According to the vertical line test for a quantal-graded response curve, if the line does not bisect both the desired therapeutic effect and the adverse effects, the therapeutic index is larger.

• A. True
• B. False
• C. Cannot determine
• D. Not applicable

Answer: (A)

Therapeutic index reflects how wide the safety margin is between the dose that gives a therapeutic effect and the dose that causes toxicity. In a quantal-graded view, you have two curves: the percent of patients achieving the desired effect as dose increases, and the percent experiencing adverse effects as dose increases. The doses at which 50% of people show therapeutic effect and 50% show toxicity are ED50 and TD50, and the TI is TD50 divided by ED50. A larger TI means a bigger gap between these two doses.

The vertical line test uses a single dose to see where you sit on both curves. If the line does not hit the 50% points on both curves at the same dose—that is, ED50 and TD50 are at different doses—then there is a safety margin and the TI is greater than 1. Only if those 50% points coincide would ED50 equal TD50 and TI equal 1. So the statement is true.