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The response of individual patients to the same drug given in the same dose varies considerably. Many patients will experience the desired drug effect, others may experience no effects, yet others may suffer from well-known adverse drug reactions, and very rarely individual patients will die from severe side effects. TDM is based on the principle that for some drugs there is a close relationship between the plasma level of the drug and its clinical effect. If such a relationship does not exit TDM is of little value. Like any diagnostic test, the measurement of plasma level is justified only when the information provided is of potential therapeutic benefit. The clinical value of plasma level monitoring depends on how precisely the treatment outcome can be defined. the extent and rate of drug absorption, distribution, tissue binding, biotransformation, and excretion, which can vary markedly between individual patients due to differences in gender, age, morbidity, smoking or eating habits, differential expression of drug metabolizing enzymes or drug transporters or other factors. Therefore drug concentrations in blood resulting after a given dose differ by tenfold or more between individual patients. The pharmacokinetic phenotype of an individual can be measured by analysis of drug concentrations in blood plasma or serum, by TDM. TDM is well established for mood stabilizers, especially for lithium. For other neuropsychiatric drugs, routine TDM is rare. TDM will be useful if the following criteria are met:
1) the drug in question has a narrow therapeutic range,
2) a direct relationship exists between the drug or drug metabolite levels in plasma and the pharmacological or toxic effects,
3) the therapeutic effect cannot be readily assessed by the clinical observation,
4) large individual variability in steady state plasma concentration exits at any given dose
5) appropriate analytic techniques are available to determine the drug and metabolite levels.
TDM is a method used to provide optimal pharmaceutical therapy. Knowing the plasma concentration of psychoactive medication helps monitor the side effects of the medication in order to prevent toxicity and increase the responsiveness of the medication. In therapeutic drug monitoring, psychoactive drugs are divided into 4 groups:
1. The most recommended group consists of: lithium and carbamazepine. Monitoring the levels of valproic acid is important in order to prevent toxicity from occurring.
2. Clozapine and similar medications are considered one group of medication whose side effects should be monitored.
3. On the other hand, medications that should be closely observed but have an elapsed response rate are included into the group monitored for response time.
4. This group of medication is often monitored in order to place a pre-diagnosis for treatment-resistant patients who may have genetic polymorphism and choose the correct medication.
More studies with adequate design are still required to obtain more data on plasma concentrations and therapeutic response relationships.