Allometric scaling exponent for clearance across body weight is typically?

• A. 1.25
• B. 0.75
• C. 0.5
• D. 1.0

Answer: (B)

Allometric scaling uses a power-law relation between clearance and body weight: CL ∝ BW^b. The value that’s typically used for clearance across different body sizes is about 0.75. This comes from the idea that metabolic rate—and thus the body's capacity to eliminate drugs—scales with body mass roughly as BW^(3/4). So as weight increases, clearance rises, but less than proportionally. For example, doubling body weight would increase clearance by 2^0.75 ≈ 1.68, not by a factor of 2. This 0.75 exponent is the standard that helps translate clearance across species and body sizes in pharmacokinetics. Using a linear scaling (b = 1) would overstate the change, while exponents like 0.5 or 1.25 would misfit observed physiological scaling.