Which statement best defines population pharmacokinetics (popPK)?

• A. The PK of a drug in a single patient.
• B. PK using linear models ignoring covariates.
• C. Population PK using nonlinear mixed-effects models to identify covariates and variability among individuals.
• D. The PK of drugs only in pediatric populations.

Answer: (C)

Population pharmacokinetics studies how drug concentrations vary across individuals and what drives that variation. It uses nonlinear mixed-effects modeling to estimate the typical PK parameters for the population while capturing between-subject variability, and to identify covariates (such as weight, age, renal function) that explain differences in parameters like clearance and volume of distribution. This approach handles nonlinear kinetics and sparse data by combining fixed effects (population means) with random effects (individual deviations) and by estimating how covariates modify those parameters. That combination—nlme modeling plus covariate effects and variability among individuals—is what makes the statement describing population PK the best definition. The other options describe single-patient PK, linear models without covariates, or use confined populations, which don’t capture the population-wide variability and covariate relationships central to popPK.