The forced oscillation technique (FOT) can be used to determine airway hyperresponsiveness, but the cut-points for changes in respiratory system conductance (Grs) and reactance (Xrs) that define a positive mannitol challenge are not known. Furthermore, the effects of changes in lung volume on these cut-points or on the repeatability of the test are unknown.

In 15 non-asthmatic and 52 asthmatic subjects, response to mannitol challenge was measured by Grs and Xrs, using FOT, and by FEV₁. The FOT variables were adjusted for inspiratory capacity (IC) at each dose. Dose response slope (DRS) was used in receiver operator characteristic (ROC) analysis to compare the ability of adjusted and unadjusted DRSGrs and DRSXrs to detect a positive challenge, defined as PD₁₅FEV₁ ≤635 mg mannitol.

Mannitol challenges were positive in 32 asthmatic and 2 non-asthmatic subjects. Both DRSGrs and DRSXrs detected positive challenges (p < 0.0001 for both), and this was not altered by adjustment for IC for either DRSGrs (p = 0.21) or DRSXrs (p = 0.90). FOT cut-points for a positive challenge were 27% fall in Grs or 0.93 cm H₂O/L/s decrease in Xrs at 635 mg. Repeatability of DRSGrs (±2.01 doubling doses) and DRSXrs (±1.95dd) was comparable with DRSFEV₁ (±1.67dd) and was not improved by adjustment for IC.

Grs and Xrs, measured by FOT, provide a sensitive, repeatable measure of response to mannitol challenge. Adjusting for lung volume does not alter the ability of these variables to detect a positive response or the repeatability of the measurement.