Interpretation of the results of methacholine inhalation challenge tests

Methacholine inhalation challenge studies are conventionally expressed as the provocative cumulative dose of methacholine that causes a 20% fall in FEV₁ (PD₂₀). This method is widely used and most suitable for expressing results for challenges in subjects with asthma. However, it is impossible to present results in this manner in other circumstances. Normal subjects and many subjects with allergic rhinitis do not have significant bronchial reactivity and will not reach a 20% fall during standard challenge protocols. This limitation precludes the use of this large population in the quantitative analysis necessary in epidemiologic, genetic, or drug studies. To compound this difficulty, a subset of patients with allergic rhinitis may demonstrate a 20% drop but have a subsequent plateau drop in FEV₁. Expressing the results of their methacholine challenge with only a PD₂₀ loses valuable information available in the test. We demonstrate that expressing results as the area beneath a dose-response curve provides useful parametric data on all studied subjects and distinguishes among subjects who might otherwise be classified as equivalent when results are expressed solely as a PD₂₀.