Exhaled nitric oxide

In medicine, exhaled nitric oxide (eNO) can be measured in a breath test for asthma or other conditions characterized by airway inflammation. Nitric oxide (NO) is a gaseous molecule produced by certain cell types in an inflammatory response. The fraction of exhaled NO (FENO) is a promising biomarker for the diagnosis, follow-up and as a guide to therapy in adults and children with asthma. The breath test has recently become available in many well-equipped hospitals in developed countries, although its exact role remains unclear. In medicine, exhaled nitric oxide (eNO) can be measured in a breath test for asthma or other conditions characterized by airway inflammation. Nitric oxide (NO) is a gaseous molecule produced by certain cell types in an inflammatory response. The fraction of exhaled NO (FENO) is a promising biomarker for the diagnosis, follow-up and as a guide to therapy in adults and children with asthma. The breath test has recently become available in many well-equipped hospitals in developed countries, although its exact role remains unclear. In humans, nitric oxide is produced from L-arginine by three enzymes called nitric oxide synthases (NOS): inducible (iNOS), endothelial (eNOS), and neuronal (nNOS). The latter two are constantly active in endothelial cells and neurons respectively, whereas iNOS' action can be induced in states like inflammation (for example, by cytokines). In inflammation, several cells use iNOS to produce NO, including eosinophils. As such, eNO has been dubbed an inflammometer. Although iNOS is thought to be the main contributor to exhaled NO in asthmatics, studies in mice also point to a role for nNOS. It was initially thought that exhaled NO derived mostly from the sinuses, which contain high levels of NO. It has subsequently been shown that the lower airways contribute most of the exhaled NO, and that contamination from the sinuses is minimal. Patients with asthma have higher eNO levels than other people. Their levels also rise together with other clinical and laboratory parameters of asthma (for example, the amount of eosinophils in their sputum). In conditions that trigger inflammation such as upper respiratory tract infections or the inhalation of allergens or plicatic acid, eNO levels rise. The eNO levels also tend to vary according to the results of lung function test results such as the degree of bronchial hyperresponsiveness. Furthermore, drugs used to treat asthma (such as inhaled glucocorticoids or leukotriene receptor antagonists) also reduce eNO levels. Clinical trials have looked at whether tailoring asthma therapy based on eNO values is better than conventional care, in which therapy is gauged by symptoms and the results of lung function tests. To date, the results in both adults and children have been modest and this technique can not be universally recommended. It has also been noted that factors other than inflammation can increase eNO levels, for example airway acidity. The fraction of eNO has been found to be a better test to identify asthmatics than basic lung function testing (for airway obstruction). Its specificity is comparable to bronchial challenge testing, although less sensitive. This means that a positive eNO test might be useful to rule in a diagnosis of asthma; however, a negative test might not be as useful to rule it out.