The absorption rate constant of a drug as determined by the Wagner-Nelson or Loo-Riegelman method, by nonlinear least-squares regression analysis, or by the method of residuals is in error when drug. removal from the absorption site is due not only to absorption but also to other competing processes resulting in reduced availability, regardless of the kinetics of such processes.
These methods overestimate the true absorption rate constant, the degree of overestimation increasing with decreasing bioavailability. Accordingly, calculation of absorption rate constants from plasma drug concentration-time data by commonly employed methods cannot be performed with any 'degree of confidence unless the drug is known to be completely absorbed as such. Keyphrasea 0 Absorption rate constants, true, apparentdrugs with incomplete availability, calculation 0 Drug absorption rate constants, true, apparent-calculation. incomplete drug availability 0 Bioavailability-absorption rate constants for drugs with reduced availability, unreliability of plasma drug concentration-time data Plasma drug concentration-time profiles-unreliability for use in calculating absorption rate constants of drugs with reduced availability
Several methods have been commonly employed in pharmacokinetics for the calculation of drug absorption rate constants from plasma drug concentration-time data. Probably the most commonly used method for the determination of absorption rate constants is that of Wagner and Nelson (1,2). The following relationship was developed to calculate the percent of an oral dose absorbed at various times after administration (2):
Program No. '7-1354.