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. Drugs that are monitored tend to have a narrow "therapeutic index" – the blood level required to be effective is close to the level that causes significant side effects and/or toxicity. 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 is evaluated, but it is not usually necessary to determine the concentration of the drug in the bloodstream. Examples of this include high blood pressure medications and many of the antibiotics given to treat bacterial infections. If the 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 are monitored therapeutically are taken for a lifetime. They must be maintained at steady concentrations year after year while the patient ages and goes through life events such as pregnancies, temporary illnesses, infections, emotional and physical stresses, accidents, and surgeries. Over time, patients 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 patient does not take the medication regularly as prescribed), identifies the effect of drug interactions (may cause drug concentrations that are higher or lower than expected at a given dosage), and helps to tailor dosages to fit the current needs of the specific patient. Along with tests such as BUN, creatinine, and liver panel to check kidney and liver function, monitoring can help identify decreases in the efficiency of and dysfunctions in the body in metabolizing and eliminating therapeutic drugs.