Diffusing Capacity of Carbon Monoxide in Assessment of COPD

Background: Diffusing capacity of the lung for carbon monoxide (DLCO) is inconsistently obtained in patients with COPD, and the added benefit of DLCO testing beyond that of more common tools is unknown. Objective: The goal of this study was to determine whether lower DLCO is associated with increased COPD morbidity independent of emphysema assessed via spirometry and CT imaging. Methods: Data for 1,806 participants with COPD from the Genetic Epidemiology of COPD (COPDGene) study 5-year visit were analyzed, including pulmonary function testing, quality of life, symptoms, exercise performance, and exacerbation rates. DLCO percent predicted was primarily analyzed as a continuous variable and additionally categorized into four groups: (1) DLCO and FEV₁ > 50% (reference); (2) only DLCO ≤ 50%; (3) only FEV₁ ≤ 50%; and (4) both ≤ 50% predicted. Outcomes were modeled by using multivariable linear and negative binomial regression, including emphysema and FEV₁ percent predicted among other confounders. Results: In multivariable analyses, every 10% predicted decrease in DLCO was associated with symptoms and quality of life (COPD Assessment Test, 0.53 [P < .001]; St. George's Respiratory Questionnaire, 1.67 [P < .001]; Medical Outcomes Study Short Form 36 Physical Function, –0.89 [P < .001]), exercise performance (6-min walk distance, –45.35 feet; P < .001), and severe exacerbation rate (rate ratio, 1.14; P < .001). When categorized, severe impairment in DLCO alone, FEV₁ alone, or both DLCO and FEV₁ were associated with significantly worse morbidity compared with the reference group (P < .05 for all outcomes). Conclusions: Impairment in DLCO was associated with increased COPD symptoms, reduced exercise performance, and severe exacerbation risk even after accounting for spirometry and CT evidence of emphysema. These findings suggest that DLCO should be considered for inclusion in future multidimensional tools assessing COPD.