What does the symbol λz represent in pharmacokinetics?

• A. The terminal first-order elimination rate constant
• B. The absorption rate constant
• C. The distribution rate constant
• D. The elimination half-life

Answer: (A)

λz is the terminal elimination rate constant—the rate at which drug concentration declines during the final, elimination-dominated phase of the concentration–time profile. It’s determined from the slope of the log(concentration) versus time plot once distribution has subsided, reflecting the ongoing removal of drug from the body.

This rate constant connects to the elimination half-life by t1/2,z = 0.693/λz, but it itself is a rate constant, not the half-life. The absorption rate constant (k_a) governs how fast the drug enters systemic circulation, not the late elimination phase. The distribution rate constant describes movement between compartments during the distribution phase, not the terminal decline. So λz specifically characterizes the terminal elimination process.