Gulsvik A, Vale J R
Eur J Respir Dis Suppl. 1986;143:1-7.
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.
体内测试的支气管反应性在一定程度上反映了下呼吸道平滑肌的反应性。体内反应性通常用PC20（导致第一秒用力呼气容积（FEV1）下降20%的激发浓度）来表示，该指标旨在通过一个连续变量提供一般剂量/反应关系的浓缩信息。然而，用相对值来表示下降并不严格符合逻辑，因为FEV1的随机实验误差呈同方差分布。此外，在反应轻微的情况下，PC20在数学上会变得不稳定，只有当观察到的下降接近20%或更多时，才能以合理的精度确定。当在临床激发试验中使用少量差异很大的剂量水平时，结果更应以在指定剂量下获得的FEV1的最大绝对变化来呈现。
