Health Encyclopedia

Therapeutic drug levels

Definition

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

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

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 (See: 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 therapeutic 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.

What abnormal results mean

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
Yheophylline: 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)

Review Date:8/28/2007
Reviewed By:Daniel R Alexander, MD, Department of Internal Medicine, St. Mary's Hospital, Leonardtown, MD. Review provided by VeriMed Healthcare Network.

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