Therapeutic drug levels

Definition

Therapeutic drug levels are lab tests to look for the amount of a drug or medicine in the blood.

Alternative Names

Therapeutic drug monitoring

How the Test is Performed

A blood sample is needed. Most of the time, blood is drawn from a vein located on the inside of the elbow or the back of the hand.

How to Prepare for the Test

You will need to prepare for some drug level tests.

How the Test will Feel

You may feel slight pain or a sting when the needle is inserted. You may also feel some throbbing at the site after the blood is drawn.

Why the Test is Performed

With most medicines, you need a certain level of the drug in your blood to get the proper effect. Some medicines 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 provider to make sure the drug levels are in the proper range.

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

Testing may also be done to determine how well your body breaks down the drug or how it interacts with other drugs you need.

Normal Results

Following are some of the drugs that are commonly checked and the normal target levels:

Normal value ranges may vary slightly among different laboratories. Talk to your provider 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 changes or be a sign that you need to adjust your dosages. Your provider may tell you to skip a dose if the values measured are too high.

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

References

Bluth MH, Pincus MR, Abraham NZ. Toxicology and therapeutic drug monitoring. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Philadelphia, PA: Elsevier; 2022:chap 24.

Clarke W. Overview of therapeutic drug monitoring. In: Clarke W, Dasgupta A, eds. Clinical Challenges in Therapeutic Drug Monitoring. Cambridge, MA: Elsevier; 2016:chap 1.

Diasio RB. Principles of drug therapy. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine. 26th ed. Philadelphia, PA: Elsevier; 2020:chap 26.

Nelson LS. Acute poisoning. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine. 26th ed. Philadelphia, PA: Elsevier; 2020:chap 102.


Review Date: 6/20/2023
Reviewed By: Jacob Berman, MD, MPH, Clinical Assistant Professor of Medicine, Division of General Internal Medicine, University of Washington School of Medicine, Seattle, WA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.
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