In the field of biochemistry, a pA2 value determines the important relationship between two drugs "competing" for effect on the same receptor. The "agonist" drug tries to affect the receptor. The "antagonist" drug attempts to block the agonist from working. The two drugs are "competitive" if increasing or reducing one drug decreases or increases the effect of the other, respectively. The pA2 value indicates the concentration of antagonist when double the agonist is required to have the same effect on the receptor as when no antagonist is present.

Calculate the base-10 logarithm of Kd. When we say log ("base 10") of x equals y, we mean that 10^y equals x. For example, log of 100 equals 2, log of 1,000 equals 3, and so forth. If Kd is 5, then use your calculator to see that log 5 approximately equals 0.7.

Multiply the result by negative one. Using our previous example, the product of 0.7 and -1 equals -0.7.

Check the result, which is the pA2 value.

#### Tip

The "p" in pA2 indicates its logarithmic scale, just like "pH." The "A" is for "antagonist." Lastly, the "2" is for dose ratio of increased agonist to original agonist concentration to counteract the antagonist.