Novel capsaicin challenge methodology: a new diagnostic endpoint?

Background: Capsaicin cough challenge is used in pre-clinical and clinical studies to evaluate mechanisms, however standard methods and endpoints discriminate poorly between healthy volunteers (HV) and refractory chronic cough (RCC) patients. Aims: We aimed to assess the ability of capsaicin dose response curves to differentiate RCC from HV using receiver operator characteristic (ROC) curve analysis. Methods: 93 HV (median age 39yrs (IQR 29-52), 47 females) and 51 RCC (age 59yrs (53-67), 31 females) performed a full dose response cough challenge, inhaling doubling doses of capsaicin (0.48-1000µmol/L) four times up to the maximum tolerated dose. The number of coughs evoked in 15s following inhalation was recorded. Emax (maximum number of coughs evoked by any dose) and ED50 (dose evoking at least half of the Emax) were derived. Results: Emax was significantly higher in the RCC group compared with HV (median(IQR) 27(21-37) coughs vs. 13(9-19), p Conclusion: ROC curve analysis suggests capsaicin evoked Emax discriminates well between RCC and HV therefore showing promise as a diagnostic test for RCC.