Predictors of late asthmatic response: Logistic regression and classification tree analyses

To identify predictors of the late asthmatic response (LAR), we reviewed data from 60 asthmatic subjects who had undergone allergen challenge over the past 5 yr (33 females, age 31.4 ± 6.7 yr [mean ± SD], FEV₁ 90% ± 14% predicted). Variables considered likely predictors of LAR included baseline FEV₁, PC₂₀ methacholine (PC₂₀), sputum eosinophil percent, and the decrease in FEV₁ within 20 min of allergen challenge. A LAR (FEV₁ ≥ 15% fall between 3 and 7 h after challenge) was documented in 57% of subjects. A variety of logistic regression methods revealed a significant inverse association between LAR and PC₂₀ (odds ratio [OR] = 0.14 [95% CI = 0.03- 0.66]) and a positive association between LAR and the decrease in FEV₁ at 20 min (OR = 1.18 [1.04-1.33]). Classification tree analysis revealed that a threshold of 0.25 mg/ml for PC₂₀ was most predictive of LAR; LAR developed in 87% of those with PC₂₀ ≤ 0.25 mg/ml (n = 23) and in 38% of those with PC₂₀ > 0.25 mg/ml (n = 37). Notably, in subjects with PC₂₀ > 0.25 mg/ml, the incidence of LAR increased from 38% to 57% if the allergen-induced decline in FEV₁ at 20 min was ≥ 27%. Surprisingly, baseline FEV₁ and percent eosinophils in induced sputum were not significantly associated with LAR. We conclude that a threshold value of 0.25 mg/ml for PC₂₀ methacholine is a good predictor of LAR. Measuring the PC₂₀ methacholine may be useful as a screening method to improve the efficiency of identifying asthmatic subjects with a LAR.