Therapeutic Drug Monitoring
Reviewed
Also called: medicine levels blood test, therapeutic drug levels
Therapeutic drug monitoring (TDM) is testing that measures the amount of certain medicines in your blood.
Specimen: blood
Why this test may be ordered
- To determine the most effective dose when you first start taking a medicine that requires close monitoring.
- To regularly check that the medicine level remains effective and safe over time, especially if your health, other medicines, or age change.
- To check if you have symptoms of a serious side effect from the medicine.
- To make sure you are taking your medicine as prescribed.
Preparation and collection
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.
A health care professional will take a blood sample from a vein in your arm, using a small needle. This usually takes less than five minutes.
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.
Understanding results
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."
- Medicine level within the therapeutic range: Your results will show if medicine levels in your blood are in a range that is medically helpful but not dangerous. Limitation: This range varies, depending on the type of medicine you are taking and your own health needs. Next step: If you have questions about your results, talk to your provider.
- Medicine level outside the therapeutic range: If your results are not in this range, your provider may need to adjust your dosages. Limitation: This range varies, depending on the type of medicine you are taking and your own health needs. Next step: If your dosages change, you may get repeated tests until your medicine levels fall into the therapeutic range.
- The therapeutic range varies depending on the type of medicine and your own health needs.
- Many factors such as aging, pregnancy, illness, infection, and other medicines can affect how quickly your body processes the medicine, leading to changes in blood levels.
- Results are interpreted by your healthcare provider, who will adjust your dosage as needed.
What to do next
If you have questions about your results, talk to your provider.
- What is my specific therapeutic range for this medicine?
- How often should I have this test?
- What symptoms of side effects should I watch for?
- Do any of my other medicines or supplements affect the results?
Evidence source
Source: U.S. National Library of Medicine (MedlinePlus). Practices and reference ranges may differ in your country.
U.S. National Library of Medicine: Therapeutic Drug Monitoring