Intrasubject between-day variability of PD20 methacholine assessed by the dosimeter inhalation test

The intrasubject between-day variability of the methacholine inhalation test (MIT) was estimated in a group of 30 patients (15 males, mean age 29 yr), representative of a wide range of degrees of nonspecific bronchial responsiveness (from normal to severely increased). In each patient, an MIT with the dosimeter method was carried out on three separate occasions within one week, keeping as constant as possible both the technical and patient-related factors (FEV1 within +/- 5 percent, no recent airway inflammation) and independently from operator-related factors (three tests, three different operators blinded with regard to previous MIT results). On each occasion, twofold increasing doses of methacholine were given from 6.25 to 3,200 micrograms as cumulative doses, at five-minute intervals by means of a dosimeter Me.far MB3 (nebulization time 0.8 +/- 0.2s, output 5 +/- 0.2 microliters/puff). The FEV1 was measured initially and 1.5 and 3 minutes after each inhalation. The test was continued until either a fall of 20 percent or more in FEV1 was obtained or the last dose was reached. The results were expressed in terms of PD20, ie, the dose of methacholine producing a 20 percent fall in FEV1. Under these carefully controlled conditions, the 95 percent confidence intervals (as based on a single determination) corresponded to +/- 0.22 on a log10 scale or, in a more meaningful way, +/- 1.66 fold-difference in PD20 from one visit to the other.