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, or infection. If you're taking a medicine that needs to be kept 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. For example, changes in your health, any other medicines you take, and aging might require changes in 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:

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.

If you have questions about your results, talk to your provider.

Learn more about laboratory tests, reference ranges, and understanding results.