On a typical dose-response curve, what is the term for the dose at which there is no detectable effect (intersection with the x-axis)?

• A. Threshold dose
• B. Response coordinate
• C. Linear gradient
• D. Average gradient

Answer: (A)

When you look at a dose-response curve, the point where the response first becomes detectable corresponds to the dose that yields zero effect on the axis—the threshold. This threshold dose marks the boundary below which there’s no observable effect and above which effects begin to appear and typically increase with dose. The other terms don’t describe that zero-effect point: a response coordinate isn’t a standard term for onset of effect, and linear or average gradient refer to the slope or rate of change, not the initial zero-response point. Therefore, the threshold dose is the correct term.