In some cases, graded dose-response curves are not well-suited to describe the relationship between drug concentration and its biological response, particularly if the response is an “either-or” event, such as prevention of arrhythmia, convulsions, or death. In this case, we can describe the dose–response relationship by depicting the proportion of a population that responds to it and these responses are known as quantal responses.
Graded responses can be converted to quantal responses by designating a predetermined level of the graded response as the point at which a response occurs or not. For example, a quantal dose–response relationship can be determined in a population taking an antidiabetic drug, glimepiride. A positive response is defined as a fall of 1mmol/Lin blood glucose levels. Quantal dose–response curves will have shapes similar to log dose–response curves, and the ED50 will represent the drug dose that causes the predetermined therapeutic response in half of the population.