Clinical, Functional, and Comorbid Characteristics of COPD Patients with Impaired Diffusing Capacity: A Cross-Sectional Study

Background: The diffusing capacity of the lung for carbon monoxide (DLCO) is a key measure of alveolar–capillary gas exchange, but its clinical significance in chronic obstructive pulmonary disease (COPD) remains incompletely defined. This study aimed to characterize the demographic, clinical, functional, and comorbid profiles of COPD patients stratified by the degree of DLCO impairment, and to evaluate the potential value of DLCO as a marker for disease severity and clinical phenotyping in a Chinese cohort. Methods: This single-center retrospective cross-sectional study enrolled 650 patients diagnosed with COPD (according to GOLD 2025 criteria) who underwent pulmonary function tests between January 2024 and February 2025 at the university-town hospital of Chongqing Medical University. Patients were stratified by predicted DLCO% into four groups: normal (≥80%), mild impairment (60–79%), moderate impairment (40–59%), and severe impairment (<40%). Demographic, clinical, laboratory, pulmonary function, echocardiographic, and chest CT data were collected. Comparisons across groups were performed using ANOVA/Kruskal–Wallis tests, chi-square or Fisher’s exact tests, and Spearman correlation analysis (IBM SPSS Statistics 25.0). Due to the exploratory nature of the study, no adjustment for multiple comparisons was applied. Results: Progressive DLCO impairment was associated with a higher proportion of male patients (69.2% to 90.9%, p = 0.018), older age (67.3 ± 9.0 to 72.9 ± 6.7 years, p < 0.001), lower BMI (median from 23.9 to 20.0 kg/m2, p < 0.001), and higher smoking prevalence (58.7% to 87.5%, p = 0.001). The prevalence of pulmonary tuberculosis rose markedly (0.58% to 9.09%, p = 0.037). All spirometric parameters declined (e.g., FEV1%pred from 67.3% to 32.6%, p < 0.001). Systemic inflammatory markers (NLR, SII) increased, while hemoglobin and albumin decreased (both p < 0.001). Respiratory failure occurred in 30.0% of the severe DLCO group (predominantly type I, p <0.001). Echocardiography revealed a decline in left ventricular ejection fraction (61.2 ± 5.0% to 59.1 ± 4.0%, p = 0.012) and a trend toward higher pulmonary hypertension risk (27.8%, p = 0.056). DLCO%pred correlated positively with FEV1%pred (r = 0.394, p < 0.001) and oxygen saturation (r = 0.151, p < 0.001), and negatively with NLR (r = −0.165, p < 0.001) and SII (r = −0.149, p < 0.001). Conclusions: In COPD, DLCO impairment is associated with distinct clinical phenotypes, including male sex, advanced age, malnutrition, increased tuberculosis risk, worse lung function, systemic inflammation, and respiratory/cardiac dysfunction. These findings support DLCO as a valuable complementary marker for disease severity characterization in COPD. Longitudinal studies are needed to confirm its prognostic value.