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Therapeutic Drug Monitoring
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Lab Tests

Therapeutic Drug Monitoring

Therapeutic drug monitoring (TDM) is a blood test used to ensure that the dosage of specific medications remains within a safe and effective range.

What is therapeutic drug monitoring (TDM)?

Therapeutic drug monitoring (TDM) is testing that measures the amount of certain medicines in your blood. It checks whether the amount of medicine you take is safe and effective.

All medicines do not require therapeutic monitoring. Usually, your health care provider can determine the right dose of medicine for you without special testing. But some medicines must maintain a certain level in your blood to work well. This is called the therapeutic dose level.

As soon as you take medicine, your body begins to process and remove it. This means the level of the medicine in your blood will vary over time. Many things can affect how quickly this process happens, including:

  • Aging
  • Pregnancy
  • Illness
  • Infection

If you are taking a medicine that needs to be kept at a certain level, it can be hard to figure out which dose provides enough medicine to treat your condition without causing dangerous side effects. TDM helps your provider figure out if you are taking the right dose.

What is it used for?

TDM helps determine the best dosages for certain hard-to-dose medicines, often those taken for a long time. Changes in your health, other medicines you take, or aging may require a dosage adjustment to avoid having too little medicine (ineffective) or too much (toxicity). TDM may also be used to ensure you are taking your medicine as prescribed.

Commonly Monitored Medicines

  • Antibiotics: Amikacin, gentamycin, vancomycin
  • Heart medicines: Amiodarone, digoxin, lidocaine, procainamide
  • Anti-seizure medicines: Phenobarbital, phenytoin
  • Autoimmune disease medicines: Cyclosporine, tacrolimus
  • Mental health medicines: Lithium, valproic acid
  • Antifungals: Itraconazole, voriconazole, posaconazole

Biologic Medicines

You may also need TDM if you are taking biologic medicines, which are made from living cells. Providers monitor these levels to ensure you do not have low levels (making the drug less effective), high levels (making the drug unsafe), or an immune response where your system rejects the drug.

Monoclonal antibodies (mAb) are common biologics used for cancer, organ transplant rejection, inflammation, autoimmune diseases, osteoporosis, and severe viral infections. Examples include adalimumab, apixaban, nivolumab, pembrolizumab, and rituximab.

Why do I need TDM?

You may need testing when you first start a medicine to find the most effective dose. Once determined, you may be tested regularly to ensure the medicine remains effective and safe. You may also need testing if you experience symptoms of a serious side effect.

What happens during TDM?

A health care professional will take a blood sample from a vein in your arm using a small needle. The process typically takes less than five minutes.

Will I need to do anything to prepare for the test?

Depending on the medicine, you may need to schedule your test before or after taking your regular dose. Inform your provider if you are taking other medicines or supplements, as these may affect your results.

Are there any risks to TDM?

There is very little risk to having a blood test. You may experience slight pain or bruising at the needle site, but symptoms usually go away quickly.

What do the results mean?

Your results will show if your medicine levels are in the therapeutic range—a range that is medically helpful but not dangerous. This range varies based on the medicine and your individual health needs. If your results are outside this range, your provider may adjust your dosage and repeat the test until your levels are stable.

Frequently Asked Questions

Several factors can lower the measured concentration: increased renal clearance (e.g., dehydration, high urine output, certain diuretics), drug interactions that speed metabolism (such as rifampin), pregnancy‑related plasma volume expansion, acute infections that alter protein binding, and genetic variations in liver enzymes. Reporting any new meds or changes in health helps the clinician adjust the dose.
For most TDM tests, timing is critical. Trough levels require the sample to be taken immediately before the next scheduled dose, and peak levels (if needed) are drawn at a specific interval after the dose (e.g., 1‑2 hours for aminoglycosides). If you miss the exact window, the result may be misleading, so coordinate with the lab or clinic to match the draw time to your dosing schedule.
The initial level is usually drawn just before the fourth dose (a “trough” sample) to see if the loading dose achieved the target range. After the dose is adjusted, repeat troughs are taken every 2‑3 days until the level stabilizes, then typically every 1‑2 weeks for long‑term therapy or sooner if kidney function changes.
Do not change the dose on your own. Contact your prescriber right away; they may hold the next dose, reduce the dose, or increase the dosing interval. In some cases, especially with drugs that can cause kidney toxicity (e.g., gentamicin), the clinician may order a repeat level within 12‑24 hours and monitor renal function labs.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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