基本介紹
EC50，半最大效應濃度(concentration for 50% of maximal effect,EC50)是指能引起50%最大效應的濃度。
EC50是藥物安全性指標。
其含義是:引起50%個體有效的劑量。
LD50/ED50、TD50/ED50、TC50/EC50等統稱為治療指數，是一類藥物的安全指標，通常其值越大越安全。需注意這些指標只反映治療作用與急性毒性的關係，並不能反映慢性毒性、過敏性。
The term half 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 EC50 of a graded dose response curve therefore represents the concentration of a compound where 50% of its maximal effect is observed. The EC50 of a quantal dose 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有許多種表達式，其中之一如圖所示:
EC50表達式及簡圖Y為觀察值，min和max分別為觀察到的最小值和最大值，Hill Slope指的是曲線最大斜率的絕對值(即曲線中點)。
EC50的求法
通常在用菌落直徑法求EC50時需要作一條回歸方程。
以菌絲淨生量計算不同質量濃度藥劑對各菌株生長的抑制率。計算公式如下：
抑制生長率( %)=（對照菌落直徑 -處理菌落直徑）/(對照菌落直徑-菌餅直徑） ×100%
EC50測真菌測得的數據以濃度取自然對數e的對數為橫坐標，通過查生物統計機率值換算表，以抑制率對應的幾率值作為縱坐標，求出每個試驗濃度的回歸方程和相關係數R，以抑制率為50%時對應的幾率值求出EC50的值以及其95%的置信區間。
機率值就是幾率的單位，共分為1-40，其實質就是常態分配的平均數加減標準誤所得的數值範圍，投射在肩線常態等差點上各個常態等差，就代表不同的死亡率，這樣就可以把死亡率的累計曲線用常態等差來計算，從而化成直線。
Limitations
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.
