The absorption of some drugs does not start immediately after oral administration. This may be due to the formulation characteristics, or due to physiological factors.
The delay time before starting drug absorption is the lag time for drug absorption.
When the method of residuals is applied in presence of lag time for absorption, the extrapolated line from the elimination phase and the line obtained from the residuals versus time plot intersect at a time point after time zero.
The x-coordinate of the point of intersection on the x axis is the lag time.
