Introduction

Therapeutic Drug Monitoring (TDM) is testing that measures the amount of certain medicines in your blood. It is done to make sure the amount of medicine you are taking is both safe and effective. Most medicines can be dosed correctly without special testing. But for certain types of medicines, it can be hard to figure out a dose that provides enough medicine to treat your condition without causing dangerous side effects. TDM helps your provider find out if you are taking the right dose of your medicine.

There are numerous variables that influence the interpretation of drug concentration data: time, route and dose of drug given, time of blood sampling, handling and storage conditions, precision and accuracy of the analytical method, validity of pharmacokinetic models and assumptions, co-medications and, last but not least, clinical status of the patient.

Many different professionals are involved with the various elements of drug concentration monitoring, which is a truly multidisciplinary process. Because failure to properly carry out any one of the components can severely affect the usefulness of using drug concentrations to optimize therapy, an organized approach to the overall process is critical.

There are a number of drugs for which desired (or toxic) effects cannot readily be assessed clinically, but are related to the amount of drug in the body. In such cases, the logical approach to control the effect of the drug is to limit the amount that is given to the patient. Pragmatically, using standard doses that will produce a satisfactory response in the majority of patients can achieve this. Such an approach has been widely used in medicine and is undoubtedly effective for a large number of drugs (e.g., penicillins).

Drugs which are monitored

When an effect, such as changes in blood pressure, pain or serum cholesterol is readily measured, the dose of a drug should be adjusted according to the response. Monitoring drug concentration is more useful when drugs are used to prevent an adverse outcome, for example, graft rejection or to avoid toxicity, as with aminoglycosides. A drug should satisfy certain criteria to be suitable for therapeutic drug monitoring. Examples include: narrow target range, significant pharmacokinetic variability, a reasonable relationship between plasma concentrations and clinical effects, established target concentration range and availability of cost-effective drug assay.

Drug assays are costly, so the reason for monitoring and the additional information to be gained (if any) should be carefully considered. For some drugs, therapeutic drug monitoring helps to increase efficacy (vancomycin), to decrease toxicity (paracetamol) and to assist diagnosis (salicylates). Routine monitoring is not advocated for most drugs. Only clinically meaningful tests should be performed.

Levels of monitored drugs are often tested frequently when a person is first put on a drug regimen. Once a person's results are in the therapeutic range and his or her clinical signs indicate that the treatment is appropriate, then the health practitioner may monitor the drug at regular intervals and as needed to accommodate changes in patient status and to ensure that the drug stays in the therapeutic range.

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Regards,

Mary Wilson,

Editorial office,

Clinical Pharmacology and Toxicology Research

E-mail: pharmatoxicol@eclinicalsci.com