Why is plasma concentration used to predict drug effect?

• A. Equilibrium between plasma and the target site
• B. Drug color
• C. Molecular weight
• D. Route of administration

Answer: (A)

Plasma concentration is used to predict drug effect because, for many drugs, the amount of drug available in the blood reflects how much can reach the site of action. The body distributes drugs between plasma and tissues until an equilibrium is reached, so changes in plasma levels tend to track changes at the target site. Since measuring the effect directly at the receptor or tissue is often impractical, clinicians rely on plasma levels as a practical surrogate to gauge how strong the effect will be and how long it will last. The relationship between plasma concentration and effect is captured in concentration–response relationships, helping to dose drugs to achieve the desired effect while avoiding toxicity. Note that the active portion is the free drug, and protein binding can influence how well plasma levels translate to effect, but plasma concentration remains the most feasible predictor of effect in many cases.