Airway responsiveness to hypertonic saline: dose-response slope or PD15?

The result of airway challenge test with hypertonic saline (HS) is expressed as the dose causing a 15% fall in forced expiratory volume in one second (FEV1; PD15). A noncensored measure, such as the dose-response slope (DRS), allows the evaluation of the risk of asthma for subjects with a fall in FEV1 <15%. The aim of this study was to assess the relationship between airway responsiveness to HS by PD15 or DRS, asthma symptoms and markers of eosinophilic inflammation. Data on current wheeze and airway responsiveness were obtained for 1,107 children (aged 8-13 yrs). Blood eosinophils and serum eosinophil cationic protein (ECP) were assessed in subsets (n = 683 and 485). PD15 was assessed if FEV1 fell > or =15%, and the DRS was calculated for all tests. Graphs were constructed to visualise relationships with current wheeze, blood eosinophils and serum ECP. Odds ratios and Spearman's correlation coefficients were calculated to quantify these relationships. Children with features of asthma had lower PD15 and higher DRS, and separation was most pronounced for DRS. Prevalence of current wheeze increased continuously over the entire range of DRS values. Blood eosinophils were significantly higher only for the highest values of DRS. In conclusion, the continuous relationship between airway responsiveness and asthma symptoms is in favour of a noncensored measure of airway responsiveness, such as the dose-response slope.