The plasma conc-time profile for drugs that follow two-compartment pharmacokinetic models can be described by:

Since α is larger than β , so the term that describes the distribution process A e^{-α t} approaches zero faster than the term that describes the elimination process, Be^{-β t}.

When the distribution process is completed and during the elimination phase the plasma concentration can be described by:

The slope of a straight line is the rate of change of the dependent variable (the variable plotted on the Y-axis) relative to the change of the independent variable (the variable plotted on the X-axis). A straight line has one slope (constant slope), however the slope of a curve is changing at different points on the curve.

To determine the slope of a straight line, take two points on the line and determine the X and Y coordinates of each of these points. The slope can be determined from the following relationship:

For the linear scale:

For the semilog scale:

The plasma conc-time profile for the drugs that follow two-compartment model can be described by:

When the distribution process is completed and during the elimination phase the plasma concentration can be described by:

The difference between the plasma conc-time curve and the extrapolated line that represents drug elimination (the residual) is equal to :

A plot of the residuals versus time will represent the distribution process. The y-intercept is equal to A and the slope is - α /2.303.