Does diurnal variation in cough reflex testing exist in healthy young adults?

- Diurnal variation in cough reflex testing has previously been considered a major confounding variable.
- No evidence of diurnal variation in cough reflex sensitivity thresholds of healthy adults was found using clinically-relevant testing methods.
- The presence of an order effect, irrespective of time of day, confirms that healthy adults can volitionally modulate their cough response.

The aim of this study was to investigate whether diurnal variation in cough reflex sensitivity exists in healthy young adults when a tidal-breathing method is used. Fifty-three participants (19-37 years) underwent cough reflex testing on two occasions: once in the morning (between 9 am - midday) and once in the afternoon (between 2-5 pm). The order of testing was counter-balanced. Within each assessment, participants inhaled successively higher citric acid concentrations via a facemask, with saline solution randomly interspersed to control for a placebo response. The lowest concentration that elicited a reflexive cough response was recorded. Morning cough thresholds (mean = 0.6 mol/L) were not different from afternoon cough thresholds (mean = 0.6 mol/L), p = 0.16, T = 101, r = −0.14. We found no evidence of diurnal variability in cough reflex testing. There was, however, an order effect irrespective of time of day, confirming that healthy participants are able to volitionally modulate their cough response.