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therapeutic drug levels

Alternative Names 
drug levels, serum drug levels, therapeutic drug monitoring

Therapeutic drug levels refers to a test that measures the amount of the drug in the bloodstream.

When a medication is given as therapy for a disease, the amount of the drug in the blood is extremely important. This is because too much of the drug may be harmful, while too little may be ineffective. The same dose of medication may help one person and harm another. This test helps doctors determine the correct dose of medication to give a person.

Who is a candidate for the test? 
Not all medications require drug levels to be measured. The doctor will let a person known if drug level testing is needed. Examples of medications that need monitoring include certain antibiotics, such as gentamicin and vancomycin, medications used to treat epilepsy, such as phenytoin, and heart medications, such as digoxin.

How is the test performed? 
To measure the amount of a medication in the bloodstream, a blood sample is needed. Blood is usually taken from a vein on the forearm or hand. First, the skin over the vein is cleaned with an antiseptic. Next, a strong rubber tube, or "tourniquet," is wrapped around the upper arm. This enlarges the veins in the lower arm by restricting blood flow through them. A fine needle is gently inserted into a vein, and the tourniquet is removed. Blood flows from the vein through the needle and is collected in a syringe or vial. The blood is then sent to the laboratory for testing. After the needle is withdrawn, the puncture site is covered for a short time to prevent bleeding.

What is involved in preparation for the test? 
A person should request specific instructions from his or her doctor. Generally, no preparation is required.

What do the test results mean? 
Each of the medications that needs to be monitored has a specific level the doctor aims for. If the level is too high, the doctor will reduce the amount of medication. If the level is too low, the doctor will increase the amount of medication. Alternatively, the doctor may change the number of times a day the medication is given.

Author: David T. Moran, MD
Reviewer: eknowhow Medical Review Panel
Editor: Dr John Hearne
Last Updated: 6/06/2005
Potential conflict of interest information for reviewers available on request

This website and article is not a substitute for independent professional advice. Nothing contained in this website is intended to be used as medical advice and it is not intended to be used to diagnose, treat, cure or prevent any disease, nor should it be used for therapeutic purposes or as a substitute for your own health professional's advice.  All Health and any associated parties do not accept any liability for any injury, loss or damage incurred by use of or reliance on the information.


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