Guidelines for the two minute tidal breathing and the 5 breath dosimeter methods for methacholine challenge have recently been published by the ATS. Although subjects are exposed to twice as much aerosol at any given concentration during the tidal breathing compared to the dosimeter method, they were thought to give equivalent results. This study was done to compare the 2 methods.
We compared the two methods in 40 subjects with stable asthma, FEV1 > 65 % predicted and a methacholine provocation causing a 20% FEV1 fall (PC20) < 16 mg/ml (tidal breathing method). Tests were done at the same time of day within a 7 day period. All aspects of the testing were identical except for aerosol generation and inhalation.
The dosimeter provocation concentration causing a 20% FEV1 fall (PC20) was almost twice that of the tidal breathing (geometric PC20, 2.4 vs. 1.3 mg/ml, p<0.00005). Three subjects with mild asthma and mild airway hyperresponsiveness (tidal breathing PC20=1.9-4.3 mg/mL) had a non-measurable PC20 (>32 mg/mL) with the dosimeter; two of these had a measurable PC20 when a modified dosimeter challenge was done with sub-maximal inhalations.
We conclude that the tidal breathing method, which exposes the subject to twice as much aerosol at each concentration, produces approximately twice the response i.e. lower PC20. The TLC manoeuvres with breathold during the dosimeter method may inhibit the response in some patients with asthma.
The equivalence of the methacholine PC20s obtained by the 2 methods requires reassessment. The TLC inhalations required for the dosimeter method may lead to falsely negative tests in some subjects with mild asthma.
D.W. Cockcroft, None.