A comparison of three measures of the response to inhaled methacholine

In studies of asthma prevalence bronchial responsiveness has usually been measured as the provocative dose of bronchoconstrictor causing a 20% fall in FEV1 (PD20FEV1). This is relatively insensitive and only 10-20% of subjects in a general population sample will show such a response. Attempts to increase sensitivity, such as the use of the provocative dose causing a 10% fall in FEV1 (PD10FEV1), have not demonstrated any overall advantage, due to poorer repeatability. It has been suggested that measurement of bronchial reactivity using flow at low lung volumes measured from a partial flow volume curve is a more sensitive index of bronchoconstriction than PD20FEV1. If equally repeatable, it would have advantages in epidemiological practice. In 20 subjects with asthma, we compared the sensitivity and repeatability of PD10FEV1, PD20FEV1, and the provocative dose causing a 40% fall in flow at 30% of vital capacity (PD40V30P) following methacholine challenge. PD40V30P was more sensitive than both PD20FEV1 and PD10FEV1 by 1.48 and 0.35 doubling doses (DD) of methacholine, respectively. PD20FEV1 and PD40V30P showed equal repeatability, the 95% range for a single estimate of both being 2.02 DD. PD10FEV1 was less repeatable, with a 95% range of 2.35 DD. Values for the intraclass correlation co-efficient, which measures the ability of a test to discriminate between subjects, were 0.63, 0.79 and 0.69 for PD10FEV1, PD20FEV1, and PD40V30P, respectively. The increased sensitivity and comparable repeatability of measurement of bronchial reactivity for PD40V30P suggest that this method may be useful for studies of asthma prevalence.