Airway responses to a diluent used in the methacholine challenge test.

Background: Inhalation of diluent is often used in performing methacholine challenge tests, but its elimination has been suggested because marked falls in FEV1 after diluent inhalation have not been documented and performing this step is time-consuming.

Objective: We investigated the frequency and magnitude of response to the inhalation of diluent, and if there were any systematic effects in determining the PC20 using the baseline and postdiluent spirometric measurements.

Methods: All methacholine challenges performed during a 6-year period (N = 3,902) were reviewed retrospectively.

Results: The maximum increase and decrease in FEV1 and FVC from baseline were 56.3% and -41.4%, respectively, and 61.7% and -40.3%, respectively. The mean absolute changes from baseline in FEV1 and FVC were -0.018 L and -0.026 L, respectively. There was a highly significant correlation (r2 = 0.96; P < .0001) between the PC20 baseline and PC20 postdiluent values, and a mean difference of 0.041 mg/mL (P < .0001), with the PC20 postdiluent being higher.

Conclusions: These data do not provide strong evidence to support either using or eliminating the diluent stage. It is clear that there are frequent and sometimes large changes in FVC and FEV1 after the inhalation of diluent containing phenol and sodium bicarbonate buffer. If a laboratory intends to report changes in airway function qualitatively (ie, positive or negative), the diluent stage may not be necessary. However, if a laboratory intends to report bronchial challenge data from inhaling methacholine in a quantitative fashion and report a continuous variable such as PC20, a diluent stage is recommended.