Therapeutic drug levels
Alternative names:
acetaminophen, amikacin, aminophylline, amitriptyline, carbamazepine, chloramphenicol, desipramine, digitoxin, digoxin, disopyramide, ethosuximide, flecainide, gentamicin, imipramine, kanamycin, lidocaine, lithium, methotrexate, nortriptyline, phenobarbital, phenytoin, primidone, procainamide, propranolol, quinidine, salicylate, serum drug levels; TDM; theophylline, therapeutic drug monitoring; tobramycin, valproic acid, serum
How the test is performed:
Adult or child:
Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (fill with blood). A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the tourniquet 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 prepared to evaluate 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 physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experience, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:
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 medications (for example, antiarrhythmics, digoxin, bronchodilators, some antibiotics, and anticonvulsants) 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.
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