In population pharmacokinetics, which covariate relationship is commonly used to scale clearance with body size?

• A. CL is proportional to weight raised to the 0.75 power (allometric scaling).
• B. CL scales with weight raised to the 2.0 power.
• C. CL scales with age raised to the power of 1.
• D. CL scales with body surface area raised to the first power.

Answer: (A)

In population pharmacokinetics, how a drug is cleared from the body often depends on body size, and this is modeled with allometric scaling. The most widely used relationship is that clearance scales with body weight raised to the 0.75 power. This sublinear scaling reflects that metabolic capacity and organ function don’t increase in direct proportion to weight, but grow more slowly with size. Across species and within humans, data support CL roughly proportional to weight^0.75, so you’d commonly express it as CL_i = CL_ref × (WT_i / WT_ref)^0.75. This keeps clearance predictions realistic for very small or very large individuals and aligns with physiological principles of metabolism and blood flow. Other options—scaling with age, or with body surface area to the first power, or with weight to an integer like 2—don’t fit the typical biology of clearance and tend to misrepresent how physiological processes scale with size.