Study objectives: Validation of test-shortening
procedures for the 2-min tidal breathing methacholine challenge
Design: Retrospective chart review.
Setting: Tertiary-care university clinical pulmonary
Patients: One thousand subjects
aged 10 to 85 years (mean ± SD, 44.5 ± 16.0 years), 44.5% male,
referred for methacholine challenge.
Two-minute tidal breathing methacholine challenge was performed, with
both physician and technician access to published test-shortening
Measurements and results: There were 315
positive test results (provocative concentration of methacholine
causing a 20% fall in FEV1 [PC20] ≤ 8
mg/mL) and 685 negative test results. The subjects with positive test
results were less likely to be male (39.1 vs 47.5%; p < 0.02) and
had lower FEV1 (91.8 ± 14.9% predicted vs
97.2 ± 13.9% predicted; p < 0.001). The average starting
PC20 was between 0.5 mg/mL and 1.0 mg/mL; the most common
PC20 was 1 mg/mL (67%). There were 431 skipped
concentrations in 380 subjects. The mean number of methacholine
inhalations was 3.7 ± 1.1 (3.9 ± 0.1 for negative test results vs
3.3 ± 1.2 for positive test results; p < 0.001). Eighteen
subjects had a ≥ 20% FEV1 fall on the first inhalation,
and 11 subjects had a ≥ 20% FEV1 fall after a skipped
concentration. In only one case (0.1%) an FEV1 fall≥
40% on the first concentration was reported, compared with no
cases after a skipped concentration and seven cases with a ≥ 40%
FEV1 fall after a routine doubling dose step-up.
Conclusions: The 2-min tidal breathing methacholine test in
clinical practice can be safely shortened to an average of less than
four inhalations using starting concentrations based on
FEV1, asthma medication, and clinical features, and by
occasionally omitting concentrations.