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Therapeutic drug levels

Alternative names

Therapeutic drug level tests are usually performed to look for the presence and the amount of specific drugs in the blood.

With most medications, you need a certain level of drug in the blood stream to obtain the desired effect. Some medications are toxic if the level rises too high and are ineffective if the levels are too low. Monitoring serum drug levels enables your health care provider to ensure that your drug levels are within an effective range.

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 puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to cause the vein to swell with blood.

A needle is inserted into the vein, and the blood is collected in air-tight vials or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

Infant or young child:
The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

The sample is then taken to the laboratory, where it is evaluated for the particular drug specified by your health care provider.

How to prepare for the test

Some drug level tests require preparation. Ask your health care provider for preparation information related to the test ordered.

Infants and children:
The preparation you can provide for this test depends on your child's age, previous experience, and level of trust. For general information regarding how you can prepare your child, see the following:
  • infant test or procedure preparation (birth to 1 year)
  • toddler test or procedure preparation (1 to 3 years)
  • preschooler test or procedure preparation (3 to 6 years)
  • schoolage test or procedure preparation (6 to 12 years)
  • adolescent test or procedure preparation (12 to 18 years)

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

Testing involves taking multiple measurements of blood levels of a drug over time. The main purpose is to determine the effective drug dosages and to prevent toxicity. 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

Often, these are drugs where the margin of safety between therapeutic levels and toxic levels is narrow. Testing may also be done to determine absorption, rate of metabolism , or interaction with other necessary drugs.

Normal Values

Following are some of the drugs that are commonly checked, and their 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
  • digitoxin: 15 to 20 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
  • mcg/ml = microgram per milliliter
  • ng/ml = nanogram per milliliter
  • mEq/L = milliequivalents per liter
  • mcmol = micromole

What abnormal results mean

Following are some of the drugs that are commonly checked, and their toxic levels:
  • 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
  • digitoxin: greater than 25 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
  • tobramycin: greater than 12 mcg/ml
  • valproic acid: greater than 100 mcg/ml
  • mcg/ml = microgram per milliliter
  • ng/ml = nanogram per milliliter
  • mEq/L = milliequivalents per liter
  • mcmol = micromole

What the risks are

Risks associated with venipuncture are slight:
  • excessive bleeding
  • fainting or feeling light-headed
  • hematoma (blood accumulating under the skin)
  • infection (a slight risk any time the skin is broken)
  • multiple punctures to locate veins

Special considerations

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.

Update Date: 11/3/2003

Sherry Woodhouse, M.D., Department of Pathology and Laboratory Medicine, Cleveland Clinic Florida, Weston, FL. Review provided by VeriMed Healthcare Network.

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Last updated: Tue, 06 Jan 2009 00:20:03 GMT