Background: The single-breath diffusing capacity of the lungs (DLCO_SB) test measures the extent to which carbon monoxide (CO) passes from the lung air sacs into the blood. The accessible alveolar volume (VA_SB) is measured by inert gas during a 10-second period. The single-breath transfer coefficient of the lung for carbon monoxide (KCO_SB) is the DLCO_SB divided by VA_SB. Cystic fibrosis (CF) disease comprises progressive airway obstruction with bronchiectasis and parenchyma fibrosis. Yet, the KCO_SB appears insignificant in the assessment of pulmonary function in CF.

Objectives: To challenge the precision of normal KCO_SB in CF.

Methods: The authors collected pulmonary function tests (PFT) data from 74 confirmed CF patients (mean age 26 ± 10 years) with various levels of pulmonary disease severity. Tests included spirometry, DLCO_BP, and body plethysmography (_BP). Anatomical dead space was calculated by deducting anatomical dead space from total lung capacity TLC(_BP) to establish alveolar volume (VA_BP) and to determine KCO_BP. We also included individual data of arterial pCO₂ blood-gas level.  

Results: KCO_SB values were normal or higher in most patients, regardless of patient FEV1 value (R² = 0.2204; P < 0.02). In contrast, the measurements of KCO_BP were low corresponding with low FEV1 values, and negatively correlated with the elevation of trapped air and pCO₂ levels (R² = 0.1383; P = 0.0133, P > 0.05, respectively).

Conclusions: The 10- second perfusion time of the inert gas during DLCO_SB represent the communicative alveolar volume in CF patients with advanced pulmonary disease. The findings justify the use of DLCO_SB with the deterioration of FEV1 and elevation of pCO₂ levels.