ANOMALOUS PARAMETER ESTIMATES IN THE ONE-COMPARTMENT MODEL WITH 1ST-ORDER ABSORPTION

A difficulty sometimes encountered in least-squares fitting of a one-c ompartment model with first-order absorption is that estimated values (k(a) and k(e)) of the rate constants of absorption and elimination ar e almost identical, are highly correlated and have very large standard deviations. This anomaly is explained by the existence of a class of data sets for which least-squares estimates of the rate constants are complex quantities. Such data sets may arise either from an unfortunat e combination of random (e.g. assay) errors in the concentration value s if k(a) and k(e) are sufficiently similar in magnitude, or from dela yed absorption.