Elimination Rate Constant, kel

The elimination rate constant is the proportionality constant relating the rate of change drug concentration and concentration OR the rate of elimination of the drug and the amount of drug remaining to be eliminated.

Defining Equations:

Units:

By inspection of Equations 5.61. or 5.6.2 it can be seen that the units for kel are time^-1, for example hr^-1, min^-1, or day^-1. In both equations the rate expression is divided by Cp or X, respectively to provide units for kel. Thus:

Determining Values of kel:

From the integrated equation presented on the previous page:

Equation 5.6.3 Ln(Cp) versus time

Plotting ln(Cp) values versus t, time should result in a straight line with a slope equal to -kel, thus kel can be calculated as:

Equation 5.6.4 kel from -slope

Thus with two value for Cp and time data a value for kel can be determined. With more than two Cp - time data points it is possible to plot the data on semi-log graph paper and draw a 'best-fit' line through the data points. This plot is shown on the previous page. The best answer for kel can be calculated by taking points at either end of the 'best-fit' line. This approach has been covered in more detail earlier in Chapter 2.

Note: We should now distinguish between the elimination rate and the elimination rate constant. The rate (tangent or slope, dCp/dt) changes as the concentration changes, however, for a linear model the rate constant (kel) is constant, it does not change.

¹ Ritschel, W.A. 1980 Handbook of Basic Pharmacokinetics, 2nd ed., Drug Intelligence Publications, p 413-426.

• Elimination rate constant has been discussed on the PharmPK listserv.

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Copyright 2001-3 David W. A. Bourne (david@boomer.org)