The links between allergen skin test sensitivity, airway responsiveness and airway response to allergen

Background: The allergen-induced early asthmatic response [provocation concentration (PC)(20), the concentration causing a 20% forced expiratory volume in 1 s (FEV)(1) fall] depends on the level of IgE sensitivity and the degree of nonallergic airway hyperresponsiveness (AHR) and can be predicted from histamine PC20 and allergen skin test endpoint. Objectives: We examined the relationships between allergen PC20, methacholine PC20, and allergen skin test endpoint and assessed the accuracy of both the histamine PC20-based prediction of allergen PC20 (using methacholine) and a new methacholine PC20-based prediction equation. Methods: From 158 allergen challenges, the allergen PC20, the methacholine PC20, and the skin test endpoint were recorded and relationships between these three were sought. We compared the measured allergen PC20 to that predicted from the previous histamine PC20-based and the new methacholine-based formulae. Results: In single regressions, allergen PC20 correlated with both methacholine PC20 (r = 0.25, P = 0.0015) and skin test endpoint (r = 0.52, P < 0.00005). The relationship was improved by multiple regression of log-allergen PC(20)vs. log-methacholine PC20 and log-endpoint (r = 0.61, P < 0.00005). The histamine-based formula predicted allergen PC20 to within 2 doubling concentrations in 80% and within 3 in 92%. The new methacholine-based formula to within 2 and 3 concentrations in 81% and 94%, respectively; only nine of 158 subjects were outside the 3 concentrations. Conclusions: We have confirmed the dependence of the allergen-induced early asthmatic response upon the level of allergic sensitivity and the degree of AHR, the latter as assessed by methacholine challenge. The allergen PC20 can be predicted to within 3 doubling concentrations in 94% of cases.