Therapeutic drug levels


Therapeutic drug level are laboratory tests to look for the presence and the amount of specific drugs in the blood.

Alternative Names

Therapeutic drug monitoring

How the Test is Performed

A blood sample is needed. For information on how this is done, see:Venipuncture

How to Prepare for the Test

Some drug level tests require preparation. Your health care provider will tell you how to prepare.

How the Test Will Feel

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the Test is Performed

With most medications, you need a certain level of drug in your bloodstream to obtain the desired effect. Some medications are harmful if the level rises too high and do not work if the levels are too low.

Monitoring the amount of the drug found in your blood allows your health care provider to make sure the drug levels are within an effective range.

Drug level testing is especially important in people taking drugs such as:

Testing may also be done to determine how well your body breaks down the drug ( metabolism), or how it interacts with other necessary drugs.

Normal Results

Following are some of the drugs that are commonly checked, followed by the normal target levels:


Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

The examples above show the common measurements for results for these tests. Some laboratories use different measurements or may test different specimens.

What Abnormal Results Mean

Values outside the target range may be due to minor fluctuations or may be a sign that you need to adjust the dose of the medicine. A dose may need to be skipped if the value measured exceeds the following levels.

Following are toxic levels for some of the drugs that are commonly checked:


Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight but may include:

Review Date: 5/12/2011
Reviewed By: Frank A. Greco, M.D., Ph.D., Director, Biophysical Laboratory, The Lahey Clinic, Burlington, MA. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.
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