Oh JY, et al. - In this prospective multicenter cohort study with retrospective analysis, researchers determined the urinary levels of L-histidine, a precursor of histamine related to inflammatory responses, as a new candidate biomarker for diagnosing asthma–COPD overlap (ACO). Urinary L-histidine levels were measured among three groups: asthma, COPD, and ACO, according to the Spanish guidelines algorithm and the metabolic profiles of the groups were discriminated. They found the significance of high urinary L-histidine level for the diagnosis of ACO even after adjusting for age, sex, and smoking amount. They observed that irrespective of the diversity of diagnostic definitions used, urinary l-histidine could have utility as a potential biomarker for ACO.