Methacholine challenge test-shortening procedures

Study objectives: Validation of test-shortening procedures for the 2-min tidal breathing methacholine challenge method. Design: Retrospective chart review. Setting: Tertiary-care university clinical pulmonary function laboratory. Patients: One thousand subjects aged 10 to 85 years (mean +/- SD, 44.5 +/- 16.0 years), 44.5% male, referred for methacholine challenge. Intervention: Two-minute tidal breathing methacholine challenge was performed, with both physician and technician access to published test-shortening procedures. Measurements and results: There were 315 positive test results (provocative concentration of methacholine causing a 20% fall in FEV1 [PC20] less than or equal to 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 greater than or equal to 20% FEV1 fall on the first inhalation, and 11 subjects had a greater than or equal to 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 greater than or equal to 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.