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Tidal-breathing methacholine challenges are now recommended by guidelines, to avoid the bronchoprotective effects of deep inhalation. This study compared different tidal breathing methacholine challenge methods; assessed the agreement between tidal dosimetric and continuous output challenges; and assessed challenge repeatability with different methods.  15 asthma patients performed dosimetric challenges and a continuous-output breath-actuated challenge, all ≥3 days apart. All subjects had a pre-bronchodilator forced expiratory volume in 1 s (FEV 1 ) ≥65% predicted, and a cumulative dose causing a 20% reduction in FEV 1  (PD 20 ) &lt;1.2 mg.  Of the dosimetric challenges, one method increased methacholine concentration (standard dosimetric challenge), and one adjusted nebuliser output time to increase dose (adjusted dosimetric challenge). The adjusted dosimetric and continuous output challenges were performed twice on separate days to assess for repeatability. All challenges were matched for dose at each dose step. The mean PD 20  ratio of the standard dosimetric challenge to the adjusted dosimetric challenge was 0.90 (95% CI 0.66–1.23, p=0.49) and intraclass correlation coefficient (ICC) was 0.82. Repeated adjusted dosimetric challenges had an ICC 0.62 for PD 20 . Repeated continuous output challenges had an ICC 0.74 for PD 20 . The adjusted dosimetric and continuous output challenges correlated (r=0.69, p=0.0043; ICC 0.65), but PD 20  was higher for the adjusted dosimetric challenge (mean PD 20  ratio 2.31, 95% CI 1.57–3.40; p=0.0004).  Tidal dosimetric methacholine challenge using adjustment of nebuliser output produces results with good repeatability. The results of this adjusted dosimetric method differed from the continuous output method, underscoring that the results of different methacholine challenge methodologies may not be directly comparable.

Methacholine challenges: comparison of different tidal breathing challenge methods