Before we begin: Therapeutic drug monitoring (TDM) measures the amount of certain medicines in your blood. TDM checks if your medicine dose is safe and effective. Learn more.
This tutorial walks you through everything you need to know about therapeutic drug monitoring. We explain it in plain language, step by step, so whether you are a student, a patient, or simply curious, you will come away with a solid understanding.
We cover what it is, why it matters, how it is diagnosed, and what you can do about it. Key facts are called out along the way.
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 don’t 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 the medicine from your body. This means that 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’re 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 of your medicine.
Other names: medicine levels blood test, therapeutic drug levels
What is it used for?
Therapeutic drug monitoring (TDM) helps determine the best dosages if you’re taking certain hard-to-dose medicines. Many medicines that require TDM are ones that you might take for a long time. While you are taking the medicines, you may have changes that could affect the medicine levels in your blood. These may include:
- Changes in your health.
- Any other medicines you take.
- Aging.
For each of these changes, you may need to change your medicine dose. Otherwise, you might have too little medicine, which might not be effective, or too much (toxicity), which might not be safe.
TDM may also be used to make sure you are taking your medicine as prescribed.
There are many different medicines that are monitored; some of the most common ones include:
| Types of Medicine | Examples |
|---|---|
| Antibiotics | |
| Amikacin, gentamycin, vancomycin | |
| Heart medicines | Amiodarone, digoxin, lidocaine, procainamide |
| Anti-seizure medicines | Phenobarbital, phenytoin |
| Medicines that treat autoimmune diseases | Cyclosporine, tacrolimus |
| Medicines that treat mental health conditions | Lithium, valproic acid |
| Medicines that treat fungal infections (antifungals) | Itraconazole, voriconazole, posaconazole |
You may also need therapeutic drug monitoring if you are taking certain biologic medicines. This type of medicine is made from living cells instead of chemicals. These cells can be used to make proteins (like insulin), antibiotics, or altered versions of your cells. By working with specific proteins or cells in your immune system, biologic medicines can often treat conditions that traditional medicine cannot (such as cancer and autoimmune diseases).
Like with other medicines that need therapeutic monitoring, however, your provider may closely monitor the levels of biologic medicine in your blood to make sure you don’t have:
- Low levels of the medicine in your blood, which could make the drug less effective.
- High levels of the medicine in your blood, which could make the drug unsafe.
- An immune response to the medicine, which is when your immune system rejects the drug.
Monoclonal antibodies (mAb) are some of the most common biologic medicines. Laboratories make monoclonal antibodies to only work with specific cells, such as those that are cancerous. Because their targets are so specific, these artificial antibodies can make treatment much more effective. Your provider may monitor mAb levels during treatment for:
- Cancer.
- Organ transplant rejection.
- Inflammation and autoimmune diseases.
- Osteoporosis.
- Severe viral infections.
- Nervous system disorders.
Other examples of biologic medicines include:
- Adalimumab.
- Apixaban.
- Nivolumab.
- Pembrolizumab.
- Rituximab.
Why do I need TDM?
You may need testing when you first start taking a medicine. This helps your provider figure out the most effective dose for you. Once that dose is determined, you may be tested regularly to make sure the medicine is still effective without being harmful. You may also need testing if you have symptoms of a serious side effect. Side effects can vary, depending on the medicine. Your provider will tell you which symptoms to watch out for.
What happens during TDM?
A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.
Will I need to do anything to prepare for the test?
Depending on the type of medicine you are taking, you may need to schedule your test before or after taking your regular dose. Be sure to tell your provider if you are taking other medicines or supplements, in case they affect the results.
Are there any risks to TDM?
There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.
What do the results mean?
Your results will show if medicine levels in your blood are in a range that is medically helpful but not dangerous. This is called the “therapeutic range.” This range varies, depending on the type of medicine you are taking and your own health needs. If your results are not in this range, your provider may need to adjust your dosages. If your dosages change, you may get repeated tests until your medicine levels fall into the therapeutic range.
Learn more about laboratory tests, reference ranges, and understanding results.
Also known as: medicine levels blood test, therapeutic drug levels.
Key Takeaways
Understanding therapeutic drug monitoring is an important part of taking charge of your health. Here is what to remember:
- Know the signs — Recognizing early symptoms can lead to earlier diagnosis and better outcomes.
- Talk to your provider — If you have concerns or a family history of this condition, bring it up at your next checkup.
- Stay informed — Medical knowledge evolves. Keep learning and asking questions.
- You are not alone — Many people deal with this condition. Support groups and educational resources can help.