Therapeutic Drug Monitoring

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Also known as: TDM
Formal name: Therapeutic Drug Monitoring

What is therapeutic drug monitoring?

Therapeutic drug monitoring is the measurement of specific drugs at timed intervals in order to maintain a relatively constant concentration of the medication in the blood. Monitored drugs tend to have a narrow "therapeutic index," – the difference between the toxic and therapeutic doses of medications. For some drugs, maintaining this steady concentration in the blood is not as simple as giving a standard dose of medication to everyone. Each person will absorb, metabolise, utilise, and eliminate drugs at different rates based upon their age, general state of health, genetic makeup, and the influence of other medications that they are taking. These factors may change over time and vary from day to day or with various disease states.

Not all medications require therapeutic drug monitoring. Most drugs have a wide therapeutic index and can be prescribed based upon standard dosing schedules. The effectiveness of these treatments has been evaluated, but routine monitoring the concentration of the drug in the bloodstream is not required. Examples of drugs that do not require concentration monitoring include high blood pressure (hypertension) medications and many of the antibiotics given to treat bacterial infections. If an infection resolves with a given antibiotic or if blood pressure is lowered with the prescribed blood pressure medication, then the treatments have been effective.

Why is it important?
Many of the drugs that require therapeutic monitoring are taken for a lifetime. They must be maintained at steady concentrations year after year while the person ages and goes through life events that may alter that individual's therapeutic level, including pregnancies, temporary illnesses, infections, emotional and physical stresses, accidents, and surgeries. Over time, people may acquire other chronic conditions that also require lifetime medication and that may affect the processing of their monitored drugs. Examples of these conditions include cardiovascular disease, kidney disease, thyroid disease, liver disease, and HIV/AIDS.

Therapeutic drug monitoring follows these changes and uses them to keep the dose of drug right. It identifies patient noncompliance (when the person does not take the medication regularly as prescribed) and the effects of other drugs, which may cause drug concentrations that are higher or lower than expected at a given dosage, and helps to personalise a dose to fit the specific needs of a patient. Along with tests such as urea, creatinine, and liver function tests, monitoring can help identify  any changes in the body's ability to metabolise and eliminate therapeutic drugs. Testing may also determine how a medication interacts with other necessary drugs.

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