Population pharmacokinetics describes the typical relationships between physiology and pharmacokinetics, the interindividual variability in these relationships, and their residual intraindividual variability. Knowledge of population kinetics can help one choose initial drug dosage, modify dosage appropriately in response to observed drug levels, make rational decisions regarding certain aspects of drug regulation, and elucidate certain research questions in pharmacokinetics. Patients with a disease for which a drug is intended are probably a better source of pharmacokinetic data than are healthy subjects. However, these types of data are contaminated by varying quality, accuracy, and precision, as well as by the fact that generally only sparse data are collected from each patient.
Although population pharmacokinetic PARAMETERS have been estimated either by fitting all individuals’ data together as if there were no kinetic differences, or by fitting each individual's data separately and then combining the individual parameter estimates, these methods have certain theoretical PROBLEMS that can only be aggravated when the deficiencies of typical CLINICAL data are present. The nonlinear mixed-effect analysis avoids many of these deficiencies and provides a flexible MEANS of estimating population pharmacokinetic parameters.