Assessment of mid flow rate measurements in patients undergoing methacholine challenge

The objective of this study is to assess the FEF_25–75 and FEF_25–75/FVC in relation to the FEV₁ in patients who have had a methacholine inhalation challenge study for a variety of clinical indications. The study is a retrospective review of methacholine challenge results at the university medical center. One hundred twenty-one consecutive patients who had a methacholine challenge performed for clinical indications were included in the study with no intervention. Methacholine was administered in successively increasing twofold concentrations in doses from 0.62 mg to a final concentration of 10 mg. A 20% drop in FEV₁compared to the prechallenge value was considered a positive test. We considered ≥25% decrease in FEF_25–75 as a significant change. The ≥25% decrease in FEF_25–75 occurred sooner than the 20% drop in FEV₁ with a positive response occurring at least one full dose sooner in 23 of the 55 subjects. Thirty two subjects reacted at the same dose. The dose at which the FEF_25–75 decreased by ≥25% was significantly different from the corresponding dose causing a 20% decrease in FEV₁. The FEF_25–75 decreases more per mg methacholine. There were no subjects in whom there was ≥20% decrease in FEV₁ without a ≥25% decrease in FEF_25–75. The mean FEF_25–75/FVC after diluent inhalation = 0.87 ± 0.27 standard deviation with a range of 0.23 to 1.67. The doses at which the FEF_25–75/FVC decreased by ≥20% and by ≥30% was significantly lower than the corresponding doses causing a 20% decrease in FEV₁. FEF_25–75 and the FEF_25–75/FVC are more sensitive but less specific than the FEV₁ as indicators of a positive response to a methacholine challenge. The FEF_25–75/FVC does not provide additional information to that provided by the FEF_25–75.