Why Measure KCO in Addition to DLCO
You should measure KCO (transfer coefficient) alongside DLCO because examining both parameters separately provides critical pathophysiological information about disease mechanisms that cannot be obtained from DLCO alone, helping distinguish between parenchymal lung disease, vascular abnormalities, and extrapulmonary restriction. 1
The Core Reasoning
DLCO is mathematically the product of two independent components: KCO and alveolar volume (VA). The same DLCO value can result from vastly different combinations of KCO and VA, each reflecting distinct disease processes 2, 3. For example, in diffuse parenchymal lung diseases with the same DLCO of 50% predicted, KCO can vary from 60-95% predicted while VA ranges from 55-85% predicted—indicating fundamentally different pathogenic mechanisms 3.
Clinical Decision Algorithm
When interpreting a low DLCO, examine the KCO pattern:
Low DLCO with High/Normal KCO
- Suggests: Extraparenchymal abnormality (pneumonectomy, chest wall restriction, neuromuscular disease)
- Mechanism: Loss of lung volume is greater than loss of diffusion capacity per unit volume
- Action: Focus evaluation on volume loss mechanisms, not parenchymal disease 1
Low DLCO with Low KCO
- Suggests: Parenchymal lung disease (emphysema, interstitial lung disease, pulmonary vascular disease)
- Mechanism: Loss of diffusion capacity exceeds volume loss, indicating true alveolar-capillary membrane or vascular pathology
- Action: Pursue parenchymal disease workup 1
Low DLCO with Elevated KCO (relative to DLCO reduction)
- Suggests: Interstitial lung disease with fibrosis
- Mechanism: Increased blood flow per alveolar-capillary unit in small, fibrotic lungs
- Action: This pattern is characteristic of restrictive ILD 4
Disease-Specific Patterns
Emphysema: KCO is more sensitive than DLCO for detecting emphysematous changes and correlates better with CT-measured emphysema volume and small vessel cross-sectional area 5. In smokers with asthma versus COPD, KCO successfully differentiates disease when airflow limitation is matched, while DLCO may not 5.
Pulmonary vascular disease: Persistent DLCO reduction after severe COVID-19 pneumonia shows predominantly reduced pulmonary capillary blood volume (Vc) rather than membrane diffusion (DmCO), which KCO assessment helps identify 6. KCO abnormalities correlate with elevated pulmonary artery pressures in children born preterm 7.
Sarcoidosis: Can show discordance between VA and total lung capacity, revealing distal airway obstruction that KCO assessment helps detect 3.
Critical Guideline Warnings
The 2005 European Respiratory Society guidelines explicitly state: "It is advisable to keep examining DLCO/VA (KCO) and VA separately, in so far as it may provide information on disease pathophysiology that cannot be obtained from their product, the DLCO." 1
The 2017 American Thoracic Society technical statement notes that KCO is optional but recommends avoiding the term "DL/VA" as it is commonly misunderstood—use "KCO" instead 8.
Common Pitfalls to Avoid
Don't normalize DLCO by dividing by VA: The relationship between DLCO and lung volume is non-linear, so simple ratios (DLCO/VA or DLCO/TLC) don't appropriately normalize for lung volume 1
Don't interpret KCO in isolation: A KCO of 100% predicted can be normal in some circumstances or pathological in others depending on the clinical context 9
Don't ignore VA measurements: Severe airflow limitation and ventilation heterogeneity cause underestimation of VA relative to true total lung capacity, affecting KCO interpretation 4
Practical Implementation
When DLCO is measured, report both KCO and VA with their predicted values. Adjust for hemoglobin when measured, particularly in patients monitored for drug toxicity or undergoing chemotherapy 1. The combined assessment allows you to determine whether reduced DLCO reflects true parenchymal/vascular disease versus volume loss from other causes—fundamentally different clinical scenarios requiring different management approaches.