In a semi-log plot of concentration versus time for a drug with elimination, what does the slope primarily indicate?

In a semi-log plot of concentration versus time for a drug with elimination, what does the slope primarily indicate?

• A. Clearance
• B. Volume of distribution
• C. Rate of elimination
• D. Bioavailability

Answer: (C)

In a semi-log plot of concentration versus time for a drug that is eliminated, a straight line indicates first-order kinetics, where the concentration declines exponentially over time. The slope of that line corresponds to the elimination rate constant (approximately -k; if you’re using base-10 logs, it’s -k/2.303). This slope tells you how quickly the drug is being eliminated from the body—the larger the magnitude, the faster the elimination. It's tied to the rate of elimination through the relationship -dC/dt = k·C, so the slope directly reflects how swiftly the drug exits, not the amount of drug that enters the system (bioavailability) or how extensively it distributes (volume of distribution) or how quickly it’s cleared in a separate sense. Clearance and volume of distribution relate to k and the slope only through the interplay Cl = k·Vd, but the slope itself mainly indicates the elimination rate constant and thus the rate of elimination.

In a semi-log plot of concentration versus time for a drug that is eliminated, a straight line indicates first-order kinetics, where the concentration declines exponentially over time. The slope of that line corresponds to the elimination rate constant (approximately -k; if you’re using base-10 logs, it’s -k/2.303). This slope tells you how quickly the drug is being eliminated from the body—the larger the magnitude, the faster the elimination.

It's tied to the rate of elimination through the relationship -dC/dt = k·C, so the slope directly reflects how swiftly the drug exits, not the amount of drug that enters the system (bioavailability) or how extensively it distributes (volume of distribution) or how quickly it’s cleared in a separate sense. Clearance and volume of distribution relate to k and the slope only through the interplay Cl = k·Vd, but the slope itself mainly indicates the elimination rate constant and thus the rate of elimination.