The methacholine challenge test performed with the tidal breathing method induces a greater fall in FEV₁ than the dosimeter method; however, the effect of the challenge method on methacholine-induced fall in FVC has not been investigated.

To determine the influence of the challenge method on methacholine-induced changes in FEV₁ and FVC.

Airway responsiveness to methacholine was determined by dosimeter method and tidal breathing method in 37 subjects with suspected asthma. The dosimeter was modified to deliver an identical volume to that obtained with the tidal breathing method and the same nebulizer model was used for the two challenges. The response was expressed by the provocative concentration of methacholine causing a 20% fall in FEV₁ (PC₂₀) and by the percent fall in FVC at the PC₂₀ value relative to FVC after saline inhalation.

The PC₂₀ values obtained with the tidal breathing method and the dosimeter method were similar, with geometric mean values of 3.15 (95%CI, 1.85–5.34 mg/mL) and 2.51 (1.37–4.61 mg/mL, P = 0.092), respectively. The percent fall in FVC at the PC₂₀ value obtained with the dosimeter was significantly greater than that obtained with the tidal breathing method, with mean values of 11.8 (95%CI, 10.0–13.5%) and 9.4 (95%CI, 8.1–10.8, P = 0.002), respectively.

Differences in methacholine PC₂₀ values obtained with the two challenge methods recommended in guidelines may be overcome by introducing some technical modifications in the dosimeter method. However, the technical factors that affect methacholine sensitivity and air trapping are at least partially different.