Fig. 3. Measured/predicted plasma drug concentrations from each of 12 subjects after oral administration of 8 mg immediate-release (IR) hydromorphone. The y-axis displays the ratio of measured to predicted drug concentrations for all subjects. The predicted concentrations were calculated from the pharmacokinetic model as estimated by NONMEM. The line drawn at y = 1 represents perfect prediction. (Top ) The model using a linear spline input function; (bottom ) data of the model assuming first-order absorption.