EC50

The termhalf maximal effective concentration(EC₅₀) refers to the concentration of a drug, antibody or toxicant which induces a response halfway between the baseline and maximum after a specified exposure time. It is commonly used as a measure of drug's potency.

The EC₅₀of agradeddose response curve therefore represents the concentration of a compound where 50% of its maximal effect is observed. The EC₅₀of aquantaldose response curve represents the concentration of a compound where 50% of the population exhibit a response, after a specified exposure duration.

It is also related to IC50 which is a measure of a compound's inhibition (50% inhibition). For competition binding assays and functional antagonist assays IC50 is the most common summary measure of the dose-response curve. For agonist/stimulator assays the most common summary measure is the EC₅₀.

In biological systems, small changes in ligand concentration typically result in rapid changes in response, following a sigmoidal function.The inflection point at which the increase in response with increasing ligand concentration begins to slow is the EC₅₀. This can be determined mathematically by derivation of the best-fit line. While relying on a graph for estimation is more convenient, this method yields less precise and less accurate results.

EC50有许多种表达式，其中之一如图所示：

EC50表达式及简图Y为观察值，min和max分别为观察到的最小值和最大值，Hill Slope指的是曲线最大斜率的绝对值（即曲线中点）。

通常在用菌落直径法求EC50时需要作一条回归方程。

以菌丝净生量计算不同质量浓度药剂对各菌株生长的抑制率。计算公式如下：

抑制生长率( %)=（对照菌落直径 -处理菌落直径）/(对照菌落直径-菌饼直径） ×100%

测真菌测得的数据以浓度取10的对数为横坐标，通过查生物统计机率值换算表，以抑制率对应的几率值作为纵坐标，求出每个试验浓度的回归方程和相关系数R，以抑制率为50%时对应的几率值求出EC50的值以及其95%的置信区间。
机率值就是几率的单位，共分为1-40，其实质就是正态分布的平均数加减标准误所得的数值范围，投射在肩线常态等差点上各个常态等差，就代表不同的死亡率，这样就可以把死亡率的累计曲线用常态等差来计算，从而化成直线。

The effects of a stressor or drug generally depend on the exposure time. Therefore, the EC₅₀(and similar statistics) will be a function of exposure time. The exact shape of this time function will depend upon the stressor (e.g., the specific toxicant), its mechanism of action, the organism exposed, etc. This time dependency hampers the comparison of potency or toxicity between compounds and between different organisms.