Terminal elimination phase

The terminal elimination phase refers to the portion of a plot of a log[drug]_p versus time graph, usually for a two-compartment drug, that is linear, indicating that absorption and distribution are complete and that elimination is solely responsible for loss of drug from the central compartment. The first order elimination rate constant for this elimination phase (β for a two-compartment drug) is smaller than the corresponding value for k_el would be (if the drug behaved with one-compartment kinetics), because of slow redistribution of drug back into the plasma from the tissues, slowing the rate at which drug could otherwise be eliminated from the central compartment. This point is discussed in more depth under the manual entry for half-life of elimination.

The discrepancy between k_el and β also has an impact upon both the meaning and of the calculated value for apparent volume of distribution, with the nature of the impact depending upon the approach chosen to calculate AV_D.