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

Pulmonary diffusing capacity for carbon monoxide (DL_CO) is a valuable pulmonary function test to evaluate the gas exchange capacity of the lungs. Generally, DL_CO 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_CO values cannot be obtained for reasons other than technical errors. Herein, we report two patients with COPD in whom DL_CO values were undetectable without prolonging the breath-holding time for the test. We discuss possible mechanisms for these peculiar findings.