The mean difference in baseline lung function between the first and second visit for the standard vs shortened AMP challenge group was 0.27 hPa/s/L (range, −1.31 to 2.57 hPa/s/L) and equates to a relative change of 1.56% (range, −26.26 to 26.52%). The mean change in respiratory system resistance (Rrs) following saline solution inhalation was 0.03 hPa/s/L (range, −1.18 to 1.82 hPa/s/L), equating to a relative change of −0.43% (range, −20.14 to 17.98%). There was a high degree of concordance between the two AMP protocols, with 17 children demonstrating agreement in responses between the two challenges, although discordance was observed in one child. Four children did not respond to either protocol, and 13 children responded to both. One child did not respond to the standard AMP protocol and responded with an increase in Rrs8 in the final stage (200 mg/mL) of the shortened AMP challenge. The mean log10 PC values to the standard and shortened AMP protocols were 1.57 ± 0.82 and 1.60 ± 0.82 mg/mL, respectively (p = 0.69), with a mean difference of 0.03 mg/mL (95% confidence interval [CI], −0.11 to 0.17 mg/mL). In addition, the responses showed a high degree of agreement (intraclass correlation coefficient, 0.94; 95% CI, 0.85 to 0.98). The mean time to complete the standard challenge was 44.3 min (range, 13 to 120 min), which was significantly (p < 0.01) longer than the shortened challenge (mean, 30.0 min; range, 11 to 60 min). For responsive children, the time difference between the standard and shortened challenge was 12.5 min (41.2 and 28.7 min, respectively), whereas in the nonresponsive children the difference was 16 min (52.8 and 36.8 min, respectively). The majority of children responding to the standard protocol (n = 13) responded with an increase in Rrs8 alone (n = 8); one child responded with audible wheeze alone, and the remaining four children responded to both FOT and wheeze response criteria at the same AMP stage. Similarly, of the children responding to the shortened protocol (n = 14), responses to FOT alone (n = 9) were most common followed by simultaneous responses in FOT and wheeze criteria (n = 3) and wheeze alone (n = 2). Persistent cough developed in three children, but this was always in conjunction with wheeze or significant change in FOT. No child responded with a decrease in Spo2 to < 90%, with the mean decrease being 1.6% (range, −1 to 8%) and 2.1% (range, −1 to 8%), respectively, following the standard and shortened AMP challenges. Full details of individual PC values and responses are presented in Table 2.