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RESEARCH PRODUCT

Differences in the Response to Methacholine Between the Tidal Breathing and Dosimeter Methods

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Background It has been postulated that differences in provocative concentration of methacholine causing a 20% fall in FEV 1 (PC 20 ) values between the dosimeter method and tidal breathing method might be due to differences in the dose of agonist delivered to the mouth. The aim of the present study was to determine the influence of the dose of aerosol delivered to the mouth on differences in the response obtained with each challenge method. Methods This study measured airway responsiveness to methacholine by dosimeter method and tidal breathing method in 27 subjects with suspected asthma. The dosimeter was modified to deliver an identical volume to that obtained with the tidal breathing method. Concentration-response curves were characterized by the PC 20 . Results The dosimeter method PC 20 was significantly higher than the tidal breathing method PC 20 , with geometric mean values of 4.03 (95% confidence interval [CI], 1.86 to 8.78 mg/mL) and 2.19 (95% CI, 1.32 to 3.64 mg/mL; p=0.04), respectively. The mean difference in the PC 20 value detected with each method was similar in subjects with tidal breathing method PC 20 values ≥ 2 mg/mL (0.77 doubling concentrations) and in those with PC 20 values Conclusions The tidal breathing method produces PC 20 values significantly lower than a modified dosimeter method, which delivers the same volume of aerosol. These results suggest that the discordant PC 20 values obtained with the two methods are not due to differences in the dose of agonist delivered to the mouth.