Bronchial challenge: interpretation of readings.

Bronchial reactivity as tested in vivo reflects to some extent the responsiveness of smooth muscle in the lower airways. The in vivo reactivity is often presented in terms of PC20 (provocation concentration leading to a 20% fall of FEV1), an index that is supposed to provide condensed information of the general dose/response relationship by a continuous variable. However, expressing the fall in relative terms is not strictly logical, as the random experimental error of FEV1 shows a homoschedastic distribution. Furthermore, PC20 becomes mathematically unstable in cases with a slight response and can be determined with reasonable precision only when the observed fall is close to 20% or more. When a small number of highly differing dose levels are used in clinical challenge testing, the results should rather be presented as the maximum, absolute change of FEV1 obtained at a specified dosage.