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Therapeutic Drug Monitoring

Therapeutic drug monitoring (TDM) is performed to ensure the blood concentration of drugs are maintained within a range where the concentration is high enough to produce the desired effect but low enough to prevent toxicity. This frequent monitoring allows for optimization of drug dosing for each individual patient. TDM is most commonly performed for drugs with a narrow therapeutic index, but may also be performed in cases where pharmacokinetics may be altered, to ensure medication compliance or to investigate a suspected drug interaction.

TDM samples must be collected at specific sampling times in order to correctly interpret the result. For most TDM measurements, samples should be collected immediately prior to the next dose of drug (predose or trough sample). Samples collected too early may have concentrations erroneously high. In addition, patients must achieve steady state before any TDM samples should be collected. Steady state refers to the equilibrium in blood concentrations achieved between dosing and excretion and typically is achieved within 4-7 half lives of the drug. Steady state must be reestablished after any change in dose or frequency of administration for a drug. Blood concentrations obtained before a steady state is achieved may be erroneously low and any increase in the dosage based on such a result could produce toxic concentrations.

Drugs that commonly are monitored by TDM include digoxin, cyclosporine, tacrolimus, sirolimus, lithium, vancomycin, gentamicin, phenobarbital and phenytoin.