Two cases of chronic obstructive pulmonary disease with undetectable diffusing capacity for carbon monoxide.

Pulmonary diffusing capacity for carbon monoxide (DL) is a valuable pulmonary function test to evaluate the gas exchange capacity of the lungs. Generally, DL values are significantly lower in patients with chronic obstructive pulmonary disease (COPD), particularly in those with a predominantly emphysema phenotype. However, it is extremely rare that DL values cannot be obtained for reasons other than technical errors. Herein, we report two patients with COPD in whom DL values were undetectable without prolonging the breath-holding time for the test. We discuss possible mechanisms for these peculiar findings.