Induced sputum in asthma: from bench to bedside.

During recent years there has been a growing interest in using non-invasive biomarkers to understand and monitor the airway inflammation in subjects with respiratory tract disorders and mainly asthma and chronic obstructive pulmonary disease (COPD). Sputum induction is generally a well-tolerated and safe procedure and a European Respiratory Society Task Force has published a comprehensive review on sputum methodology. Induced sputum cell count and, to a lesser extent, mediator measurements have been particularly well validated. In asthma, the sputum and the cell culture supernatant can be used for the measurement of a variety of soluble mediators, including eosinophil-derived proteins, nitric oxide (NO) derivatives, cytokines and remodelling-associated proteins. Sputum eosinophilia (> 3%) is a classic feature of asthma although half of the patients seems to be non eosinophilic. Measuring the percentage of sputum eosinophils has proved to be useful in the clinical arena in helping to predict short term response to inhaled corticosteroids (ICS) and tailor the dose of ICS in the severe patients but there is scope for the application of other induced sputum markers potentially useful in clinical practice. The widespread application of induced sputum in asthma across the spectrum of disease severity has given insight into the relationship between airway function and airway inflammation, proposed new disease phenotypes and defined which of these phenotypes respond to current therapy, and perhaps most importantly provided an additional tool to guide the clinical management of asthmatic patients. To date sputum induction is the only non-invasive measure of airway inflammation that has a clearly proven role in asthma management.