Capsaicin cough threshold test in diagnostics

Among patients with chronic unexplained cough, there is a recognized subgroup with respiratory symptoms induced by environmental irritants like chemicals and odours. The diagnosis of sensory hyperreactivity (SHR) has been suggested for this group of patients and can be made using a tidal breathing capsaicin inhalation test. The aim of the present study was to evaluate the ability of a single-breath, dose-response capsaicin threshold test to discriminate such patients from control subjects.

A total of 46 patients with chronic cough and SHR who had previously shown a positive reaction in accordance with limits set for a tidal breathing capsaicin test were tested once with a single-breath, dose-response capsaicin cough threshold test, assessing capsaicin concentrations to evoke 2 (C2), 5 (C5) or 10 (C10) coughs. Twenty-nine subjectively healthy control subjects were also included and tested with the threshold method.

Patients had significantly lower C2, C5 and C10 in comparison to controls. From the results among patients and controls, sensitivity and specificity were calculated, and a receiver operating characteristic curve was constructed, showing excellent ability for C5 and C10 to discriminate patients from control subjects.

For patients with SHR and chronic cough, capsaicin cough sensitivity was once again confirmed to be increased, in this case, using the single-breath dose-response method. Limits set for cough reactions regarded as more sensitive than normal can be useful in diagnostics and further research. C5 seems to be the best measure to use in research and differential diagnostics.