Am. J. Respir. Crit. Care Med., Volume 161, Number 6, June 2000, 2092-2095

Predictors of Late Asthmatic Response
Logistic Regression and Classification Tree Analyses

PEDRO C. AVILA, MARK R. SEGAL, HOFER H. WONG, HOMER A. BOUSHEY, and JOHN V. FAHY

Cardiovascular Research Institute, Department of Medicine, and the Department of Epidemiology and Biostatistics, University of California at San Francisco, San Francisco, California

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.

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