Asthma-COPD Overlap Syndrome (ACOS): Single disease entity or not? Could exhaled nitric oxide be a useful biomarker for the differentiation of ACOS, asthma and COPD?

Asthma and chronic obstructive pulmonary disease (COPD) represent two major public health problems. However, there is a significant proportion of patients with a mixed asthma-COPD phenotype. This condition is defined as asthma-COPD overlap syndrome (ACOS). Since there are no internationally accepted criteria for the diagnosis of that syndrome, its management remains difficult. Given the fact that patients with ACOS have an increased risk of exacerbation and hospitalization, there is a pressing need for a more targeted approach and better management. We propose that fractional exhaled nitric oxide (FeNO), a marker of eosinophilic inflammation, could help clinicians differentiate ACOS from asthma and COPD. We evaluate this hypothesis, using data derived from the existing literature.