Rudi Hayden
Urologist
Blood-tests Explained
Disclaimer:
This website is intended to assist with patient education and should not be used as a diagnostic, treatment or prescription service, forum or platform. Always consult your own healthcare practitioner for a more personalised and detailed opinion
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.
How are these tests used?
Not all drug levels need to be monitored. These tests are used to monitor blood levels of particular drugs that have a narrow range in which the drug is effective but not toxic. In addition, some drugs require monitoring because the amount of drug given does not correlate well with the amount of drug that may reach the bloodstream. Sometimes, the way that a particular drug is absorbed and metabolized can vary from person to person, or the physical or health status of a person can affect the drug level in the blood.
Through years of testing, the optimum therapeutic ranges for drugs have been determined. In these ranges, most people will be effectively treated without excessive side effects or symptoms of toxicity. The drug dosage necessary to reach this level must be determined for each individual. When a person starts on a monitored drug (or returns to it after an absence), the health practitioner adjusts the dose upwards and tests blood concentrations frequently until the appropriate steady level is achieved. If someone's levels are too high, the health practitioner will lower the dosage. Often, each different dosage level will take a short period of time to stabilize, so these corrections up and down may take place over a few days or weeks. It is important that people work closely with their health practitioner during this process and not make their own adjustments or stop taking their medication. Abrupt changes can sometimes worsen conditions and cause acute symptoms.
When are they ordered?
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. The frequency of testing required will depend on the drug and on the needs of the patient. If treatment does not appear to be fully effective, or if the person has either excessive side effects or signs of toxicity, then the health practitioner will order testing aimed at adjusting the drug dosage and maintaining levels within the therapeutic range. Sometimes, the health practitioner may need to re-evaluate the use of a specific medication and consider switching to another type of drug to better fit the person's condition.
The timing of blood collection is an important part of therapeutic drug monitoring. When a person takes a dose of drug, the amount in the blood rises for a period of time, peaks, and then begins to fall, usually reaching its lowest level, or trough, just before the next dose. To be effective, peak levels should be below toxic concentrations and trough levels should remain in the therapeutic range. Through experience and studies, health practitioners know when to expect peaks and troughs and will request blood sample collections as either trough levels (usually drawn just before the next dose), peak levels (for which timing varies depending on the drug), or sometimes as a randomly timed level. Consistent and accurate interpretation of the results depends on the timing of sample collection. If someone is unable to take his or her medication or have blood drawn at the appropriate time interval, then he or she should talk to their health practitioner before the sample is collected.
Monitored Drugs by Category
| DRUG CATEGORY | DRUGS | TREATMENT USE |
|---|---|---|
| Cardiac drugs | Digoxin, digitoxin, amiodarone, lidocane, quinidine, procainamide, N-acetyl-procainamide (a metabolite of procainamide) | Congestive heart failure, angina, arrhythmias |
| Antibiotics | Aminoglycosides (gentamicin, tobramycin, amikacin), vancomycin, chloramphenicol | Infections with bacteria that are resistant to less toxic antibiotics |
| Antiepileptics | Phenobarbital, phenytoin, valproic acid, carbamazepine, ethosuximide, sometimes gabapentin, lamotrigine, levetiracetam, topiramate, zonisamide, eslicarbazepine acetate, felbamate, lacosamide, oxcarbazepine, pregabalin, rufinamide, stiripentol, tiagabine, vigabatrin | Epilepsy, prevention of seizures, sometimes to stabilize moods |
| Bronchodilators | Theophylline, caffeine | Asthma, chronic obstructive pulmonary disorder (COPD), neonatal apnea |
| Immunosuppressants | Cyclosporine, tacrolimus, sirolimus, mycophenolate mofetil, azathioprine | Prevent rejection of transplanted organs, autoimmune disorders |
| Anti-cancer drugs | Methotrexate, all cytotoxic agents | Psoriasis, rheumatoid arthritis, various cancers, non-hodgkin lymphomas, osteosarcoma |
| Psychiatric drugs | Lithium, valproic acid, some antidepressants (imipramine, amitriptyline, nortriptyline, doxepin, desipramine) | Bipolar disorder (manic depression), depression |