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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:

  • Procainamide or digoxin used to treat abnormal beating of the heart
  • Dilantin or valproic acid used to treat seizures
  • Gentamicin or amikacin, antibiotics used to treat infections

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 Values

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

  • Acetaminophen: varies with use
  • Amikacin: 15 to 25 mcg/mL
  • Aminophylline: 10 to 20 mcg/mL
  • Amitriptyline: 120 to 150 ng/mL
  • Carbamazepine: 5 to 12 mcg/mL
  • Chloramphenicol: 10 to 20 mcg/mL
  • Desipramine: 150 to 300 ng/mL
  • Digoxin: 0.8 to 2.0 ng/mL
  • Disopyramide: 2 to 5 mcg/mL
  • Ethosuximide: 40 to 100 mcg/mL
  • Flecainide: 0.2 to 1.0 mcg/mL
  • Gentamicin: 5 to 10 mcg/mL
  • Imipramine: 150 to 300 ng/mL
  • Kanamycin: 20 to 25 mcg/mL
  • Lidocaine: 1.5 to 5.0 mcg/mL
  • Lithium: 0.8 to 1.2 mEq/L
  • Methotrexate: greater than 0.01 mcmol
  • Nortriptyline: 50 to 150 ng/mL
  • Phenobarbital: 10 to 30 mcg/mL
  • Phenytoin: 10 to 20 mcg/mL
  • Primidone: 5 to 12 mcg/mL
  • Procainamide: 4 to 10 mcg/mL
  • Propranolol: 50 to 100 ng/mL
  • Quinidine: 2 to 5 mcg/mL
  • Salicylate: 100 to 250 mcg/mL
  • Theophylline: 10 to 20 mcg/mL
  • Tobramycin: 5 to 10 mcg/mL
  • Valproic acid: 50 to 100 mcg/mL

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 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:

  • 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
  • Chloramphenicol: greater than 25 mcg/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 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
  • Propranolol: greater than 150 ng/mL
  • Quinidine: greater than 10 mcg/mL
  • Salicylate: greater than 300 mcg/mL
  • Theophylline: greater than 20 mcg/mL
  • Yobramycin: greater than 12 mcg/mL
  • Valproic acid: greater than 100 mcg/mL

What the risks are

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:

  • Excessive bleeding
  • Fainting or feeling light-headed
  • Hematoma (blood accumulating under the skin)
  • Infection (a slight risk any time the skin is broken)

Updated: 5/12/2011

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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