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This article waslast modified on 24 May 2021.

What is therapeutic drug monitoring?

Therapeutic drug monitoring is the measurement of specific drug concentrations in the blood at timed intervals, in order to maintain a relatively constant concentration of the medication in the circulation. 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 cannot be achieved by simply 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 using standard dosing schedules. The effectiveness of these treatments has been evaluated, but routine monitoring of 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 drug level, including pregnancies, temporary illnesses, infections, emotional and physical stresses, accidents, and surgical operations. Over time, people may acquire other chronic conditions that also require lifetime medication and these 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 the effects of these changes and is used to keep the dose of drug right. It can identify patient noncompliance (when the person does not take the medication regularly as prescribed) and can detect the effects of other drugs, which may cause drug concentrations that are higher or lower than expected at a given dosage. Thus, thereapeutic drug monitoring helps to personalise dosage to fit the specific needs of a patient. Along with tests such as urea, creatinine, and liver function tests, monitoring can help to identify  any changes in the body's ability to metabolise and eliminate therapeutic drugs. Testing can also determine how a medication interacts with other prescribed drugs.

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About Therapeutic Drug Monitoring
  • How are these tests used?

    Not all drug levels need to be monitored. These tests are used to monitor blood concentrations of particular drugs that have a narrow dose range in which the drug is effective but not toxic. In addition, some drugs require monitoring because the amount of drug administered does not correlate well with the amount of drug that reaches the bloodstream. Sometimes, the way that a particular drug is absorbed and metabolised 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 effective drug concentrations in the blood have been determined. Within these concentration ranges, most people will be effectively treated without excessive side effects or symptoms of toxicity. The drug dosage necessary to reach this concentration must be determined for each individual. When a person starts on a monitored drug (or returns to it after an absence), the healthcare professional adjusts the dose upwards and tests blood concentrations frequently until the appropriate steady level is achieved. If someone's levels are too high, the healthcare professional will lower the dosage. Often, each different dosage level will take a short period of time to stabilise, so these corrections up and down may take place over a few days or weeks. It is important that people work closely with their healthcare professional 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 requested?
    Concentrations of monitored drugs are often tested frequently when a person is first put on a drug. Once a person's results are in the therapeutic range and his or her clinical signs indicate that the treatment is appropriate, then the healthcare professional may monitor the drug at less frequent intervals as needed to ensure that the drug concentration 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 excessive side effects or signs of toxicity, then the healthcare professional will request testing aimed at adjusting the drug dosage and maintaining levels within the therapeutic range. Monitoring may also take place during illness or other changed circumstances such as pregnancy. Sometimes, the healthcare professional 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. After a person takes a dose of drug, the amount in the blood rises for a short period of time, reaches a peak and then begins to fall, usually reaching its lowest level (the ‘trough level’) just before the next dose. For the drug to be most effective, peak levels should be below toxic concentrations and trough levels should remain in the therapeutic range. Through experience and studies, healthcare professionals know when to expect peaks and troughs and will request blood sample collections as either trough levels (usually collected 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, so it is important to be clear when your blood sample needs to be taken in relation to when you took the drug. If you are unable to take your medication or have blood collected at the appropriate time, then you should talk to your healthcare professional before the sample is collected.


  • Monitored Drugs by Category
    There are several categories of drugs that may require monitoring, as summarised here.
    Drug Category Drugs Treatment Use
    Cardiac drugs Digoxin, digitoxin, amiodarone Congestive heart failure, angina, arrhythmias
    Antibiotics Aminoglycosides (gentamicin, tobramycin, amikacin, vancomycin, chloramphenicol) Infections with bacteria that are resistant to less powerful antibiotics
    Antiepileptics Phenobarbital/phenobarbitone, phenytoin, valproic acid, carbamazepine, (rarely: ethosuximide, gabapentin, lamotrigine, levetiracetam, topiramate, zonisamide, eslicarbazepine, felbamate, lacosamide, oxcarbazepine, pregabalin, rufinamide, stiripentol, tiagabine, vigabatrin) Epilepsy, prevention of seizures, sometimes to stabilise moods

    Theophylline, caffeine


    Asthma, Chronic obstructive pulmonary disorder (COPD), neonatal apnoea
    Immunosuppressants Ciclosporin, tacrolimus, sirolimus, mycophenolate mofetil 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 Bipolar disorder (manic depression), depression
  • Common Questions

    1. How does the doctor work out how much drug to give me?
    There are many factors to consider. Some of them are your weight, body composition, age, and general health and nutritional status. Other factors to consider are whether you have any acute or chronic disease such as kidney, liver, or heart conditions, of if you have suffered burns, shock, or trauma. Your healthcare professional takes these into account when prescribing a dosage quantity and frequency and then tailors your medication based upon the results of therapeutic monitoring.

    2. What should I do if I forget to take my medication on time?
    Do not double your dose the next time. Ask your doctor or pharmacist to find out what you should do.

    3. Can I monitor the drugs myself at home?
    No. Blood must be collected at particular times and tests done on special laboratory equipment.