Nomenclature

• DLCO (diffusing capacity of the lung for carbon monoxide) and TLCO (transfer factor for carbon monoxide) represent the identical physiological measurement; DLCO is the preferred term in American practice while TLCO is used in European practice. 8

Official Abbreviation

• The correct scientific abbreviation is DL,CO (with subscript L and CO), although the condensed form DLCO is commonly employed in clinical documentation. 8

Synonymous Terminology Across Guidelines

• European guidelines refer to the measurement as the “transfer factor,” whereas American guidelines label it the “diffusing capacity”; both terms describe the same test. 8