The diffusing capacity in adult cystic fibrosis.

The value of adjusting the diffusing capacity for the lung volume has been demonstrated in a large number of patients with other lung diseases but has not been validated in patients with cystic fibrosis (CF). Pulmonary function test results on a cohort of 52 adult CF patients were analyzed to determine whether the diffusing capacity of carbon monoxide by single breath method (DLCO(SB)) when adjusted for alveolar volume (V(A)%), correlated with the severity of pulmonary dysfunction. The DLCO(SB) remained within the reference range except in those with severe lung impairment (61.88 +/- 15.48%). DLCO(SB) has a significant (P < 0.05) positive correlation (0.70, 0.67, 048, 0.69 and 0.31, respectively) with measures of airflow limitation (FVC%, FEV1%, FEV1/FVC%, MVV%, and sGaw) and negative correlation (-0.36 and -0.21, respectively) with measures of air trapping (RV% and RV/TLC%). DLCO(SB)/V(A) remained above 100% of predicted despite worsening lung disease and did not correlate with other measures of lung function. On the other hand, the DLCO(SB) and DLCO(SB)/V(A), when adjusted for V(A)%, decreased and were significantly correlated with worsening airflow limitation and, to a lesser extent, air trapping. The relatively preserved adjusted DLCO(SB) and DLCO(SB)/V(A) values in CF patients up until late in its course may be explained the predominant airway involvement, minimal loss of alveolar-capillary units, and enhanced V/Q relationship due to claustration in CF.