Exhaled carbon monoxide in airway diseases: from research findings to clinical relevance

Breath tests have gained increasing interest in recent years mainly driven by the unmet clinical need to monitor airway diseases and to obtain information of unravelled aspects of respiratory disorders. Carbon monoxide is present in the exhaled breath and has been suggested to reflect ongoing oxidative stress, even if there are some confounding factors limiting its clinical usefulness. Increased concentration of exhaled carbon monoxide has been demonstrated in different acute and chronic airway diseases including allergic rhinitis, asthma, bronchiectasis, and post transplant bronchiolitis obliterans syndrome. Although exhaled carbon monoxide might not prove as a clinically useful biomarker of airway diseases, its measurement has helped to understand the place of heme oxygenase activity in allergic and non-allergic airway diseases. The scope of this review is the exciting field of exhaled carbon monoxide in airway diseases.