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 bloodstream. Monitored drugs tend to have a narrow "therapeutic index," a ratio between the toxic and therapeutic doses of medications. For some drugs, maintaining this steady state is not as simple as giving a standard dose of medication. Each person will absorb, metabolize, utilize, and eliminate drugs at different rates based upon their age, general state of health, genetic makeup, and the interference of other medications that they are taking. These rates may change over time and vary from day to day. Changes in the rate may also occur in various disease states or through interaction with other medications.

Not all medications require therapeutic monitoring. Most drugs have a wide therapeutic index and can be prescribed based upon pre-established dosing schedules. The effectiveness of these treatments has been evaluated, but monitoring the concentration of the drug in the bloodstream is not required for dosing. Examples of drugs that do not require 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 accommodates them. It identifies patient noncompliance (when the person does not take the medication regularly as prescribed) and the effect of drug interactions, which may cause drug concentrations that are higher or lower than expected at a given dosage, and helps to personalize a dose to fit the specific needs of a patient. Along with tests such as BUN, creatinine, and liver function tests, monitoring can help identify decreases in the efficiency of and dysfunctions in the body's ability to metabolize and eliminate therapeutic drugs. Testing may also determine how a medication interacts with other necessary drugs. 

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