Therapeutic drug levels

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

How is the Test 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.

Preparation for the Test

You will need to prepare for some drug level tests.

  • Your health care provider will tell you if you need to change the times you take any of your medicines.
  • Do not stop or change your medications without talking to your doctor first.

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 is the Test 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 health care 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 for Therapeutic drug levels

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

Note:

  • mcg/mL = microgram per milliliter
  • ng/mL = nanogram per milliliter
  • mEq/L = milliequivalents per liter
  • mcmol = micromole

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 changes or be a sign that you need to adjust your dosages. Your doctor 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:

  • Acetaminophen: greater than 250 mcg/mL
  • Amikacin: greater than 25 mcg/mL
  • Aminophylline: greater than 20 mcg/mL
  • Amitriptyline: greater than 500 ng/mL
  • Carbamazepine: greater than 12 mcg/mL
  • Cyclosporine: greater than 400 ng/mL
  • Desipramine: greater than 500 ng/mL
  • Digoxin: greater than 2.4 ng/mL
  • Disopyramide: greater than 5 mcg/mL
  • Ethosuximide: greater than 100 mcg/mL
  • Flecainide: greater than 1.0 mcg/mL
  • Gentamicin: greater than 12 mcg/mL
  • Imipramine: greater than 500 ng/mL
  • Kanamycin: greater than 35 mcg/mL
  • Lidocaine: greater than 5 mcg/mL
  • Lithium: greater than 2.0 mEq/L
  • Methotrexate: greater than 10 mcmol/L over 24-hours
  • Nortriptyline: greater than 500 ng/mL
  • Phenobarbital: greater than 40 mcg/mL
  • Phenytoin: greater than 30 mcg/mL
  • Primidone: greater than 15 mcg/mL
  • Procainamide: greater than 16 mcg/mL
  • Quinidine: greater than 10 mcg/mL
  • Salicylate: greater than 300 mcg/mL
  • Theophylline: greater than 20 mcg/mL
  • Tobramycin: greater than 12 mcg/mL
  • Valproic acid: greater than 100 mcg/mL

References

Diasio RB. Principles of drug therapy. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Saunders Elsevier; 2011:chap 28.

Ford MD. Acute poisoning. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Saunders Elsevier; 2011:chap 110.

Review Date: 4/29/2013
Reviewed By: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Bethanne Black, and the A.D.A.M. Editorial team.
The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed medical professional should be consulted for diagnosis and treatment of any and all medical conditions. Links to other sites are provided for information only -- they do not constitute endorsements of those other sites. © 1997- 2014 A.D.A.M., Inc. Any duplication or distribution of the information contained herein is strictly prohibited.
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