Therapeutic drug monitoring (TDM) is utilized to monitor and adjust drug doses based on measured plasma concentrations. It is desirable that these drug concentrations fall within a narrow therapeutic range. An underdose or overdose may easily occur and is undesirable. TDM is employed when clinical findings, such as vital signs, or results of other laboratory testing do not indicate whether an optimal drug concentration has been achieved. The pharmacologic effect of the drug may depend upon achieving a specific concentration. Overdosage may have serious toxicities. If the level is too low, then the drug may not have efficacy. In general, the ideal range between highest and lowest effective and safe level is the therapeutic range of the drug.
Unless a drug is being administered by continuous infusion, there are variations in concentration depending upon dosing intervals. Depending upon the route of administration, a peak (highest) concentration occurs at an interval following administration. For injection, this depends upon the volume of distribution of the drug. For oral ingestion, rate of absorption and distribution is important. The half-life of the drug and the ability of the body to metabolize or excrete the drug determine the trough (lowest) concentration of drug before the next dose.
It may be necessary to determine how a patient's renal or hepatic status may affect drug concentrations. For example, with drugs such as aminoglycoside antibiotics excreted via the kidney, the patient's renal function is important, and a serum creatinine level may be obtained. The very young and older persons may have different levels of metabolism or volume of distribution, so age may play a role in determining proper drug administration.
The timing of phlebotomy to obtain patient samples is critical in relationship to the timing of drug dosing.
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- Examples:
For some antibiotics, such as aminoglycosides, there is risk for renal toxicity and ototoxicity. Based upon dosing intervals, a peak drug level is measured at a specified time following the last dose, while a trough level is measured at a specified interval before the next dose. The dose is adjusted to maintain the drug level within defined peak and trough ranges.
For coumadin, the desired effect is anticoagulation. Since this drug antagonizes vitamin K dependent coagulation factor synthesis, then the prothrombin time is used to gauge the efficacy of coumadin therapy. The desired range is a prothrombin time of 2.5 (in a range of 2 to 3) greater than the normal range. To adjust varability among laboratory testing methods for prothrombin time, an international normalized ratio (INR) value is reported.
Random drug testing is most often employed for detecting usage of drugs of abuse.
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