Asthma–COPD Overlap (ACO) remains one of the most diagnostic and therapeutic "gray zones" in respiratory medicine. While the term describes patients with features of both diseases, the clinical reality is a spectrum: on one end, asthma with fixed airflow obstruction; on the other, COPD with reversible components. The challenge? Spirometry often looks identical at both ends of this spectrum. To bridge this gap, recent evidence suggests that Diffusing Capacity of the Lung for Carbon Monoxide (DLCO) may be the "tie-breaker" clinicians need.
The Physiological Logic While spirometry measures mechanics (how fast air moves), DLCO measures integrity (how well gas moves across the membrane). Asthma: Typically preserves the alveolar-capillary interface. COPD/Emphysema: Leads to structural destruction and a reduced surface area for gas exchange. In ACO, the DLCO value acts as a surrogate marker…