Monitoring plasma concentrations during drug therapy is used to optimize the dosage regimen of a drug in a patient. The establishment of effective plasma concentrations (within the therapeutic range) will in most cases result in effective drug response. By monitoring the plasma concentration, modifications can be made in the amount of drug administered to provide the best therapeutic response from the drug for a given patient. The concentration of drug required in the plasma to produce adequate therapeutic response have been established for several drugs and is referred to as the therapeutic range of the drug. Concentrations below this range are referred to as sub – therapeutic and concentrations higher are referred to as being in the toxic range.

The usefulness of information obtained from measuring the concentration of a drug in the plasma is highly dependent on how long the drug has been given at a specific dose and whether or not the blood samples were collected after establishment of “steady state” concentrations in this animal. Steady state is a term which means that the drug has been given long enough so that the plasma concentrations will remain the same with each subsequent dose. Obviously, the drug concentration will rise and fall between each dose, but the concentrations will remain within a certain range with each dose. The dosing time required for establishment of steady state is 5 to 6 times the half – life of the drug. This is the most appropriate time to collect samples to determine whether a patient has adequate plasma concentrations. The collection of the sample must also be timed with the time of the last dose. Most protocols will require collection of the sample at specific times after the last dose (peak or through samples).

DoNOT send whole blood.
Plasma (EDTA) or serum can be collected.

It is recommended that Heparin NOT be used as an anticoagulant. It is important to remove the cells from the sample as soon as possible since some drugs are known to redistribute from inside red blood cells. This would result in higher plasma concentrations.

See Appendix D

Revised 1-11-01 website designer Lou Ann Miller