When it became clear that asthma symptoms could not be depended on to predict the presence or absence of airway hyper-responsiveness and inflammation, the hallmarks of asthma, the necessity for a diagnostic test for asthma was created. It has also been acknowledged that there is a need to identify asthmatic patients who have been overtreated or undertreated with inhaled corticosteroids (ICS). 

To satisfy this demand, dry-powder mannitol is inhaled as a bronchial provocation test. Mannitol is an osmotic substance that induces mediators to be released, causing the airways to constrict. This mannitol-induced airway response is dependent on the presence of inflammatory cells, and it is decreased or even entirely prevented by ICS therapy. Mannitol has a high specificity (95%) for detecting asthma and may be used to monitor ICS therapy in large populations, according to studies. Importantly, when mannitol responsiveness is lowered by ICS medication, all other measures of airway inflammation are reduced, as are clinical symptoms. 

In both adults and children, the response to mannitol is greater than the responsiveness to exercise. Mannitol responsiveness is not always related to other airway inflammatory indicators such as increased exhaled nitric oxide and/or sputum eosinophilia. It might be because mast cells are essential for the mannitol response. In adults, a negative mannitol test result is typical in asthmatic patients receiving ICS, and reverse titration of the dosage should be considered to confirm the diagnosis and avoid the dangers of overtreatment.

Reference:journals.lww.com/clinpulm/Abstract/2016/11000/Inhaled_Mannitol_in_the_Diagnosis_and_the.3.aspx