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Therapeutic Drug Monitoring

What is therapeutic drug monitoring (TDM)?

Therapeutic drug monitoring (TDM) is testing that measures the amount of certain medicines in your blood. It is done to make sure the amount of medicine you are taking is both safe and effective.

Most medicines can be dosed correctly without special testing. But for certain types of medicines, it can be hard to figure out a dose that provides enough medicine to treat your condition without causing dangerous side effects. TDM helps your provider find 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) is used to determine the best dosages for people taking certain types of hard-to-dose medicines. Below are some of the most common medicines that should be monitored.

Types of Medicine Medicine Names
vancomycin, gentamicin, amikacin
Heart drugs digoxin, procainamide, lidocaine
Anti-seizure drugs phenytoin, phenobarbital
Drugs treat autoimmune diseases cyclosporine, tacrolimus
Drugs that treat bipolar disorder lithium, valproic acid

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 vary depending on the medicine. Your health care provider will let you know 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 for before or after you take your regular dose.

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. The range varies depending on the type of medicine and your own health needs. If your results are not in this range, your provider may need to adjust your dosages. If your dosages are changed, you may get repeated tests until your medicine levels fall into the therapeutic range.

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

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


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The information on this site should not be used as a substitute for professional medical care or advice. Contact a health care provider if you have questions about your health.