Sputum Tests and Exhaled NO in the Diagnosis and Monitoring of Asthma

Since 1987, with the identification of nitric oxide (NO) as the previously uncharacterised endothelial-derived relaxing factor, it has become apparent that this endogenous multifunctional regulatory molecule, which is widely distributed throughout the body, plays a significant role in human physiology [1, 2]. In 1998, Furchgott, Ignarro, and Murad were awarded the Nobel Prize for their work with NO which had, several years earlier, been recognised by Science as 1992s “Molecule of the Year” [3]. In the respiratory system, NO is a neurotransmitter for bronchial non-adrenergic, non-cholinergic neurons and promotes dilation of blood vessels and relaxation of bronchial smooth muscle, thus regulating vascular and bronchial tone [4–6]. Its cytotoxic and bacteriostatic properties play a role in host defense [4–6]. NO also promotes mucus secretion and is instrumental in coordinating the ciliary beat frequency of airway epithelial cells, thus affecting mucociliary clearance [4–6]. The discovery by Gustafsson et al. in 1991 that NO can be detected in the exhaled breath of animals and humans, and the subsequent observation by Alving and coworkers that NO is elevated in the exhaled air of patients with asthma has led to evolving research evaluating the significance of varying concentrations of this molecule in exhaled air in inflammatory airway disease [7, 8].