Exhaled breath condensate in chronic obstructive pulmonary disease: Methodological challenges and clinical application

Collection of exhaled breath condensate (EBC) is a non-invasive method to sample the airway lining fluid. The composition of EBC may reflect physiological and pathophysiological processes within the lower airways, which could otherwise be investigated only with more invasive techniques. Hence, analysis of the condensate fluid seems feasible in chronic obstructive pulmonary disease (COPD) to monitor disease processes and facilitate personalized therapy. In the past two decades, a multitude of molecules has been assessed in EBC samples from patients with COPD, characterizing inflammation, oxidative and nitrative stress in this disorder. Recently, multimarker profiling with sensitive metabolomic or proteomic approaches, optimized for the EBC matrix, has also been applied, which could overcome the pitfalls of single marker detection using commercial assay systems. In this review, we describe the theoretical background of EBC formation, systematically discuss technical and methodological difficulties of sample collection and analysis and summarize data on EBC biomarkers in COPD. Finally, based on previous findings and our experience, we propose potential future directions for the EBC research community, which could pave the way for introducing EBC analysis in clinical practice.